Methionine, Homocysteine and Sulfur Amino Acids: Diet, Folate and B Vitamins
Methionine is an essential sulfur amino acid and homocysteine is made from it. See what B vitamins do, what trials of lowering homocysteine found, and where methionine restriction stands.
Content last updated . This page has not been medically reviewed.
Quick answer
Methionine is an essential amino acid: the body cannot make it, so it comes from food. [1] Homocysteine is a different amino acid that the body makes from methionine along the way; it is not a nutrient you eat. [2] Vitamin B12, vitamin B6 and folate (vitamin B9) help the body break homocysteine down. [2] High homocysteine is mostly a lab finding to be explained by a clinician. Lowering it with vitamins has not been shown to prevent heart attacks. [3]
How the sources define these terms
- Essential. MedlinePlus lists methionine among the nine essential amino acids. It lists cysteine among the nonessential amino acids and also among the conditionally essential ones, needed mainly in illness and stress. [1] Methionine and cysteine are the two "sulfur amino acids" (they contain sulfur); a 2024 trial used that term for a diet low in both. [4]
- Not all views agree on the labels. One review notes that nutritional, metabolic and functional views of "essential" lead to somewhat different lists. [5]
- Food. Amino acids come from animal foods such as meats, milk, fish and eggs, and plant foods such as soy, beans, legumes, nut butters and some grains. [6] We did not retrieve a food-by-food methionine table from a food-composition database this round, so none is given.
Methionine, homocysteine and B vitamins
Normally homocysteine levels are low because the body uses B12, B6 and folate to break it down. [2] A high level may mean the B vitamins are low or the process is not working well. [2] Higher-than-normal levels can also occur with osteoporosis, chronic kidney disease, hypothyroidism and some other conditions, and age, sex and smoking affect the result. [2] A rare inherited condition, homocystinuria, makes it hard to break homocysteine down, so homocysteine and methionine build up in the blood. [7] In the US most newborns have routine screening for it. [2] Details on the vitamins: Vitamin B12, Vitamin B6 and Folate.What research found
| Question | What the source found | Label |
|---|---|---|
| Does lowering homocysteine with B6, B9 or B12 prevent heart attacks? | Cochrane review (2017): 15 trials, 71,422 people. No difference versus placebo in myocardial infarction or death from any cause. [3] | Established: no benefit found for heart attack |
| Stroke | In the same review, stroke was 4.3% with homocysteine-lowering versus 5.1% with comparator (RR 0.90, 95% CI 0.82 to 0.99). [3] | Promising but limited (borderline, small difference) |
| Cognitive decline | A 2022 meta-analysis of 95 studies (25 trials, 20 cohort and 50 cross-sectional studies) linked B vitamin supplementation to slower cognitive decline, especially with early, long intervention. [8] MedlinePlus says a provider may recommend homocysteine-lowering vitamins for people with homocystinuria or signs of cognitive decline. [2] | Mixed (pooled designs; small average effect) |
| Methionine-restricted diets in people | One 8-week trial: 59 adults with overweight or obesity ate a plant-based diet low (about 2 g a day) or high (about 5.6 g a day) in sulfur amino acids; the low group lost about 20% more weight. [4] The authors say more research is needed. [4] | Promising but limited (one short trial) |
| Methionine restriction and lifespan | Restriction has extended lifespan in houseflies and rodents; the same review reports long-term restriction can cause bone disorders, stunted growth and high homocysteine. [9] Another review describes the evidence as coming from animal models. [10] | Preclinical |
MedlinePlus puts it this way: studies show that in most cases lowering homocysteine does not reduce the risk of heart attack or stroke. [2] The Cochrane stroke result is small and should be read with that.
Official intake levels
We did not retrieve an official methionine, or methionine plus cysteine, intake table from NIH, EFSA, the NHS or Health Canada, so no figure is quoted. For protein targets by age see Protein intake levels.Testing and its limits
A homocysteine test is a blood test, may require fasting for 8 to 12 hours, and can be affected by medicines and supplements, especially B vitamins. [2] MedlinePlus says routine testing to screen for heart disease risk is not recommended for everyone. [2] Results do not diagnose a vitamin deficiency or a disease by themselves. See Vitamin deficiency testing.Frequently asked questions
Is methionine bad for you?
Not at ordinary food intakes. A review of human studies found no adverse effects from moderate fluctuations in dietary methionine, while much higher intakes raised homocysteine. [11]
Does high homocysteine mean I have heart disease?
No. A high level may reflect low B12 or folate, kidney disease or other conditions, and MedlinePlus says screening everyone is not recommended. [2]
Do B vitamins that lower homocysteine prevent heart attacks or stroke?
A Cochrane review found no difference in heart attack or death, and a small reduction in stroke. [3]
Is a methionine-restricted diet proven to extend life in humans?
No. The lifespan findings are from animals, and the one human trial we read lasted 8 weeks and measured weight and biomarkers. [9][4]
Should I take methionine as a supplement?
We found no evidence that healthy people need to. High intakes can raise homocysteine, so ask a clinician or pharmacist first. [11]
More details: evidence, limits and cautions
The details behind this page: cautions, limits of the evidence and where sources disagree. Open any note to read it.
How to read the evidence labels
Labels used on this page: established nutritional role, mixed, promising but limited, preclinical, insufficient evidence.
Supplemental methionine: what is known about safety
- A review of human studies found no adverse effects from moderate changes in dietary methionine, but intakes higher than 5 times normal raised homocysteine; B6, B12, C and folic acid moderated the effect. [11]
- In a trial of 15 healthy older adults taking free methionine capsules for 4 weeks at rising amounts, homocysteine rose at the highest amount. [12] These are study amounts, not advice.
- The same review notes methionine is known to worsen symptoms in people with schizophrenia. [11]
- Authors of a 2023 review say national health agencies have not set upper limits for individual amino acids. [13] That review was prompted by studies run under an amino acid industry nonprofit. [13]
Safety: who should ask first
- Do not stop any medicine because of a test result; MedlinePlus says to continue medicines unless your provider tells you otherwise. [2]
- People with kidney disease, a known homocysteine disorder, schizophrenia, or who are pregnant or breastfeeding should talk to a clinician or dietitian before using methionine or B-vitamin products or changing their eating pattern. Pregnancy safety of methionine supplements is not described in the sources we read; see Pregnancy and breastfeeding.
- Medicines: see How interactions work.
- Quality varies between products: Quality testing and certifications. Cysteine and NAC are covered in the conditionally essential library; all nine essential amino acids are on Essential Amino Acids.
References
- MedlinePlus Medical Encyclopedia. Amino acids. US National Library of Medicine. Read 2026-10-22.
- MedlinePlus. Homocysteine Test. US National Library of Medicine. Read 2026-10-22.
- Martí-Carvajal AJ, et al. Homocysteine-lowering interventions for preventing cardiovascular events. Cochrane Database Syst Rev. 2017;8:CD006612. PubMed 28816346.
- Dietary sulfur amino acid restriction in humans with overweight and obesity: a translational randomized controlled trial. J Transl Med. 2024;22:11. PubMed 38195568.
- Reeds PJ. Dispensable and indispensable amino acids for humans. J Nutr. 2000;130(7):1835S. PubMed 10867060.
- MedlinePlus Medical Encyclopedia. Protein in diet. US National Library of Medicine. Read 2026-10-22.
- MedlinePlus Genetics. Homocystinuria. US National Library of Medicine. Read 2026-10-22.
- B vitamins and prevention of cognitive decline and incident dementia: a systematic review and meta-analysis. Nutr Rev. 2022. PubMed 34432056.
- Methionine as a double-edged sword in health and disease: current perspective and future challenges. Ageing Res Rev. 2021. PubMed 34700006.
- Amino acid restriction, aging, and longevity: an update. Front Aging. 2024. PubMed 38757144.
- Toxicity of methionine in humans. J Nutr. 2006;136(6):1722S. PubMed 16702346.
- Tolerance to increased supplemented dietary intakes of methionine in healthy older adults. Am J Clin Nutr. 2017;106(2). PubMed 28637772.
- Tolerable Upper Intake Level for Individual Amino Acids in Humans: A Narrative Review of Recent Clinical Studies. Adv Nutr. 2023. PubMed 37062432.
Education, not medical advice. This page is for general information. It does not diagnose or treat any condition and is not a substitute for care from a qualified health professional. Natural does not automatically mean safe. Ask a clinician or pharmacist before changing your care, especially if you take medication, are pregnant or breastfeeding, or have a health condition.