Methionine

Methionine is an essential sulfur-containing amino acid whose role goes far beyond being a protein building block. It starts every protein the body makes — in human cells, protein synthesis begins at the AUG “start” codon, which codes for methionine. More importantly, methionine is the source of S-adenosylmethionine (SAM), the body’s main methyl donor, which more than 200 enzymes use for tasks ranging from DNA methylation and neurotransmitter production to making phospholipids and creatine. Methionine can also be converted into cysteine, which the body needs to make glutathione, its main antioxidant inside cells. These roles make methionine one of the most metabolically important amino acids — with links to antioxidant defense, homocysteine and heart health, and even ageing research.

Classification

• Type: Essential amino acid (EAA) — can’t be made by the body; must come from the diet
• Side chain: Contains a sulfur atom (a thioether) — one of the two sulfur-containing amino acids used in proteins, along with cysteine, which the body can make from methionine
• Metabolism: Purely glucogenic — its breakdown through the transsulfuration pathway produces cysteine and alpha-ketobutyrate, which is converted into succinyl-CoA and can be used to make glucose
• Protein synthesis: The start signal for making proteins — every new protein chain in human cells begins with methionine, although it’s often removed afterwards
• Limiting amino acid: Methionine and cysteine (the “sulfur amino acids”) are often the limiting amino acids in legumes, which are rich in lysine — the reverse of cereals, which are relatively rich in sulfur amino acids but low in lysine

Key Mechanisms

SAM — the body’s main methyl donor:

Methionine combines with ATP, using the enzyme methionine adenosyltransferase, to form S-adenosylmethionine (SAM). SAM then donates its methyl group to a huge range of molecules, in reactions carried out by methyltransferase enzymes:

• DNA methylation: DNA methyltransferases add methyl groups to DNA, which generally silences the genes involved — one of the main mechanisms of epigenetic gene regulation.
• Histone methylation: SAM provides methyl groups for lysine and arginine units in histone proteins, which can switch genes on or off.
• Creatine production: the final step in making creatine — guanidinoacetate + SAM → creatine — uses a methyl group from SAM. Creatine production is one of the largest users of the body’s methyl groups, with some estimates as high as 40%.
• Noradrenaline → adrenaline: the adrenal glands use a methyl group from SAM to convert noradrenaline into adrenaline.
• Phosphatidylcholine production: in the liver, SAM is used to make phosphatidylcholine — a key component of cell membranes and of the particles the liver uses to export fat. This pathway provides an alternative to getting choline from food, linking methionine to liver fat metabolism.
• Polyamine production: a modified form of SAM is used to make polyamines such as spermidine and spermine, which are needed for cell growth and to stabilize DNA. Spermidine has attracted interest in ageing research for its ability to trigger autophagy, the cell’s recycling process.

After giving up its methyl group, SAM becomes S-adenosylhomocysteine, which is then converted into homocysteine.

The methionine cycle and homocysteine:

Homocysteine sits at a metabolic crossroads with two possible fates:

1. Recycling back into methionine: the enzyme methionine synthase transfers a methyl group from the active form of folate (5-methyltetrahydrofolate) to homocysteine, using vitamin B12 as a cofactor. In the liver, betaine (trimethylglycine) can also provide the methyl group, through a different enzyme (BHMT).

2. Conversion into cysteine (transsulfuration): cystathionine beta-synthase, which needs vitamin B6, combines homocysteine with serine to form cystathionine, which another B6-dependent enzyme then splits into cysteine and alpha-ketobutyrate.

High blood homocysteine is linked with a higher risk of heart disease, stroke, and cognitive decline. The most common causes are low folate, vitamin B12, or vitamin B6, kidney disease, certain medicines, and genetic factors — rather than eating protein. Notably, large trials that lowered homocysteine with B vitamin supplements didn’t reduce heart attacks, though some found a modest reduction in strokes. So homocysteine is best seen as a marker of B vitamin status and overall risk, not as a direct target for most people.

Cysteine and glutathione:

Transsulfuration provides cysteine — the limiting building block for glutathione, which cells make from cysteine, glutamate, and glycine. Glutathione is the main antioxidant inside cells: it neutralizes hydrogen peroxide and other reactive compounds, helps recycle vitamin C (which in turn helps recycle vitamin E), and supports immune function. Cysteine also comes directly from food protein, so methionine is one of two sources rather than the only one.

Taurine production:

Cysteine can also be converted into taurine. Taurine isn’t used to build proteins, but it helps regulate cell volume, has calming effects in the brain, is used to form bile salts, and supports muscle and heart function. The body makes some taurine this way, and meat, fish, and shellfish also provide it.

Methionine restriction and ageing research:

In yeast, fruit flies, and rodents, restricting methionine in the diet — without cutting overall calories — consistently extends lifespan. Proposed mechanisms include lower production of reactive oxygen species in mitochondria, reduced IGF-1 signalling, more autophagy, and higher levels of FGF21, a hormone linked with metabolic health. Human evidence is limited to short-term studies, so whether this matters for people is unknown — but it’s one reason ageing researchers are interested in the lower methionine content of plant-based diets.

Recommended Intake

Requirements are set for methionine and cysteine together (the sulfur amino acids), because cysteine from food reduces the amount of methionine the body needs to convert into cysteine.

Found In (per 100g)

Brazil nuts stand out: about 8% of their protein is methionine — far more than any other common food — on top of their very high but variable selenium content. Sesame seeds are another good plant source (about 0.59g per 100g in USDA data). Egg white is particularly rich in sulfur amino acids relative to its protein.

The last column tells the more important story, because requirements are set for methionine and cysteine together. The World Health Organization’s reference for adults is 2.2% of protein (22mg per gram). Animal proteins provide about 3–4%, and grains are also rich — oats and rice provide around 4.5–5%. Legumes are the lowest: lentils sit right at the reference, while black beans, chickpeas, and peanuts are slightly above it. (USDA’s entry for firm tofu shows an unusually low figure of 1.6%; cooked soybeans come in at about 2.7%, in line with other legumes.) That’s why legumes and grains complement each other so well: legumes supply the lysine grains lack, and grains supply the sulfur amino acids legumes are lower in.

Helps With

• Supplying methyl groups (via SAM) for DNA methylation and gene regulation
• Creatine production, which uses a methyl group from SAM
• Adrenaline production, through SAM-dependent methylation of noradrenaline
• Phosphatidylcholine production and fat export from the liver
• Glutathione production, through conversion to cysteine
• Taurine production, through cysteine
• Polyamine production for cell growth
• Starting protein synthesis — methionine is coded by the start codon
• Carnitine production, together with lysine

Methionine for Athletes and Active People

Methionine connects to several systems that matter for training — energy, antioxidant defense, and adaptation — but athletes eating enough protein get plenty of it.

Creatine production: the body makes about 1g of creatine a day from arginine, glycine, and a methyl group from SAM, and this is one of the largest uses of the body’s methyl groups. Taking creatine supplements reduces the body’s own creatine production, which in theory frees up methyl groups for other uses — but studies on whether creatine supplements lower homocysteine have produced mixed results, so this remains a possible bonus rather than a reason to take creatine.

Glutathione and oxidative stress: hard training increases oxidative stress, and glutathione is a key part of the body’s defense. Methionine (through cysteine) and cysteine from food both support glutathione production. There’s no evidence that athletes eating enough protein — including plant-based athletes who eat varied protein sources — are short of the building blocks for glutathione. High-dose antioxidant supplements may even blunt some training adaptations, so more isn’t automatically better.

Training and gene regulation: exercise changes DNA methylation in muscle, helping switch on genes involved in adaptations such as building new mitochondria. This is an active area of research, but there’s no evidence that eating more methionine — beyond meeting your needs — improves training adaptations.

Homocysteine and high-protein diets: athletes eating 1.6–2.2g of protein per kg get more methionine than sedentary people, and processing it produces more homocysteine. With adequate folate, vitamin B12, and vitamin B6, the body clears this efficiently. Athletes on very meat-heavy diets with few vegetables, fruits, and legumes — the main sources of folate — and plant-based athletes not taking vitamin B12 are the groups most likely to have high homocysteine.

SAMe for joints: SAMe, the activated form of methionine, is sold for joint pain. In head-to-head trials in osteoarthritis, it relieved pain about as well as anti-inflammatory painkillers (NSAIDs) such as ibuprofen or naproxen, although it worked more slowly, taking about a month to reach the same effect. The placebo-controlled evidence is less certain: a Cochrane review of four trials involving 656 people found a small reduction in pain (about 0.4 points on a 10-point scale) that wasn’t statistically certain, and concluded that the effects could be clinically relevant but needed confirming in larger trials. SAMe may be worth discussing with a doctor for persistent osteoarthritis pain — particularly for people who can’t take NSAIDs — but it hasn’t been studied for athletes’ overuse injuries.

Methionine Deficiency

Methionine deficiency is uncommon in people eating enough protein. Situations where intake or use can fall short include:

• Vegan diets that rely heavily on legumes with few grains, nuts, or seeds, especially when total protein is low
• Severe protein-energy malnutrition
• Malabsorption conditions

Homocystinuria: in this rare inherited disorder, usually caused by a lack of cystathionine beta-synthase, homocysteine and methionine build up to very high levels instead of being converted into cysteine. Without treatment, it can cause dislocated eye lenses, blood clots, skeletal problems, and learning difficulties. It’s managed with vitamin B6 (which helps some people), betaine, and a low-methionine diet, under specialist care.

Methionine, choline, and fatty liver: phosphatidylcholine is needed to package fat for export from the liver, and the body can make it either from dietary choline or using SAM from methionine. In animals, a diet lacking both methionine and choline reliably causes fatty liver — a standard research model for fatty liver disease. In people, low choline intake is the better-documented cause of fatty liver.

Signs of inadequate methionine (and protein) intake:

• Poor growth in children
• Muscle loss and negative nitrogen balance
• Weakened immunity and poor wound healing
• Hair, skin, and nail changes, as with other forms of protein deficiency

Excessive Doses of Methionine

No formal Tolerable Upper Intake Level has been set. Methionine from food is safe, but high-dose L-methionine supplements carry some specific risks:

Raised homocysteine: a large dose of methionine raises blood homocysteine for several hours — so much so that a “methionine loading test” was once used to check how well people process homocysteine. People with low B vitamin status or inherited homocysteine disorders are most affected.

Liver disease: in people with severe liver disease, high methionine intake can worsen hepatic encephalopathy (brain dysfunction caused by liver failure), partly through the sulfur compounds produced when it’s broken down.

Nausea and unpleasant taste: methionine has a sulfurous smell and taste, and high doses often cause nausea and stomach upset.

The bigger picture: based on the animal research on methionine restriction, some researchers suggest that very high methionine intakes may not be ideal for long-term health, although this hasn’t been shown in people. The sensible goal is to get enough methionine from a varied diet, along with adequate B vitamins to process it — not to maximize it.

Types and Forms

• L-Methionine (free form): the standard form. It’s used medically in some countries as an oral treatment for paracetamol (acetaminophen) poisoning — it provides cysteine to restore the liver’s glutathione — although acetylcysteine is the standard antidote. It isn’t generally needed as a supplement.
• SAMe (S-adenosylmethionine): the activated, methyl-donating form of methionine, sold as a supplement in some countries and as a prescription medicine in others. The strength of the evidence depends on the use. Depression has the strongest case: a 2024 meta-analysis of 23 trials involving 2,183 people found that SAMe taken on its own, by mouth or by injection, reduced depressive symptoms more than placebo, and its authors concluded that it could be considered a treatment option. They also noted that whether SAMe works as well as standard antidepressants, or adds benefit when taken with them, remains unresolved, because the comparison trials were few and mostly small — and an earlier Cochrane review rated the evidence low quality. For osteoarthritis, small head-to-head trials found pain relief similar to NSAIDs, though slower to start, while a Cochrane review of placebo-controlled trials found a small effect that needs confirming in larger trials. For liver disease — relevant because SAMe is made and used mainly in the liver, where it also supplies cysteine for glutathione — a 2015 meta-analysis of 12 trials involving 705 people found that SAMe lowered bilirubin and AST (a liver enzyme), but not ALT, and made no difference to long-term outcomes; a Cochrane review likewise found no effect on survival in alcohol-related liver disease. SAMe is used in some countries to treat cholestatic liver conditions, under medical supervision. SAMe is unstable, so it’s sold as stabilized, enteric-coated tablets.
• N-Acetylcysteine (NAC): provides cysteine directly, skipping the conversion from methionine. It’s the standard antidote for paracetamol poisoning and reliably raises glutathione levels, which makes it the most practical choice when glutathione support is the goal.
• Glutathione: available as standard and liposomal supplements. Much of standard oral glutathione is broken down in the gut, and evidence that liposomal forms raise body levels meaningfully comes from small studies.

Which form for your goal:

How to Take Methionine

Through diet: any varied diet with enough protein — from animal foods, or from plant foods that combine legumes with grains, nuts, and seeds — meets methionine needs comfortably. Brazil nuts and sesame seeds are especially rich plant sources, and eggs are an excellent vegetarian one.

If you use SAMe: trial doses have ranged from 400mg to 1,600mg a day, usually started low and taken in the morning with food, because it can be stimulating and cause insomnia or agitation. Don’t combine it with antidepressants or other serotonergic medicines unless your doctor approves, as there have been reports of serotonin syndrome. Importantly, SAMe can trigger mania in people with bipolar disorder, so they shouldn’t use it without medical supervision.

If you use NAC: typical doses are 600–1,800mg a day, in divided doses with food. It can cause digestive upset and has a sulfurous smell.

B vitamins: make sure your diet provides enough folate, vitamin B12, and vitamin B6, especially if you eat a lot of protein, follow a vegan diet (which needs a B12 supplement), or have been told your homocysteine is high.

Interactions

Vitamin B12 and folate are needed to recycle homocysteine back into methionine, and deficiency of either raises homocysteine. Nitrous oxide (“laughing gas”), whether used medically or recreationally, inactivates vitamin B12 and blocks this recycling step — one reason heavy recreational use causes serious nerve damage. Vitamin B6 is needed for both steps of transsulfuration. Betaine provides an alternative way of recycling homocysteine and can lower it when other routes are limited. Adequate choline intake reduces the liver’s use of SAM for making phosphatidylcholine. Lysine and methionine are both needed to make carnitine. Zinc methionine is one of several chelated forms of zinc used in supplements.

Synergy

• Works with folate, vitamin B12, and vitamin B6 — the cofactors of the methionine cycle and transsulfuration pathway, which together keep homocysteine in check.
• Works with arginine and glycine in creatine production — arginine and glycine form the backbone, and SAM provides the methyl group.
• Works with lysine in carnitine production.
• Works with selenium in antioxidant defense — methionine (via cysteine) supports glutathione production, and selenium-dependent glutathione peroxidases use glutathione to neutralize peroxides.
• Works with choline and betaine — all three contribute to the body’s supply of methyl groups.

Antagonistic Interactions

• Low folate, vitamin B12, or vitamin B6 allows homocysteine to build up.
• High methionine intake can worsen hepatic encephalopathy in people with severe liver disease.
• Nitrous oxide inactivates vitamin B12, blocking the recycling of homocysteine into methionine.
• SAMe combined with serotonergic medicines may contribute to serotonin syndrome, and SAMe can trigger mania in bipolar disorder.

Food values: USDA FoodData Central, SR Legacy — Brazil nuts, dried, unblanched (FDC ID 170569)*; parmesan, hard (170848); chicken breast, roasted (171477); yellowfin tuna, cooked (172006); turkey breast, roasted (174516); pork top loin chop, broiled (167841); sardines, canned in oil (175139); ground beef, 80% lean, cooked (171797); Atlantic salmon, farmed, cooked (175168); pumpkin and squash seed kernels, dried (170556)*; cheddar, sharp, sliced (170899)*; egg white, raw (172183)*; egg, hard-boiled (173424); peanuts, raw (172430)*; cottage cheese, low-fat, 2% (172182)*; tofu, firm, calcium-set (172475); oats, dry (173904); black beans, cooked (173735); chickpeas, cooked (173757); quinoa, cooked (168917); lentils, cooked (172421); white rice, cooked (168878). * SR Legacy values used as fill-ins for the Foundation Foods entries for Brazil nuts (2515373), pumpkin seeds (2515380), cheddar (328637), egg white (747997), peanuts (2515376), and cottage cheese (328841), whose amino acid data aren’t included in our tracker. Percentages of protein are calculated from the same USDA records. Requirements from the US National Academies and the World Health Organization (2007).