Vegetables · Vegetable
Kale
Brassica oleracea · also called Curly kale, borecole, col rizada (Spanish)
Kale is a leafy green in the cabbage family. The leaves are eaten raw or cooked. One raw cup has only about 7 calories but is very high in vitamin K. Large studies link leafy greens as a group with lower heart disease risk, but that is a link, not proof, and trials of kale itself are few and small. The main caution is for people who take warfarin, because vitamin K changes how it works.
General information, not medical advice. Last substantive update · Not medically reviewed. A named clinician has not signed off on this page. How this page is made
About Kale
Kale is an everyday leafy green that is low in calories and very high in vitamin K. Research on eating kale itself is thin. Most evidence is about leafy greens as a group, plus a few small studies of kale juice, bars and capsules. The practical safety points are vitamin K with warfarin, goitrogens and the thyroid, and washing raw leaves.
What it is
Kale is a leafy vegetable of the species Brassica oleracea, in the cabbage family, Brassicaceae. The leaves are eaten raw or cooked.
One raw cup (21 g) provides about 7 calories, 0.9 g of fibre, 82 micrograms of vitamin K, 19.6 mg of vitamin C and 53 mg of calcium. Per 100 g, USDA lists about 390 micrograms of vitamin K and about 6,260 micrograms of lutein plus zeaxanthin. The per-cup figures are our arithmetic from USDA's 100 g values. USDA lists cooked kale as a separate food.
What the research shows
Large studies that followed people over time link higher intake of green leafy vegetables with slightly lower rates of heart disease and stroke. A 2021 meta-analysis of 17 studies found a pooled relative risk of 0.93 for cardiovascular events. These studies group many greens together and show a link, not cause.
One small study gave 150 mL of kale juice a day for 12 weeks to 32 men with high cholesterol. LDL fell 10% and HDL rose 27%, but the abstract describes no comparison group.
In a 12-week randomised trial of 30 adults with type 2 diabetes, freeze-dried kale bars (about 341 g of fresh kale a day) lowered HbA1c compared with placebo bars. Weight and calorie intake also fell, and one small trial is not enough to draw conclusions.
A trial in 78 women found no change in a urine estrogen ratio with a Brussels sprout and kale capsule. A food diary study of 960 older adults linked leafy greens, not named as kale, with slower decline in thinking skills.
Whole kale versus juice, bars, powders and chips
The studies used different products: juice, freeze-dried bars, a mixed capsule, and a different species (Ethiopian kale). The results do not carry over from one form to another, or to ordinary meals.
Dried products such as chips can concentrate contaminants, as the thallium case report shows.
At a glance
Traditional uses history, not proof
- Eaten raw or cooked as a leaf vegetable
- No source we read documents a traditional medical use
Studied for research
- Heart disease and stroke in diets rich in green leafy vegetables, low certainty
- Blood cholesterol with kale juice in one small study, very low certainty
- Blood sugar in type 2 diabetes with freeze-dried kale bars, low certainty
- Memory and thinking in older age with leafy greens, low certainty
- Estrogen breakdown in urine, no effect found
Main compounds
- Vitamin K (about 82 micrograms in a raw cup)
- Vitamin C, calcium, potassium and folate
- Lutein and zeaxanthin, plant pigments
- Glucosinolates, sulfur-containing plant compounds
Common forms
- Fresh raw leaves (the form the nutrition figures describe)
- Cooked leaves, steamed, sauteed or boiled
- Kale juice
- Freeze-dried kale in bars or powder (not the same as eating the leaf)
- Kale chips (dried products can concentrate contaminants)
Research links: MedlinePlus: Warfarin · Linus Pauling Institute: Iodine (goitrogens) · FDA: Selecting and Serving Produce Safely · Freeze-dried kale bar in type 2 diabetes, randomised trial (Nutrients, 2024) · Green leafy vegetables and cardiovascular disease, meta-analysis (2021) · Thallium and commercial kale chips, case report (2025) · Research Library · Genes & Biomarkers Library
What it is and its history
- Accepted scientific name
- Brassica oleracea
- Naming authority
- L. (GBIF accepted, usageKey 3042845). Kale is the variety sabellica (curly kale) or the Acephala group of this species.
- Synonyms
- Brassica oleracea var. sabellica L. (GBIF lists it as a synonym of Brassica oleracea L., usageKey 3042870); Brassica oleracea var. acephala (the name used in the kale juice and kale bar studies); Brassica oleracea (Acephala Group) (the name used by USDA)
- Common names
- Kale (English); Curly kale (English); Borecole (English); Col rizada (Spanish)
- Family or group
- Brassicaceae (the cabbage or mustard family).
- Classification
- Plantae > Tracheophyta > Magnoliopsida > Brassicales > Brassicaceae > Brassica > Brassica oleracea
- Native range
- Not verified from a source read.
- Habitat
- Not verified.
- What it looks like
- Roots: Not verified.; Stems: Not verified.; Leaves: The leaves are the part eaten. Curly kale has frilly-edged leaves (description from the existing summary, not a source read).; Flowers: Not verified.; Fruit: Not verified.
- Cultivation
- Not verified from a source read.
- Harvest and processing
- Eaten raw or cooked; also sold as juice, chips, powder and freeze-dried products (forms named in the studies read and the existing summary).
- Look-alikes and adulterants
- Name: Ethiopian kale (Brassica carinata); Note: A different species. One human trial in this review used it, so its results do not apply to curly kale.
- Sustainability and conservation
- Not assessed.
Photographs
Traditional use and history history, not clinical proof
| Tradition or era | Documented use | Source |
|---|---|---|
| Everyday leafy vegetable | Eaten as a cooked or raw leaf vegetable. No source read documents its history or any traditional medical use. | NOT verified (no source read) |
Traditional use shows how it was used. It does not show that it works or is safe today.
What is inside, and how it may work
What it contains and how those compounds act in cells and animals. Laboratory and animal mechanisms are not proven effects in people.
Main compounds
| Compound | Family | Where it is concentrated | Activity studied | Level of evidence |
|---|---|---|---|---|
| Vitamin K (phylloquinone) | vitamin | Leaves About 390 micrograms per 100 g raw; about 82 micrograms in one raw cup of 21 g (USDA, cup value is our arithmetic) | Needed for blood clotting. Affects how warfarin works. 16 15 | Composition data and authoritative guidance |
| Vitamin C | vitamin | Leaves 93.4 mg per 100 g raw; about 19.6 mg per raw cup (USDA, cup value is our arithmetic) | Provides vitamin C in the diet. Not tested here as a treatment. 16 | Composition data |
| Lutein and zeaxanthin | carotenoids (plant pigments) | Leaves About 6,260 micrograms per 100 g raw (USDA) | Studies of lutein intake in general link it with lower risk of some eye diseases, but kale was not tested. 16 6 | Composition data; observational links for lutein in general |
| Beta-carotene | carotenoids (plant pigments) | Leaves About 2,870 micrograms per 100 g raw (USDA) | A source of vitamin A in the diet. 16 | Composition data |
| Dietary fibre | carbohydrate | Leaves 4.1 g per 100 g raw; about 0.9 g per raw cup (USDA, cup value is our arithmetic) | Provides fibre in the diet. Not tested here as a treatment. 16 | Composition data |
| Glucosinolates | sulfur-containing plant compounds of the cabbage family | Leaves (not verified) | Turned into isothiocyanates when the leaf is chopped or chewed. Most research is from the lab and animals. The existing summary states this and was not re-checked against a source. 10 | Lab and animal; summary only |
| Oxalic acid | organic acid | Leaves 0.01 g in 150 g of kale, compared with 1.27 g in 150 g of spinach (one 2008 study) | Can bind minerals. Added to a kale meal, it did not change iron absorption. 11 | Human measurement, small study |
Mechanisms, receptors and pathways
| Target or pathway | Gene or receptor | What was found | Level |
|---|---|---|---|
| Vitamin K and blood clotting with warfarin | Vitamin K in food can change how warfarin works, per MedlinePlus. Kale is very high in vitamin K (USDA). 15 16 | Authoritative guidance | |
| Goitrogens and thyroid iodine use | Goitrogens in cruciferous vegetables can interfere with how the thyroid uses iodine. They matter mainly in large amounts or with low iodine, per the Linus Pauling Institute. 14 | Authoritative guidance | |
| Plant compounds and DNA damage in blood cells | Raw Ethiopian kale contains allyl isothiocyanate and cooked does not. Both raw and cooked preparations changed a lab measure of DNA damage after aflatoxin B1 exposure. How they did it was not shown. 10 | Human marker (different species) |
Absorption, metabolism and elimination
- Absorption
- Vitamin K: modelled bioavailability from a 400 g kale meal with 30 g oil was 4.7% in six volunteers, peaking in plasma between 6 and 10 hours.
- Distribution
- Plasma and tissue half-times for vitamin K were 8.8 and 215 hours in the same study.
- Metabolism
- Not researched.
- Elimination
- Not researched.
- Bioavailability
- Vitamin K 4.7% in that small study. Other nutrients not researched.
Isolated compounds versus whole extracts
The kale studies used different products. The cholesterol study used kale juice, the diabetes trial used freeze-dried kale bars at about 341 g of fresh kale a day, and the hormone trial used a capsule of dried Brussels sprouts and kale. The leafy-green heart studies are about food eaten in meals. The thallium case involved dried kale chips, where drying may concentrate contaminants. Results from one form should not be carried over to another.
Nutritional composition
Per 100 g of raw kale (USDA FoodData Central, SR Legacy record 168421, last updated 2019): 35 kcal, 89.6 g water, 2.9 g protein, 1.5 g fat, 4.4 g carbohydrate and 4.1 g fibre. It is very high in vitamin K (about 390 micrograms), and has 93.4 mg vitamin C, 254 mg calcium, 348 mg potassium and about 6,260 micrograms lutein plus zeaxanthin. One raw cup is 21 g, so the per-cup figures are our arithmetic from the 100 g values. USDA lists cooked kale as a separate food; these figures are for raw kale only. Values vary with variety, ripeness and storage.
6 low certainty2 very low certainty1 not enough evidence
What was looked at
The 9 benefits and uses checked for Kale, each with how strong the evidence is. Tap one to open it.
- Heart disease and stroke (diets rich in green leafy vegetables)Low certainty
- Blood cholesterol (kale juice, one small before-and-after study)Very low certainty
- Blood sugar control in type 2 diabetes (freeze-dried kale bars, one small trial)Low certainty
- Memory and thinking in older age (green leafy vegetables)Low certainty
- Estrogen breakdown (urinary 2:16 hydroxyestrone ratio) - no effect foundLow certainty
- Iron absorption and oxalate (kale compared with spinach)Low certainty
- How much vitamin K the body absorbs from kaleLow certainty
- A lab DNA-damage marker (Ethiopian kale, a different species)Very low certainty
- Cancer, eye disease, weight loss, detox and other claimsNot enough evidence
Lab markers tracked in the studies
HbA1cInsulin resistanceFasting blood glucoseLDL cholesterolHDL cholesterolBody weightUrinary 2:16 hydroxyestrone ratioNon-haem iron absorptionPlasma vitamin K (phylloquinone)DNA damage in blood cells (comet assay, lab-induced)
What is Kale good for?
The main studied uses and how strong the human evidence is for each. Most results are links seen in large groups of people, or come from small studies of kale juice, bars or capsules, not from eating kale in ordinary meals.
Benefits, one by one
Each card is a research question, not a promise. Tap a goal to filter. Open a card to see the studies, doses and limits.
Heart disease and stroke (diets rich in green leafy vegetables)
Low certaintyIn large groups of people followed over time, those who ate more green leafy vegetables had a slightly lower rate of heart disease and stroke. Kale was not singled out in the reviews we read, so this is a link for leafy greens as a group, not proof that kale protects the heart.
2 studiesGreen leafy vegetables eaten as food. Kale alone was not separated.
Studies, doses and limits
What the trials found
A 2021 meta-analysis of 17 cohort studies found green leafy vegetable intake went with fewer cardiovascular events. A 2021 review of 24 meta-analyses reported that each 100 g a day of green leafy vegetables went with about 25% lower risk of death from any cause, coronary heart disease and stroke.
Results
Slightly lower risk in people who ate more leafy greens. Kale is not named in either abstract.
Effect sizes as reported
Pooled relative risk of cardiovascular events 0.93 (95% CI 0.92-0.95, 17 studies); cerebral infarction 0.92 (0.88-0.96); heart disease 0.93 (0.87-0.99). The umbrella review reports a drop of about 25% per 100 g a day (abstract gives no confidence interval).
Why this rating
Observational data only. People who eat more leafy greens may differ in other ways. The studies group many different greens together, and the umbrella review reports a much larger effect than the cohort meta-analysis, which the abstracts do not explain.
Form and dose studied
Green leafy vegetables eaten as food. Kale alone was not separated.
Who was studied
Adults in cohort studies.
Limits
Association, not cause. Leafy greens as a group, not kale. Only abstracts were read.
Why researchers looked
Kale is a commonly eaten leafy green, and leafy greens are often named in healthy eating patterns.
Proposed mechanism Preliminary mechanism
Possible routes include fibre, potassium, folate, carotenoids and plant compounds. These ideas are unproven in people.
A proposed mechanism does not show that the effect happens in people.
Kale is very high in vitamin K, which matters for people on warfarin. See the Safety tab.
Blood cholesterol (kale juice, one small before-and-after study)
Very low certaintyIn one small study, 32 men with high cholesterol drank kale juice every day for 12 weeks. Their LDL (bad) cholesterol fell and their HDL (good) cholesterol rose. The study had no comparison group described, so the results are weak evidence. This was juice, not whole kale.
1 studiesKale juice, 150 mL a day for 12 weeks. Not the same as eating the who…
Studies, doses and limits
What the trials found
A 2008 study in Korea gave 32 men with total cholesterol above 200 mg/dL 150 mL of kale juice a day for 12 weeks and measured blood before and after.
Results
HDL cholesterol rose 27% and LDL cholesterol fell 10%. The HDL-to-LDL ratio rose 52% and the atherogenic index fell 24.2%. Body mass index and nutrient intake did not change. The authors say responses depended on smoking status.
Effect sizes as reported
HDL +27% (P<0.0001); LDL -10% (P=0.0007); HDL/LDL ratio +52% (P<0.0001); atherogenic index -24.2% (P<0.0001). No confidence intervals in the abstract.
Why this rating
A small study of men only. The abstract describes measurements before and after, with no placebo or control group, so changes cannot be credited to the juice alone. It has not been repeated in our sources.
Form and dose studied
Kale juice, 150 mL a day for 12 weeks. Not the same as eating the whole leaf.
Who was studied
32 men with high cholesterol, Korea.
Limits
No control group described, men only, short, small, juice. Funding is not stated in the abstract.
Why researchers looked
Researchers wanted to know whether a leafy green could improve blood fats in men with high cholesterol.
Proposed mechanism Unknown
The abstract does not test or prove a mechanism.
A proposed mechanism does not show that the effect happens in people.
Do not use food to replace cholesterol medicine without asking your doctor.
Blood sugar control in type 2 diabetes (freeze-dried kale bars, one small trial)
Low certaintyIn one 12-week trial of 30 adults with type 2 diabetes, people who ate three freeze-dried kale bars a day (about 341 g of fresh kale) had a lower HbA1c than people on placebo bars. They also lost weight and ate fewer calories. That is far more kale than most people eat, in bar form, and one small trial is not enough to draw conclusions.
1 studiesFreeze-dried kale in a bar, equal to about 341 g fresh kale a day. No…
Studies, doses and limits
What the trials found
A 2024 randomised, double-blind, placebo-controlled trial in Denmark gave 30 adults with type 2 diabetes either placebo bars or kale bars (three a day, each with 26.25 g freeze-dried kale) for 12 weeks.
Results
Compared with placebo, the kale group had a significant fall in HbA1c, insulin resistance, body weight and calorie intake. Fasting blood glucose and LDL cholesterol showed only positive trends. Total fat mass and the 240-minute glucose curve during a sugar-drink test did not differ significantly.
Effect sizes as reported
The abstract gives significance but no HbA1c values or confidence intervals.
Why this rating
It is a proper randomised, blinded trial, but only 30 people, a single short trial, a very large amount, and the kale group also ate fewer calories and lost weight, which could explain part of the effect. The abstract gives no numbers.
Form and dose studied
Freeze-dried kale in a bar, equal to about 341 g fresh kale a day. Not ordinary kale servings.
Who was studied
30 adults with type 2 diabetes, Denmark.
Limits
Small, one site, 12 weeks, processed product. Funding is not stated in the abstract; no conflict of interest was declared.
Why researchers looked
Kale is high in fibre and low in sugar, so researchers asked whether a large daily amount might help people with type 2 diabetes.
Proposed mechanism Authors hypothesis
Not established. The authors point to kale's fibre and plant compounds, but the trial did not test how it worked.
A proposed mechanism does not show that the effect happens in people.
People with diabetes who take blood sugar medicine should ask their care team before changing their diet. A very large kale intake also means a large vitamin K intake, which matters with warfarin.
Memory and thinking in older age (green leafy vegetables)
Low certaintyIn one study that followed 960 older adults for about five years, those who ate the most green leafy vegetables lost thinking skills more slowly. The abstract does not name kale, and a food diary study cannot show cause.
1 studiesGreen leafy vegetables eaten as food.
Studies, doses and limits
What the trials found
A 2018 prospective study of 960 participants of the Memory and Aging Project in Chicago, ages 58-99, used food questionnaires and repeated memory tests over a mean of 4.7 years.
Results
People in the top fifth for leafy green intake (median 1.3 servings a day) declined more slowly than the others. The authors compare this to being 11 years younger. Higher intake of each nutrient except beta-carotene also went with slower decline.
Effect sizes as reported
Highest versus lowest fifth: beta 0.05 standardised units (P=0.0001). Individual nutrients ranged from beta 0.02 to 0.05.
Why this rating
Observational. People who eat more greens may differ in other ways, even after adjustment, and food questionnaires are imperfect. Kale is not named in the abstract.
Form and dose studied
Green leafy vegetables eaten as food.
Who was studied
960 older adults (58-99 years) in Chicago.
Limits
Association, not cause. Leafy greens as a group. Only the abstract was read.
Why researchers looked
Kale and other leafy greens are rich in vitamin K, lutein, folate and nitrate, which have each been studied in relation to brain ageing.
Proposed mechanism Authors interpretation
The authors looked at individual nutrients in leafy greens (vitamin K, lutein, folate, vitamin E, nitrate, kaempferol) and found each went with slower decline, but this does not show which one matters or whether any is the cause.
A proposed mechanism does not show that the effect happens in people.
None specific. People on warfarin need steady vitamin K intake (Safety tab).
Estrogen breakdown (urinary 2:16 hydroxyestrone ratio) - no effect found
Low certaintyA trial in 78 women tested whether capsules of dried Brussels sprouts and kale, or a daily serving of broccoli or Brussels sprouts, would shift a urine hormone ratio some researchers link to breast cancer risk. After eight weeks, none did.
1 studiesDried Brussels sprout and kale capsules, or whole broccoli or Brussel…
Studies, doses and limits
What the trials found
A 2024 randomised, partly blinded, placebo-controlled trial in 78 healthy premenopausal women aged 38 to 50 with a low starting ratio compared six capsules a day of dried Brussels sprouts and kale (550 mg each), 40 g a day of broccoli or Brussels sprouts, or placebo for eight weeks.
Results
No treatment effect on the ratio (P=0.9). The ratio changed a little over time in all groups.
Effect sizes as reported
No treatment effect (P=0.9; treatment-by-time P=0.6).
Why this rating
A properly randomised trial, but one trial in healthy women, eight weeks, and a marker that is not a disease outcome. The capsule mixed kale with Brussels sprouts, so kale is not tested alone.
Form and dose studied
Dried Brussels sprout and kale capsules, or whole broccoli or Brussels sprouts.
Who was studied
78 healthy premenopausal women, USA.
Limits
Kale only in a mixed capsule. Biomarker, not cancer. Funding not stated in the abstract.
Why researchers looked
Earlier studies linked cruciferous vegetable intake with a higher urinary 2:16 ratio, which has been proposed as a marker of breast cancer risk.
Proposed mechanism Hypothesis
Indole compounds from cruciferous vegetables were thought to change how estrogen is broken down. This trial did not show that.
A proposed mechanism does not show that the effect happens in people.
None specific.
Iron absorption and oxalate (kale compared with spinach)
Low certaintySpinach has a lot of oxalic acid, which some people worry blocks iron. A small Swiss study found a kale meal has very little oxalic acid, and adding oxalic acid to the kale meal did not change how much iron the women absorbed.
1 studies150 g kale in a wheat roll meal (the abstract does not say how the ka…
Studies, doses and limits
What the trials found
Two randomised crossover studies in Zurich used stable iron isotopes in 16 women, 13 of whom finished both. Test meals were wheat rolls with 150 g spinach (1.27 g oxalic acid) or 150 g kale (0.01 g), and kale with oxalate added.
Results
Iron absorption from the kale meal (10.7%) was about the same as from kale with oxalate added (11.5%). Absorption from the spinach meal was 24% lower than from the kale meal, but this was not statistically significant.
Effect sizes as reported
Kale 10.7% versus kale plus oxalate 11.5% (P=0.86); spinach meal 24% lower than kale meal (P>0.16, not significant).
Why this rating
Small, healthy women, a single test meal, and results normalised to a spinach reference meal.
Form and dose studied
150 g kale in a wheat roll meal (the abstract does not say how the kale was prepared).
Who was studied
16 healthy women aged 18-45, under 60 kg.
Limits
Small, one meal, healthy women. It tests oxalate and iron, not kidney stones.
Why researchers looked
Kale and spinach are both leafy greens, but they differ a lot in oxalate, so the study used them to test whether oxalate blocks iron.
Proposed mechanism Human measurement
Oxalate can bind minerals, but this study found it did not reduce non-haem iron absorption from the meal.
A proposed mechanism does not show that the effect happens in people.
See the Safety tab for kidney stones; this study did not look at them.
How much vitamin K the body absorbs from kale
Low certaintyIn seven healthy volunteers who ate one large kale serving with oil, only a small share of its vitamin K reached the blood: about 5%. This measures absorption, not a health benefit.
1 studies400 g of kale (the abstract does not say whether it was cooked) with …
Studies, doses and limits
What the trials found
A 2010 US study fed seven healthy volunteers one 400 g serving of kale with 30 g of vegetable oil and tracked the labelled vitamin K (phylloquinone) in plasma. Six showed detectable amounts.
Results
Blood levels peaked between 6 and 10 hours. Modelling put the bioavailability of vitamin K from kale at 4.7%.
Effect sizes as reported
Mean bioavailability 4.7% (n=6); average peak 2.1 nmol/L; plasma half-time 8.8 hours.
Why this rating
Seven volunteers, one meal, with added oil. It describes absorption in this setting and does not show a benefit.
Form and dose studied
400 g of kale (the abstract does not say whether it was cooked) with 30 g of oil. This is a research meal, not advice.
Who was studied
Seven healthy adult volunteers.
Limits
Tiny, one meal, labelled kale grown for the study.
Why researchers looked
Kale is a rich source of vitamin K, so researchers wanted to measure how much of it the body takes up.
Proposed mechanism Human measurement
The kale was grown to contain carbon-13-labelled vitamin K so it could be tracked in blood.
A proposed mechanism does not show that the effect happens in people.
Because kale holds so much vitamin K, steady intake matters for people on warfarin (Safety tab).
A lab DNA-damage marker (Ethiopian kale, a different species)
Very low certaintyIn 22 healthy volunteers, five days of eating cooked Ethiopian kale made their blood cells less sensitive to DNA damage from a mould toxin in a lab test. Ethiopian kale is a different plant from the kale sold in most shops, and this was a lab marker, not a disease.
1 studiesRaw and cooked Ethiopian kale leaf preparations.
Studies, doses and limits
What the trials found
A 2018 randomised crossover trial in 22 healthy people compared five days of raw and five days of cooked Brassica carinata leaves. Blood cells were then exposed to aflatoxin B1 in the lab and DNA damage was measured.
Results
After cooked leaves, DNA damage fell 35% (with liver enzyme mix) and 33% (without), compared with baseline. After raw leaves it fell 21% with the enzyme mix, which was not significant (P=0.08). A blood marker of inflammation (PGE2) fell after raw leaves. Total antioxidant capacity did not change.
Effect sizes as reported
Cooked: -35% (+S9) and -33% (-S9), P<=0.01; raw: -21% (+S9), P=0.08.
Why this rating
A lab marker, a different species (Brassica carinata, not curly kale), five days only, and no disease outcome.
Form and dose studied
Raw and cooked Ethiopian kale leaf preparations.
Who was studied
22 healthy adults, Germany.
Limits
Different species, marker only, short. The results do not apply to curly kale.
Why researchers looked
Researchers wanted to know whether cooking changes the protective effect of Brassica leaves against the mould toxin aflatoxin B1.
Proposed mechanism Human biomarker
Raw leaves contain allyl isothiocyanate; cooked leaves do not. The authors discuss how cooking changes the plant compounds, but a mechanism in people was not shown.
A proposed mechanism does not show that the effect happens in people.
None specific.
Cancer, eye disease, weight loss, detox and other claims
Not enough evidenceKale is often promoted for cancer prevention, eye health, weight loss and detox. We found no trial of kale as a food for any of these. Lutein, a plant pigment in kale, has been linked to lower eye disease risk in studies of lutein intake in general, but those studies did not test kale.
1 studiesNot applicable.
Studies, doses and limits
What the trials found
A 2021 review of 24 meta-analyses reported that dietary lutein went with lower risk of age-related macular degeneration, cataracts and some other outcomes. It did not test kale.
Results
No kale-specific human trial for these claims was found in the sources read.
Why this rating
No kale-specific human evidence was found for cancer, eye disease, weight or detox.
Limits
Lutein intake studies are observational and cover many foods and supplements.
Why researchers looked
Kale contains lutein, carotenoids and sulfur-containing plant compounds that are studied in the lab.
Proposed mechanism Lab and animal
Lab and animal work on plant compounds is common, but it is not evidence of an effect in people.
A proposed mechanism does not show that the effect happens in people.
None specific.
Connections to the rest of health
How Kale connects to the rest of our health knowledge graph. Every link says what kind of connection it is. A safety link (for example to the liver) is a warning, not a benefit. A study that merely mentions a topic never creates a benefit link.
Explore every connection: goals, body systems, organs, symptoms, genes
A. Wellness goals
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| Heart Health | Low certainty | Heart disease and stroke (diets rich in green leafy vegetables): In large groups of people followed over time, those who ate more green leafy vegetables had a slightly lower rate of heart disease and stroke. Kale was not singled out in the reviews we read, so this is a link for leafy greens as a group, not proof that kale protects the heart. | 9 6 | |
| Cholesterol Balance | Very low certainty | Blood cholesterol (kale juice, one small before-and-after study): In one small study, 32 men with high cholesterol drank kale juice every day for 12 weeks. Their LDL (bad) cholesterol fell and their HDL (good) cholesterol rose. The study had no comparison group described, so the results are weak evidence. This was juice, not whole kale. | 5 | |
| Heart Health | Very low certainty | Blood cholesterol (kale juice, one small before-and-after study): In one small study, 32 men with high cholesterol drank kale juice every day for 12 weeks. Their LDL (bad) cholesterol fell and their HDL (good) cholesterol rose. The study had no comparison group described, so the results are weak evidence. This was juice, not whole kale. | 5 | |
| Blood Sugar Balance | Low certainty | Blood sugar control in type 2 diabetes (freeze-dried kale bars, one small trial): In one 12-week trial of 30 adults with type 2 diabetes, people who ate three freeze-dried kale bars a day (about 341 g of fresh kale) had a lower HbA1c than people on placebo bars. They also lost weight and ate fewer calories. That is far more kale than most people eat, in bar form, and one small trial is not enough to draw conclusions. | 4 | |
| Healthy Aging | Low certainty | Memory and thinking in older age (green leafy vegetables): In one study that followed 960 older adults for about five years, those who ate the most green leafy vegetables lost thinking skills more slowly. The abstract does not name kale, and a food diary study cannot show cause. | 7 | |
| Preventive Wellness | Low certainty | Estrogen breakdown (urinary 2:16 hydroxyestrone ratio) - no effect found: A trial in 78 women tested whether capsules of dried Brussels sprouts and kale, or a daily serving of broccoli or Brussels sprouts, would shift a urine hormone ratio some researchers link to breast cancer risk. After eight weeks, none did. | 3 | |
| Preventive Wellness | Low certainty | Iron absorption and oxalate (kale compared with spinach): Spinach has a lot of oxalic acid, which some people worry blocks iron. A small Swiss study found a kale meal has very little oxalic acid, and adding oxalic acid to the kale meal did not change how much iron the women absorbed. | 11 | |
| Preventive Wellness | Low certainty | How much vitamin K the body absorbs from kale: In seven healthy volunteers who ate one large kale serving with oil, only a small share of its vitamin K reached the blood: about 5%. This measures absorption, not a health benefit. | 8 | |
| Cellular Protection | Very low certainty | A lab DNA-damage marker (Ethiopian kale, a different species): In 22 healthy volunteers, five days of eating cooked Ethiopian kale made their blood cells less sensitive to DNA damage from a mould toxin in a lab test. Ethiopian kale is a different plant from the kale sold in most shops, and this was a lab marker, not a disease. | 10 | |
| Preventive Wellness | Not enough evidence | Cancer, eye disease, weight loss, detox and other claims: Kale is often promoted for cancer prevention, eye health, weight loss and detox. We found no trial of kale as a food for any of these. Lutein, a plant pigment in kale, has been linked to lower eye disease risk in studies of lutein intake in general, but those studies did not test kale. | 6 | |
| Weight Management | Not enough evidence | Cancer, eye disease, weight loss, detox and other claims: Kale is often promoted for cancer prevention, eye health, weight loss and detox. We found no trial of kale as a food for any of these. Lutein, a plant pigment in kale, has been linked to lower eye disease risk in studies of lutein intake in general, but those studies did not test kale. | 6 |
B. Body systems
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| Cardiovascular System | Biological association | Low certainty | Studied for: Heart disease and stroke (diets rich in green leafy vegetables) Studied for: Blood cholesterol (kale juice, one small before-and-after study) | 9 6 5 |
| Blood Sugar Support | Biological association | Low certainty | Studied for: Blood sugar control in type 2 diabetes (freeze-dried kale bars, one small trial) | 4 |
| Endocrine System | Biological association | Low certainty | Studied for: Blood sugar control in type 2 diabetes (freeze-dried kale bars, one small trial) Studied for: Estrogen breakdown (urinary 2:16 hydroxyestrone ratio) - no effect found | 4 3 |
| Nervous System | Biological association | Low certainty | Studied for: Memory and thinking in older age (green leafy vegetables) | 7 |
| Digestive System | Biological association | Low certainty | Studied for: Iron absorption and oxalate (kale compared with spinach) Studied for: How much vitamin K the body absorbs from kale | 11 8 |
| Cellular | Biological association | Very low certainty | Studied for: A lab DNA-damage marker (Ethiopian kale, a different species) | 10 |
| Eyes And Vision | Biological association | Not enough evidence | Studied for: Cancer, eye disease, weight loss, detox and other claims | 6 |
| Metabolism Weight | Biological association | Not enough evidence | Studied for: Cancer, eye disease, weight loss, detox and other claims | 6 |
C. Body parts and organs
I. Biomarkers
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| HbA1c | Biological association | Low certainty | Measured in trials: lowered. A lab change is not the same as a health benefit. | 4 |
| Insulin resistance | Biological association | Low certainty | Measured in trials: lowered. A lab change is not the same as a health benefit. | 4 |
| Fasting blood glucose | Biological association | Low certainty | Measured in trials: no clear change. A lab change is not the same as a health benefit. | 4 |
| LDL cholesterol | Biological association | Very low certainty | Measured in trials: lowered. A lab change is not the same as a health benefit. | 5 4 |
| HDL cholesterol | Biological association | Very low certainty | Measured in trials: raised. A lab change is not the same as a health benefit. | 5 |
| Body weight | Biological association | Low certainty | Measured in trials: lowered. A lab change is not the same as a health benefit. | 4 |
| Urinary 2:16 hydroxyestrone ratio | Biological association | Low certainty | Measured in trials: no clear change. A lab change is not the same as a health benefit. | 3 |
| Non-haem iron absorption | Biological association | Low certainty | Measured in trials: no clear change. A lab change is not the same as a health benefit. | 11 |
| Plasma vitamin K (phylloquinone) | Biological association | Low certainty | Measured in trials: raised. A lab change is not the same as a health benefit. | 8 |
| DNA damage in blood cells (comet assay, lab-induced) | Biological association | Very low certainty | Measured in trials: lowered. A lab change is not the same as a health benefit. | 10 |
L. Active compounds
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| Thallium | Adverse effect or risk | Documented risk (case report) | One household cluster of raised urine thallium was traced to a kale chip product. Preparation or compound: Commercial kale chips |
2 |
| Vitamin K (phylloquinone) | Biological association | Composition data and authoritative guidance | Needed for blood clotting. Affects how warfarin works. Preparation or compound: Leaves |
16 15 |
| Vitamin C | Biological association | Composition data | Provides vitamin C in the diet. Not tested here as a treatment. Preparation or compound: Leaves |
16 |
| Lutein and zeaxanthin | Biological association | Composition data; observational links for lutein in general | Studies of lutein intake in general link it with lower risk of some eye diseases, but kale was not tested. Preparation or compound: Leaves |
16 6 |
| Beta-carotene | Biological association | Composition data | A source of vitamin A in the diet. Preparation or compound: Leaves |
16 |
| Dietary fibre | Biological association | Composition data | Provides fibre in the diet. Not tested here as a treatment. Preparation or compound: Leaves |
16 |
| Glucosinolates | Biological association | Lab and animal; summary only | Turned into isothiocyanates when the leaf is chopped or chewed. Most research is from the lab and animals. The existing summary states this and was not re-checked against a source. Preparation or compound: Leaves (not verified) |
10 |
| Oxalic acid | Biological association | Human measurement, small study | Can bind minerals. Added to a kale meal, it did not change iron absorption. Preparation or compound: Leaves |
11 |
Each connection is stored with the ingredient, the connected record, the kind of connection, the evidence status, the supporting studies, the preparation or compound involved, and a review date (last reviewed October 26, 2026).
The evidence
The research centre for Kale. Every record here was checked against PubMed or the publisher's page; we never list a study we have not verified.
Evidence by outcome
Bars show how many studies we found for each outcome, not how well it works. People are added up across trials only (reviews are not added, to avoid double counting).
How we rate evidence
High certainty
Several well-run trials or reviews agree; further research is unlikely to change the picture much.
Moderate certainty
Good trials point the same way, but some doubt remains.
Low certainty
Small, short or inconsistent trials. The finding could change with better research.
Very low certainty
Very little or very weak human data. Treat as a possibility only.
Mixed findings
Trials or reviews disagree with each other.
Not enough evidence
Not enough human research to say either way.
Evidence against
Good research found no benefit, or found harm.
We take the reviewers' own risk-of-bias and GRADE statements where they exist, and otherwise judge from study size, design, consistency, directness and whether a product matches what is sold. Many publications do not make evidence high. Method note: We started from our existing kale summary, re-read the USDA FoodData Central record, and searched PubMed for human trials, reviews and case reports. Only abstracts were read for studies. Items we could not verify are flagged in the research gaps.
Study library: every study we checked 16
| Study | Design | People | Product, dose, length | Result |
|---|---|---|---|---|
| Chartoumpekis 2019 Broccoli sprout beverage is safe for thyroid hormonal and autoimmune status: Results of a 12-week randomized trial. Food Chem Toxicol · PMID 30735751 · DOI |
Analysis within a randomised clinical trial |
45 45 female participants |
Broccoli sprout beverage enriched in sulforaphane and glucoraphanin (not kale) sprouts 84 days · vs Control beverage in the randomised trial |
Levels were not affected by the beverage and thyroid autoimmunity status did not change. No change in thyroid measures Methods, limits and fundingOutcomes: Thyroid-stimulating hormone, free thyroxine, thyroglobulin, thyroid autoimmunity Limitations: A different cruciferous food and preparation from kale, women only, 12 weeks. Funding and conflicts: Not stated in the abstract Read: abstract |
| Choudhury 2025 Thallium Exposure Secondary to Commercial Kale Chip Consumption: California Case Highlights Opportunities for Improved Surveillance and Toxicological Understanding. Int J Environ Res Public Health · PMID 40869821 · DOI |
Case report with public health investigation |
4 One household: a mother and three young children |
Commercial kale chip product leaves (dried chips) |
Peak urine thallium was 5.6 micrograms per gram creatinine in the mother (adult reference up to 0.4) and 10.5 in the children (child reference up to 0.8). Two kale chip samples contained 1.98 and 2.15 mg/kg thallium. Levels and symptoms were monitored after the product was removed. The authors say thallium can build up in brassica vegetables such as kale, that processing such as dehydration can change its concentration, and that food surveillance data are limited. Urine thallium several times above reference ranges in the household; kale chips 1.98 and 2.15 mg/kg Methods, limits and fundingOutcomes: Urine thallium levels, symptoms, thallium in food samples Limitations: One household. The abstract does not report symptoms or outcomes after removal. Funding and conflicts: No conflicts of interest declared Read: abstract |
| Davis 2024 Urinary 2- to 16alpha-hydroxyestrone ratio did not change with cruciferous vegetable intake in premenopausal women. Int J Vitam Nutr Res · PMID 37335576 · DOI |
Randomised, parallel arm, placebo controlled, partly blinded trial |
78 78 healthy premenopausal women aged 38-50 with a starting 2:16 ratio of 3.0 or less |
Dried Brussels sprout and kale capsules (550 mg each, six a day), or 40 g a day of broccoli or Brussels sprouts leaves and sprouts (dried in capsules, or whole vegetables) As above 8 weeks · vs Placebo |
No treatment effect (P=0.9) and no treatment-by-time interaction (P=0.6). A time effect appeared in all groups. Per-protocol analysis agreed. No treatment effect (P=0.9) Methods, limits and fundingOutcomes: Urinary 2:16 hydroxyestrone ratio Limitations: Kale only in a mixed capsule; a marker, not a disease outcome. Funding is not stated in the abstract. Funding and conflicts: Not stated in the abstract Read: abstract |
| Jeppesen 2024 Beneficial Effects of a Freeze-Dried Kale Bar on Type 2 Diabetes Patients: A Randomized, Double-Blinded, Placebo-Controlled Clinical Trial. Nutrients · PMID 39519473 · DOI |
Randomised, double blind, placebo controlled trial |
30 30 adults with type 2 diabetes, Aarhus, Denmark |
Freeze-dried kale (Brassica oleracea var. acephala) in a bar leaves (freeze-dried) Three bars a day, each with 26.25 g freeze-dried kale (about 341 g fresh kale a day) 12 weeks · vs Placebo bar |
Significant reductions in HbA1c, insulin resistance, body weight and calorie intake compared with placebo. Positive trends in fasting glucose and LDL cholesterol. No significant difference in total fat mass or the 240-minute glucose curve. Significant differences versus placebo; no effect sizes in the abstract Methods, limits and fundingOutcomes: HbA1c, insulin resistance, body weight, calorie intake, fasting glucose, LDL cholesterol, body fat, glucose tolerance test Limitations: Only 30 people, 12 weeks, very large amount, processed product; calorie intake and weight also fell. The authors call for larger studies. Funding and conflicts: No conflicts of interest declared; funding not stated in the abstract Read: abstract |
| Kim 2008 Kale juice improves coronary artery disease risk factors in hypercholesterolemic men. Biomed Environ Sci · PMID 18548846 · DOI |
Single group before and after intervention (no control group described in the abstract) |
32 32 men with high cholesterol (above 200 mg/dL), university staff, Korea |
Kale (Brassica oleracea acephala) juice leaves (juice) 150 mL a day 12 weeks |
HDL cholesterol rose 27%, HDL/LDL ratio rose 52%, LDL fell 10% and the atherogenic index fell 24.2%. Glutathione peroxidase activity and serum selenium rose; malondialdehyde did not change. Weight measures and nutrient intakes were unchanged. Responses depended on smoking status. HDL +27% (P<0.0001); LDL -10% (P=0.0007); atherogenic index -24.2% (P<0.0001) Methods, limits and fundingOutcomes: Blood lipids, antioxidant enzyme activity, body measures, diet Limitations: Small, men only, no control group described, juice not whole kale, short. Funding and conflicts: Not stated in the abstract (listed as non-US government research support in PubMed) Read: abstract |
| Li 2021 Green leafy vegetable and lutein intake and multiple health outcomes. Food Chem · PMID 34034049 · DOI |
Umbrella review of meta analyses |
24 meta-analyses covering 29 health outcomes | Green leafy vegetable intake and dietary lutein intake (kale not named in the abstract) leaves Varied · vs Lower intake |
Per 100 g a day of green leafy vegetables, risk of all-cause death, coronary heart disease and stroke was about 25% lower. Benefits were also reported for bladder and oral cancer. Dietary lutein went with lower risk of age-related macular degeneration, cataracts, coronary heart disease, stroke and some other outcomes. The authors warn about contamination with disease-causing organisms, specifically E. coli. About 25% lower risk per 100 g a day (all-cause mortality, coronary heart disease, stroke); no interval in the abstract Methods, limits and fundingOutcomes: All-cause mortality, cardiovascular disease, cancers, eye disease and others Limitations: Review of other reviews, mostly observational; the abstract gives no details of study quality. Kale is not named. Funding and conflicts: Not stated in the abstract Read: abstract |
| Morris 2018 Nutrients and bioactives in green leafy vegetables and cognitive decline: Prospective study. Neurology · PMID 29263222 · DOI |
Prospective cohort study |
960 960 participants of the Memory and Aging Project, ages 58-99, mean follow-up 4.7 years |
Usual intake of green leafy vegetables and their nutrients (kale not named in the abstract) leaves Mean 4.7 years · vs Lowest fifth of intake |
Higher intake of green leafy vegetables went with slower decline. Higher intake of phylloquinone, lutein, folate, alpha-tocopherol, nitrate and kaempferol each went with slower decline; beta-carotene did not reach significance. Top versus bottom fifth: beta 0.05 standardised units (P=0.0001); phylloquinone 0.02; lutein 0.04; folate 0.05; alpha-tocopherol 0.03; nitrate 0.04; kaempferol 0.04; beta-carotene 0.02 (P=0.08) Methods, limits and fundingOutcomes: Rate of cognitive decline Limitations: Observational, food questionnaire, adjusted for several factors but not proof of cause. Kale is not named in the abstract. Funding and conflicts: US government research funding (NIH); no further detail in the abstract Read: abstract |
| Novotny 2010 Vitamin K absorption and kinetics in human subjects after consumption of 13C-labelled phylloquinone from kale. Br J Nutr · PMID 20420753 · DOI |
Controlled clinical (isotope kinetics) study |
7 Seven healthy volunteers (six with detectable labelled vitamin K) |
Kale with carbon-13-labelled phylloquinone, eaten with vegetable oil leaves One 400 g serving with 30 g oil (research meal) Single meal, blood sampled over time |
Plasma 13C-phylloquinone peaked between 6 and 10 hours at an average of 2.1 nmol/L. Modelled bioavailability of vitamin K from kale was 4.7% (n=6). Plasma and tissue half-times were 8.8 and 215 hours. Mean bioavailability 4.7% Methods, limits and fundingOutcomes: Plasma labelled vitamin K over time Limitations: Seven people, one meal, with oil. Funding and conflicts: US government (USDA) research; no further detail in the abstract Read: abstract |
| Ojagbemi 2021 Dietary intakes of green leafy vegetables and incidence of cardiovascular diseases. Cardiovasc J Afr · PMID 34128951 · DOI |
Systematic review and meta analysis (observational cohort studies) |
17 cohort studies of green leafy vegetable intake and cardiovascular events | Usual dietary intake of green leafy vegetables (kale not named in the abstract) leaves Follow-up varied by study · vs Lower intake |
Higher green leafy vegetable intake went with fewer cardiovascular events overall and for cerebral infarction, heart disease and other events. The authors used a fixed-effect model. Pooled RR 0.93 (95% CI 0.92-0.95) for cardiovascular events; cerebral infarction 0.92 (0.88-0.96); heart disease 0.93 (0.87-0.99); other 0.95 (0.93-0.98) (0.92-0.95 (all events)) Methods, limits and fundingOutcomes: Incident cardiovascular disease events Limitations: Observational association. Leafy greens as a group; kale not separated. Funding is not stated in the abstract. Funding and conflicts: Not stated in the abstract Read: abstract |
| Schlotz 2018 Are Raw Brassica Vegetables Healthier Than Cooked Ones? A Randomized, Controlled Crossover Intervention Trial on the Health-Promoting Potential of Ethiopian Kale. Nutrients · PMID 30400138 · DOI |
Randomised controlled crossover trial |
22 22 healthy adults, Freiburg, Germany |
Brassica carinata (Ethiopian kale) leaf preparations, raw or cooked leaves Daily preparation (amount not in the abstract) 5 days each · vs Raw versus cooked, and baseline |
Cooked leaves reduced DNA damage by 35% (with S9 mix) and 33% (without), P<=0.01. Raw leaves reduced S9-activated damage by 21% (P=0.08). PGE2 fell after raw leaves. No change in plasma antioxidant capacity. Cooked: -35% and -33% (P<=0.01); raw: -21% (P=0.08) Methods, limits and fundingOutcomes: DNA damage after aflatoxin B1 exposure in blood cells (comet assay), plasma antioxidant capacity, prostaglandin E2 Limitations: A different species from curly kale, a lab marker, five days. Funding and conflicts: No conflict of interest declared Read: abstract |
| genannt Bonsmann 2008 Oxalic acid does not influence nonhaem iron absorption in humans: a comparison of kale and spinach meals. Eur J Clin Nutr · PMID 17440529 · DOI |
Two randomised crossover stable isotope absorption studies |
16 16 healthy women (13 completed both studies), Zurich |
Wheat rolls with 150 g spinach, 150 g kale, or kale plus added potassium oxalate leaves 150 g vegetable per meal Single test meals, iron measured 14 days later · vs Spinach reference meal; kale with and without added oxalate |
Kale meal 10.7% versus kale plus oxalate 11.5% (P=0.86). Absorption from the spinach meal was 24% lower than from the kale meal but not significantly different (P>0.16). Native oxalic acid: kale 0.01 g, spinach 1.27 g per 150 g. 10.7% versus 11.5% (P=0.86) Methods, limits and fundingOutcomes: Non-haem iron absorption Limitations: Small, healthy women, single meals. Funding and conflicts: Not stated in the abstract Read: abstract |
| s-fda-produce-safely FDA: Selecting and Serving Produce Safely · Source |
Official consumer guidance n/a |
Methods, limits and fundingRead: full-text | ||
| s-gbif-brassica-oleracea GBIF Backbone: Brassica oleracea L. (usageKey 3042845); Brassica oleracea var. sabellica L. (usageKey 3042870, synonym) · Source |
Reference database n/a |
Methods, limits and fundingRead: full-text | ||
| s-lpi-iodine Linus Pauling Institute: Iodine · Source |
University reference article n/a |
Methods, limits and fundingRead: full-text | ||
| s-medlineplus-warfarin MedlinePlus: Warfarin · Source |
Official health guidance n/a |
Methods, limits and fundingRead: full-text | ||
| s-usda-fdc-168421 USDA FoodData Central: Kale, raw (FDC 168421, SR Legacy) · Source |
Food composition database n/a |
Methods, limits and fundingRead: full-text |
What we do not know yet
- No trial has tested kale eaten as ordinary food for heart disease, stroke, diabetes or memory.
- The large studies group kale with other leafy greens, so a kale-only effect is not known.
- The kale juice study had no comparison group described, and the kale bar trial had only 30 people and a very large amount.
- Pregnancy, breastfeeding, children, allergy and pesticide residues were not covered by any source we read.
- Only abstracts were read for the studies.
How new research is added
New studies are found by a saved PubMed search and placed in a review queue before the page changes.
Lab markers
What changed in blood or saliva tests during trials. A changed number is not the same as feeling or being healthier, and we give no "normal ranges".
Metabolic 4
HbA1c
Lowered in trials
A blood test that shows average blood sugar over the past two to three months.
Low certaintyResults and what it means
Why it was measured
The standard long-term marker of blood sugar control in diabetes.
Statistical vs. clinical meaning
One small trial at a very large amount; the size of the change is not in the abstract.
Safety implication
People on diabetes medicine should not change doses on their own.
| Study | Result | People | Product |
|---|---|---|---|
| 4 | Significant reduction versus placebo (no values given in the abstract) | 30 adults with type 2 diabetes | Freeze-dried kale bars, about 341 g fresh kale a day, for 12 weeks |
LDL cholesterol
Lowered in trials
The so-called bad cholesterol carried in the blood.
Very low certaintyResults and what it means
Why it was measured
A common heart-risk marker.
Statistical vs. clinical meaning
Both studies were small, and the first had no comparison group.
Safety implication
Do not stop cholesterol medicine without asking your doctor.
HDL cholesterol
Raised in trials
The so-called good cholesterol carried in the blood.
Very low certaintyResults and what it means
Why it was measured
Higher levels are generally seen as favourable, but raising HDL has not been shown to prevent heart disease.
Statistical vs. clinical meaning
A large percentage change in a small uncontrolled study; whether it matters for heart health is unknown.
Safety implication
None specific.
| Study | Result | People | Product |
|---|---|---|---|
| 5 | +27% (P<0.0001) after 12 weeks of kale juice, with no control group described | 32 men with high cholesterol | Kale juice, 150 mL a day |
Body weight
Lowered in trials
Total body weight.
Low certaintyResults and what it means
Why it was measured
Weight change can itself improve blood sugar, so it matters when reading the HbA1c result.
Statistical vs. clinical meaning
The weight and calorie change may explain part of the blood sugar change.
Safety implication
None specific.
| Study | Result | People | Product |
|---|---|---|---|
| 4 | Significant reduction versus placebo, together with lower calorie intake (no values in the abstract) | 30 adults with type 2 diabetes | Freeze-dried kale bars, 12 weeks |
Other 6
Insulin resistance
Lowered in trials
How poorly the body responds to its own insulin.
Low certaintyResults and what it means
Why it was measured
A driver of type 2 diabetes.
Statistical vs. clinical meaning
Same single small trial.
Safety implication
None specific.
| Study | Result | People | Product |
|---|---|---|---|
| 4 | Significant reduction versus placebo (measure and values not given in the abstract) | 30 adults with type 2 diabetes | Freeze-dried kale bars, 12 weeks |
Fasting blood glucose
No clear change in trials
Blood sugar measured after not eating overnight.
Low certaintyResults and what it means
Urinary 2:16 hydroxyestrone ratio
No clear change in trials
A urine test comparing two breakdown products of estrogen.
Low certaintyResults and what it means
Why it was measured
Proposed as a marker of breast cancer risk; this is a hypothesis, not a proven test.
Statistical vs. clinical meaning
No change found.
Safety implication
None specific.
| Study | Result | People | Product |
|---|---|---|---|
| 3 | No treatment effect (P=0.9) after eight weeks | 78 healthy premenopausal women | Brussels sprout and kale capsules, or broccoli or Brussels sprouts, versus placebo |
Non-haem iron absorption
No clear change in trials
How much iron from plant foods is taken up into red blood cells.
Low certaintyResults and what it means
Why it was measured
Shows how well iron from a meal is absorbed.
Statistical vs. clinical meaning
Oxalate did not change absorption in this meal.
Safety implication
None specific.
| Study | Result | People | Product |
|---|---|---|---|
| 11 | 10.7% from the kale meal versus 11.5% with oxalate added (P=0.86) | 13 healthy women | Wheat rolls with 150 g kale, with and without added oxalate |
Plasma vitamin K (phylloquinone)
Raised in trials
The main plant form of vitamin K measured in blood.
Low certaintyResults and what it means
Why it was measured
Shows how much vitamin K from food reaches the blood.
Statistical vs. clinical meaning
Describes absorption only.
Safety implication
Matters for people on warfarin.
| Study | Result | People | Product |
|---|---|---|---|
| 8 | Labelled vitamin K peaked at 6-10 hours; bioavailability 4.7% (n=6) | Seven healthy volunteers | 400 g labelled kale with 30 g oil, one meal |
DNA damage in blood cells (comet assay, lab-induced)
Lowered in trials
A lab test that measures breaks in DNA after cells are exposed to a damaging agent, here aflatoxin B1.
Very low certaintyResults and what it means
Why it was measured
A research marker of how well cells resist damage.
Statistical vs. clinical meaning
A lab marker in a different species; not a health outcome.
Safety implication
None specific.
| Study | Result | People | Product |
|---|---|---|---|
| 10 | -35% (with liver enzyme mix) and -33% (without) after cooked Brassica carinata; -21% after raw (P=0.08) | 22 healthy adults | Raw or cooked Ethiopian kale for five days |
Safety
What studies and official sources say about side effects, medicines and who should check first. Each point shows what kind of evidence it comes from.
Main points to know 4
- Kale is very high in vitamin K, which matters if you take warfarin. USDA lists about 390 micrograms of vitamin K in 100 g of raw kale, which is roughly 82 micrograms in one raw cup (21 g). MedlinePlus says foods that contain vitamin K can affect how warfarin works, and advises eating consistent amounts of vitamin K foods from week to week. It also advises against eating large amounts of leafy green vegetables and says to talk to your doctor before changing your diet. The MedlinePlus page does not name kale; kale is a leafy green and is very high in vitamin K. The aim is a steady intake and telling your anticoagulation clinic, not necessarily avoiding kale. Other blood thinners work differently; ask your pharmacist. Authoritative guidance 15 16
- Thallium was found in a commercial kale chip product in one case. A 2025 case report from California describes a mother and her three young children with raised urine thallium (a toxic metal). Investigators traced it to a commercial kale chip product, in which thallium measured 1.98 and 2.15 mg/kg. The authors say thallium can build up in brassica vegetables such as kale, that manufacturing steps such as drying can raise its concentration, and that thallium is not regulated in food. This is one case report and does not show that ordinary kale is harmful. It does show that dried products can concentrate contaminants. Case report 2
- Raw leafy greens can carry germs such as E. coli. A 2021 review of leafy greens and lutein warns about contamination with disease-causing organisms, specifically E. coli. The FDA says harmful bacteria from soil, water or handling can contaminate fresh produce, that washing reduces but does not eliminate bacteria, and that children, older adults, pregnant women and people with weakened immune systems are more vulnerable to foodborne illness. Authoritative guidance 12 6
- Kale contains goitrogens; they matter mainly in large amounts or with low iodine. The Linus Pauling Institute lists kale among cruciferous vegetables that contain goitrogens, substances that can interfere with how the thyroid uses iodine. It says most such goitrogens are not of clinical importance unless eaten in large amounts or when iodine is also low. It gives no amounts for kale. People with a thyroid condition or on thyroid medicine should ask their clinician about their own diet. Authoritative guidance 14
Side effects 4
| Effect | How often | Severity and recovery | Evidence |
|---|---|---|---|
| Allergic reaction to kale No source read describes how common allergy to kale is. Any food can cause an allergic reaction. We make no claim about kale specifically. | Not established | mild to severe; severe reactions need emergency care Not described in any source read. | Absence of evidence |
| Raised urine thallium after eating commercial kale chips One household cluster (a mother and three children). The authors call for better surveillance of thallium in food. | Not established | not stated in the abstract Levels were monitored after the product was removed; the abstract does not report the outcome. | Case report 2 |
| Food poisoning from contaminated raw leaves FDA lists vomiting, diarrhoea, abdominal pain and flu-like symptoms. No kale-specific outbreak was read. | Not established | usually mild but can be serious in vulnerable people Most people recover within a short period, per the FDA. | Authoritative guidance 12 6 |
| Changes in warfarin control when vitamin K intake changes suddenly Applies to people taking warfarin. Kale is not named on the MedlinePlus page. | Not established | can matter for clotting control Managed by the anticoagulation clinic. | Authoritative guidance 15 |
Liver
No liver injury linked to eating kale was found in the sources read. LiverTox was not checked.
How often. No frequency found.
This is a safety connection. It is not evidence that Kale supports liver health.
Medicines and supplements 4
| With | Why it matters | Importance | Evidence | What to do |
|---|---|---|---|---|
| Warfarin (a blood thinner) prescription | Vitamin K in food can change how warfarin works, per MedlinePlus. Kale is very high in vitamin K (USDA). | MedlinePlus advises consistent amounts of vitamin K foods week to week and a talk with your doctor before diet changes. Kale is not named on that page. In one small study, about 4.7% of the vitamin K from a large kale meal reached the blood in six volunteers; absorption in people on warfarin was not studied. | Authoritative guidance Moderate certainty 15 16 8 | Do not start, stop or greatly change how much kale you eat without telling your anticoagulation clinic. |
| Other blood thinners (not warfarin) prescription | These work differently from warfarin and are not described on the MedlinePlus warfarin page. | No source read describes an interaction between kale and these medicines. | None found Not enough evidence | A pharmacist can check your own medicines. |
| Thyroid medicine and low iodine intake prescription | Goitrogens can interfere with iodine use. The Linus Pauling Institute says this matters mainly with large amounts or low iodine. | No kale-specific study of thyroid medicine was read. A 12-week trial of a broccoli sprout drink in 45 women found no change in thyroid hormones or thyroid autoimmunity; that is a different food and a different preparation. | Authoritative guidance Not enough evidence 14 1 | If you have thyroid disease or take thyroid medicine, ask your clinician what is right for you. |
| Other prescription and over-the-counter medicines prescription | No other medicine interaction with eating kale was found in the sources read. Kale juice, powders, bars and capsules are processed products and were not assessed for interactions. | None found Not enough evidence | A pharmacist can check your own medicines, especially before using concentrated kale products. |
Who should check first 5
| Situation | Advice | Evidence |
|---|---|---|
| Known allergy to kale or to a related brassica vegetable that caused swelling or trouble breathing | Avoid it and ask an allergy specialist. Follow your own emergency plan. | Absence of evidence |
| Taking warfarin | Kale is not forbidden. MedlinePlus advises consistent amounts of vitamin K foods and a talk with your doctor before changing your diet. | Authoritative guidance 15 |
| Thyroid disease, thyroid medicine or low iodine intake | Ask your clinician about large or regular amounts of kale. The Linus Pauling Institute says goitrogens matter mainly in large amounts or with low iodine. | Authoritative guidance 14 |
| History of kidney stones | A small study measured very little oxalic acid in kale (0.01 g in 150 g, against 1.27 g in the same weight of spinach). No study of kale and kidney stones was read, so ask a clinician what to eat. | Human measurement 11 |
| Pregnancy, breastfeeding and young children | No source read gave kale-specific advice for pregnancy, breastfeeding or children, and no warning was found. The FDA lists pregnant women and children among people more vulnerable to foodborne illness, so washing and chilling matter. Ask your clinician or midwife about your own diet. | Authoritative guidance 12 |
Overdose, product quality and regulators
Summary. Kale is an ordinary food. The one toxicity concern in the sources read is a case report of thallium in a commercial kale chip product. The vitamin K point concerns people who take warfarin.
Animal. Not reviewed.
Human doses studied. A 12-week trial gave freeze-dried kale equal to about 341 g fresh kale a day; the abstract does not mention any safety problem or give a safety analysis. Kale juice at 150 mL a day for 12 weeks was studied in 32 men; the abstract does not mention a safety problem or give a safety analysis.
Overdose response. Not found.
Genotoxicity. Not reviewed.
Kale is a leafy green that can carry germs like any raw produce. Brassica vegetables such as kale can take up thallium from soil, and drying may concentrate it. No source on pesticide residues in kale was read.
Thallium in kale chips (California, 2025). Two samples of a commercial kale chip product measured 1.98 and 2.15 mg/kg thallium. The authors say thallium is not regulated in food and that surveillance data are limited. 2
Germs on raw leafy greens. A 2021 review warns about E. coli contamination of green leafy vegetables. The FDA says harmful bacteria can reach produce in soil or water, or after harvest. 6 12
| Body | Action | Date |
|---|---|---|
| MedlinePlus (US National Library of Medicine) (United States) | Warfarin patient page: eat consistent amounts of vitamin K foods week to week; do not eat large amounts of leafy green vegetables; talk to your doctor before changing diet source | Last revised 03/15/2026 (page read 2026-10-26) |
| Linus Pauling Institute, Oregon State University (United States) | Iodine article: cruciferous vegetables including kale contain goitrogens that are not of clinical importance unless eaten in large amounts or with low iodine source | Page read 2026-10-26 |
| FDA (US Food and Drug Administration) (United States) | Consumer advice on selecting, washing and storing produce safely source | Content current as of 03/05/2024 (page read 2026-10-26) |
When to get medical help
- Swelling of the lips, face, tongue or throat after eating
- Trouble breathing, wheezing or a tight throat
- Feeling faint, dizzy or collapsing after eating
- Spreading hives after eating
- Any unusual bleeding or bruising if you take a blood thinner: tell your clinic
- Vomiting, diarrhoea, abdominal pain or fever after eating raw greens: the FDA says to contact your healthcare provider
Stop using the product and contact a clinician or emergency service. In the US, Poison Control is 1-800-222-1222.
General information only, not medical advice. Ask a clinician or pharmacist before use, especially with medicines, in pregnancy or with a health condition. See the Natural Remedy Safety guide.
Frequently asked questions
What is kale good for?
Can I eat kale if I take warfarin?
How much vitamin K is in kale?
USDA lists about 390 micrograms per 100 g of raw kale, which works out to about 82 micrograms in one raw cup (21 g). Cooked kale is a different food in USDA's database and was not read for this page.
Does kale lower cholesterol?
One small study of 32 men with high cholesterol found LDL fell 10% and HDL rose 27% after 12 weeks of kale juice, but it described no comparison group. We found no trial of eating whole kale for cholesterol.
Does kale help with blood sugar or diabetes?
One 12-week trial of 30 adults with type 2 diabetes found lower HbA1c with freeze-dried kale bars than with placebo bars. The amount equalled about 341 g of fresh kale a day, and weight and calorie intake also fell. It is a promising but very small result. People on diabetes medicine should ask their care team before changing their diet.
Is kale bad for the thyroid?
Kale contains goitrogens, which can interfere with how the thyroid uses iodine. The Linus Pauling Institute says they are not of clinical importance unless eaten in large amounts or when iodine is also low. If you have thyroid disease or take thyroid medicine, ask your clinician.
Is kale high in oxalate? What about kidney stones?
A small 2008 study measured 0.01 g of oxalic acid in 150 g of kale, against 1.27 g in the same weight of spinach. We read no study of kale and kidney stones, so if you form stones, ask a clinician what to eat.
Does kale block iron absorption?
In a small Swiss study of 13 women, adding oxalic acid to a kale meal did not change how much iron they absorbed. The study was small and used single test meals.
Is raw or cooked kale healthier?
We found no trial that compares raw and cooked curly kale. One trial of Ethiopian kale, a different species, tested raw and cooked leaves on a lab DNA-damage marker; both changed it, cooked more clearly. That does not settle the question for curly kale.
Does kale protect the heart or the brain?
Are kale chips, juice and powders as good as the leaf?
They are different products, and the studies of them cannot be applied to the leaf. A 2025 case report found thallium, a toxic metal, in a commercial kale chip product linked to raised urine levels in a family, and the authors say drying can change contaminant levels. It is one case.
How should I wash kale?
The FDA advises washing all produce under running water, not with soap, detergent or commercial produce wash, and keeping pre-cut or packaged produce refrigerated. Packs labelled pre-washed and ready-to-eat do not need further washing. Washing reduces bacteria but does not remove them all.
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Editorial transparency
- Written by
- AlternativeMed editorial team. Research compiled with AI assistance, then checked against the sources listed.
- Medical review
- None. This page has not been reviewed by a physician or pharmacist.
- First published
- 2026-10-26
- Last substantive update
- 2026-10-26
- How we research
- We started from our existing kale summary, re-read the USDA FoodData Central record, and searched PubMed for human trials, reviews and case reports. Only abstracts were read for studies. Items we could not verify are flagged in the research gaps.
- How we rate evidence
- See "How we rate evidence" in the Evidence tab.
- Independence
- No product is promoted on this page. No affiliate links are shown.
Sources 16
Each source was opened and checked. Where we read only the abstract, it says so. The small numbers in the text point here.
- Chartoumpekis DV, Ziros PG, Chen JG, et al.. Broccoli sprout beverage is safe for thyroid hormonal and autoimmune status: Results of a 12-week randomized trial.. Food Chem Toxicol. 2019. PMID 30735751. DOI 10.1016/j.fct.2019.02.004. (abstract)
- Choudhury A, Fowles J, Bartlett R, et al.. Thallium Exposure Secondary to Commercial Kale Chip Consumption: California Case Highlights Opportunities for Improved Surveillance and Toxicological Understanding.. Int J Environ Res Public Health. 2025. PMID 40869821. DOI 10.3390/ijerph22081235. (abstract)
- Davis SJ, Arscott SA, Goltz S, et al.. Urinary 2- to 16alpha-hydroxyestrone ratio did not change with cruciferous vegetable intake in premenopausal women.. Int J Vitam Nutr Res. 2024. PMID 37335576. DOI 10.1024/0300-9831/a000785. (abstract)
- Jeppesen PB, Dorner A, Yue Y, et al.. Beneficial Effects of a Freeze-Dried Kale Bar on Type 2 Diabetes Patients: A Randomized, Double-Blinded, Placebo-Controlled Clinical Trial.. Nutrients. 2024. PMID 39519473. DOI 10.3390/nu16213641. (abstract)
- Kim SY, Yoon S, Kwon SM, et al.. Kale juice improves coronary artery disease risk factors in hypercholesterolemic men.. Biomed Environ Sci. 2008. PMID 18548846. DOI 10.1016/S0895-3988(08)60012-4. (abstract)
- Li N, Wu X, Zhuang W, et al.. Green leafy vegetable and lutein intake and multiple health outcomes.. Food Chem. 2021. PMID 34034049. DOI 10.1016/j.foodchem.2021.130145. (abstract)
- Morris MC, Wang Y, Barnes LL, et al.. Nutrients and bioactives in green leafy vegetables and cognitive decline: Prospective study.. Neurology. 2018. PMID 29263222. DOI 10.1212/WNL.0000000000004815. (abstract)
- Novotny JA, Kurilich AC, Britz SJ, et al.. Vitamin K absorption and kinetics in human subjects after consumption of 13C-labelled phylloquinone from kale.. Br J Nutr. 2010. PMID 20420753. DOI 10.1017/S0007114510001182. (abstract)
- Ojagbemi A, Okekunle AP, Olowoyo P, et al.. Dietary intakes of green leafy vegetables and incidence of cardiovascular diseases.. Cardiovasc J Afr. 2021. PMID 34128951. DOI 10.5830/CVJA-2021-017. (abstract)
- Schlotz N, Odongo GA, Herz C, et al.. Are Raw Brassica Vegetables Healthier Than Cooked Ones? A Randomized, Controlled Crossover Intervention Trial on the Health-Promoting Potential of Ethiopian Kale.. Nutrients. 2018. PMID 30400138. DOI 10.3390/nu10111622. (abstract)
- genannt Bonsmann SS, Walczyk T, Renggli S, et al.. Oxalic acid does not influence nonhaem iron absorption in humans: a comparison of kale and spinach meals.. Eur J Clin Nutr. 2008. PMID 17440529. DOI 10.1038/sj.ejcn.1602721. (abstract)
- . FDA: Selecting and Serving Produce Safely. . . (full-text)
- . GBIF Backbone: Brassica oleracea L. (usageKey 3042845); Brassica oleracea var. sabellica L. (usageKey 3042870, synonym). . . (full-text)
- . Linus Pauling Institute: Iodine. . . (full-text)
- . MedlinePlus: Warfarin. . . (full-text)
- . USDA FoodData Central: Kale, raw (FDC 168421, SR Legacy). . . (full-text)
