Collard Greens: Nutrients, What Research Shows and Safety

Collard greens are the large, smooth, dark green leaves of Brassica oleracea var. viridis, a member of the cabbage family (Brassicaceae). The leaf is eaten, usually cooked. People eat them for their vitamins and fiber. This page keeps the food apart from supplements and from hype, and says what the research does and does not show.

Read this first

• Warfarin: collard greens are very high in vitamin K, which affects how the blood thinner warfarin works. Steady intake matters more than avoiding them, and the prescriber or warfarin clinic should guide you. [2][3]
• Allergy: any food can cause an allergic reaction. For trouble breathing or swelling of the face or throat, call 911. See food allergy basics and emergency signs.
• Thyroid: goitrogens in cabbage-family vegetables matter mainly at large intakes combined with low iodine. [4]
• Evidence in one line: collards are a nutrient-dense vegetable; studies of cabbage-family vegetables as a group are mostly observational and do not prove that collards prevent disease. [7][8][10]

What is in it (USDA values)

Source: USDA FoodData Central item 170406, "Collards, raw" (SR Legacy). [1] % Daily Value uses the FDA 2016 Daily Values; values vary with variety, ripeness and storage. The serving column is simple arithmetic from the USDA household portion and is information, not a prescription.

USDA also lists lutein plus zeaxanthin (4,323 µg per 100 g) and beta-carotene (2,991 µg per 100 g). [1] These are carotenoids, plant pigments. USDA measured them; we did not find a trial in the sources we read showing that eating collards changes a health outcome through them. Cabbage-family vegetables also contain glucosinolates (sulfur compounds); most evidence on their breakdown products comes from lab and animal work. [5]

What research says

Eaten as food vs taken as a supplement or extract. The studies above ask about eating cabbage-family vegetables, not about pills. Observational studies cannot prove cause: people who eat more vegetables often differ in other ways. We did not find a trial in the sources we read that tested collard greens specifically for a disease outcome: proper evidence not found in the sources we read. Vitamin K and folate supplements are separate products and are not covered here.

How much is a normal serving

• USDA household portion in our data file: 1 cup, chopped (36 g) of raw collards. [1]
• NHS: 80 g of fresh, canned or frozen vegetables counts as one portion (last reviewed 9 June 2026). [17] WHO: at least 400 g of fruits and vegetables a day for people older than 10 (fact sheet dated 26 January 2026). [16] This is information, not a prescription.
• Cooking and folate: studies of other vegetables show cooking can lower folate. Steamed leafy vegetables in a 2026 study from Ghana lost between 8.6% and 78.5% of folate compared with fresh. [12] A 2022 study found sous-vide kept more folate than boiling in broccoli and spinach. [14] Another 2026 study found small losses in spinach and larger ones in beetroot and asparagus. [13] We did not find folate retention data for collards themselves.
• Cooking and glucosinolates: cooking inactivates myrosinase, the enzyme that makes isothiocyanates; light steaming under about 5 minutes keeps some of it, and gut bacteria can do part of the breakdown. [5]
• Cooking and vitamin K: the sources we read did not give cooking losses for vitamin K in collards.
• Frozen and canned: see fresh, frozen and canned produce.

Safety, allergy and medicines

• Warfarin (blood thinner). MedlinePlus says people on warfarin may need to eat about the same amount of vitamin K every day, because sudden changes can change the medicine's effect. [2] The NHS says foods high in vitamin K can affect warfarin, that you do not have to avoid them, and to ask your warfarin clinic for advice; vitamin K supplements should not be taken during treatment. [3] One 24-week randomized trial (conference abstract, 2019) found that a diet raising vitamin K intake improved the stability of warfarin control in people who had been unstable. [11] That is one small abstract; do not change your diet without your clinic. See foods and medicine interactions.
• Other blood thinners. Newer anticoagulants do not work by blocking vitamin K, but the sources we read did not discuss collard greens with them. Ask a pharmacist.
• Goitrogens and thyroid. The Linus Pauling Institute says cabbage-family vegetables contain goitrogens, substances that can interfere with iodine use, and that most goitrogens matter clinically only in large amounts or when iodine intake is already low. [4] Rat work suggests cooking may not remove all goitrogenic compounds. [18] We did not find human trials on collards and thyroid function. People with thyroid disease or on thyroid medicine should ask their clinician; see thyroid conditions and iodine.
• Allergy. Allergy to leafy greens is not described in the sources we read; any food allergy can be serious. For trouble breathing or swelling of the face or throat, call 911.
• Potassium and kidneys. Collards are not high in potassium per serving (2% DV per cup, raw). [1] The sources we read did not discuss collards with kidney disease; people on restricted diets should follow their clinician's list.
• Oxalate, FODMAP and pesticide residues. We did not read a source on these for collards, so we make no claim.
• Pregnancy. The sources we read did not raise any food-specific caution for collards. Folic acid advice for pregnancy comes from health professionals. [6]

Choosing, storing and food safety

• Wash produce under running water before eating or cooking; do not use soap or produce wash. Washing reduces but does not eliminate bacteria. [15]
• Keep perishable produce and pre-cut or packaged produce in the refrigerator at 40 degrees F or below. [15]
• Buy bagged greens only if they are refrigerated or on ice. [15]

Related pages

• Cruciferous vegetables
• Leafy greens
• Foods and medicine interactions: vitamin K, grapefruit and others
• Sulforaphane and glucosinolates in cruciferous vegetables
• Fresh, frozen and canned produce
• Thyroid conditions and iodine