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Methionine

Supplement amino acid general nutrition
Aka: L-methionine
Brands seen in Singapore: NOW Foods, Swanson, Nutricost, Nutricology, Source Naturals
Not medical advice. Check with your doctor or pharmacist before you start, stop or change the timing of anything you take.
How to use
What it’s for
An indispensable amino acid · the body's main methyl donor
Take with
B6, B12, folate and c moderate its homocysteine effect
Pros & cons
Pros
  • Unique among essential amino acids for its sulfur atom
  • Precursor of cysteine and of glutathione
  • Minimum need in adults 60+ measured at 5.1 mg/kg/day
Cons
  • Intakes over ~5x normal raised homocysteine levels
  • A mistaken 10-fold overdose resulted in death
  • Known to worsen symptoms in schizophrenia
Sources
  1. Toxicity of methionine in humans (Garlick PJ, J Nutr 2006; PMID 16702346) · read 2026-08-27
    what it says
    “Nutritional and metabolic studies have employed amounts of methionine … both below and above the requirement and have not reported adverse effects in adults and children … intakes higher than 5 times normal resulted in elevated homocysteine levels”
  2. Toxicity of methionine in humans (Garlick PJ, J Nutr 2006) — loading dose and overdose · read 2026-08-27
    what it says
    “A "loading dose" of methionine (0.1 g/kg) has been given … However, a 10-fold larger dose, given mistakenly, resulted in death.”
  3. Toxicity of methionine in humans (Garlick PJ, J Nutr 2006) — psychiatric effects · read 2026-08-27
    what it says
    “Although methionine is known to exacerbate psychopathological symptoms in schizophrenic patients, there is no evidence of similar effects in healthy subjects.”
  4. Toxicity of methionine in humans (Garlick PJ, J Nutr 2006) — B vitamin moderation · read 2026-08-27
    what it says
    “These effects of methionine on homocysteine and vascular function are moderated by supplements of vitamins B-6, B-12, C, and folic acid.”
  5. Toxicity of methionine in humans (Garlick PJ, J Nutr 2006) — main concern · read 2026-08-27
    what it says
    “The role of methionine as a precursor of homocysteine is the most notable cause for concern.”
  6. Toxicity of methionine in humans (Garlick PJ, J Nutr 2006) — infants · read 2026-08-27
    what it says
    “In infants, methionine intakes of 2-5 times normal resulted in impaired growth and extremely high plasma methionine levels, but no adverse long-term consequences were observed.”
  7. Methionine Nutrition and Metabolism: Insights from Animal Studies to Inform Human Nutrition (Elango R, J Nutr 2020; PMID 33000159) · read 2026-08-27
    what it says
    “Methionine is a nutritionally indispensable amino acid, and is unique among indispensable amino acids due to its sulfur atom.”
  8. Methionine Nutrition and Metabolism (Elango R, J Nutr 2020) — functions · read 2026-08-27
    what it says
    “Methionine is involved in cysteine synthesis via the transsulfuration pathway, which is rate limiting for the key antioxidant molecule, glutathione. Methionine is also the primary methyl donor in the body”
  9. The Minimum Methionine Requirement for Adults Aged >=60 Years Is the Same in Males and Females (Paoletti et al., Nutrients 2023; PMC10574673) · read 2026-08-27
    what it says
    “the data were pooled to generate a mean of 5.1 mg/kg/d … This is the first study to estimate the minimum methionine requirement in healthy older adults”
  10. HSA Health Supplements Guidelines GL-CHPB-3-001 Rev. No. 020 (January 2026) — s3.1 working definition and clause 4.8 (Tables 4-5 cover vitamins and minerals only) · read 2026-08-27
    what it says
    “Vitamins, minerals, amino acids, fatty acids, enzymes, probiotics and other bioactive substances … HS for use by the general adult population should not contain vitamins and minerals exceeding the stipulated limits stated in Tables 4 and 5.”
  11. HSA Guidelines on Prohibited and Restricted Ingredients in HS and TM, GL-CHPB-1-012 Rev. No. 001 (Oct 2025), Annex A/B — full text searched 27 Aug 26, no row for methionine · read 2026-08-27
    what it says
    “The objective of this guideline is to highlight the substances of concern when used in HS and TM and is not meant to be an exhaustive list.”
  12. iHerb Singapore category page "L-Methionine: Essential Amino Acid for Liver Support", default best-selling order, first 5 distinct brands · read 2026-08-27
    what it says
    “L-Methionine: Essential Amino Acid for Liver Support”
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Updated 2026-08-29. Every line above traces to a numbered source. Kozen-X is not a medical device and does not give medical advice. It lists what published sources have written down — nothing more. A pair not listed is unrecorded, not cleared. Before you start, stop or re-time anything, talk to your doctor or pharmacist.