Cysteine

Cysteine is a sulfur-containing amino acid. It is one of the 4 common sulfur amino acids, with methionine, homocysteine and taurine, according to a review in the Journal of Nutrition. Only cysteine and methionine are built into proteins, the review adds. Cysteine is conditionally essential: the body makes it from methionine and serine, but under some conditions not fast enough.

The WHO/FAO/UNU report of 2007 lists cysteine among the conditionally indispensable amino acids. So does the US National Academies report of 2005, which names prematurity as an example. Yet the safety chapter of the same report calls L-cysteine dispensable.

What the body makes from it matters more than the label. Cysteine becomes glutathione, taurine and coenzyme A, and it forms the disulfide bridges that help hold proteins in shape.

Most people do not need to think about it. The WHO sets one figure for methionine and cysteine together: 15 mg per kg of body weight a day. That is about 1,050 mg for a 70 kg adult. It lists the split as 10 mg for methionine and 4 mg for cysteine.

US survey data cited by the National Academies put the average cysteine intake from food and supplements at 1.0 g a day. In our USDA ranking, soybeans, chicken, sardines and lentils lead per serving, and hemp and sunflower seeds lead per 100 g.

One thing most readers do not know: the best-known cysteine supplement, N-acetylcysteine (NAC), was a drug before it was a supplement. The FDA says NAC is excluded from the dietary supplement definition because it was approved as a drug first. It also says it intends to use enforcement discretion for certain NAC products sold as supplements, which means it will generally not act against them.

NAC is also poorly absorbed by mouth. In 6 volunteers, 4.0% of an oral dose reached the blood as reduced NAC and 9.1% as total NAC.

Health benefits of cysteine

• ★★No rise in glutathione from glycine plus NACIn 114 healthy adults (average age 65), glycine plus N-acetylcysteine (NAC) for 2 weeks did not raise total glutathione, a compound made with cysteine. Only in a subset picked after the fact, with high oxidative stress and low starting glutathione, did medium and high doses raise it.
• ★★Mucus thinners kept more people free of flare-upsIn 38 trials of 10,377 people with COPD or chronic bronchitis, mucus-thinning drugs including NAC gave 1.73 times the odds of a flare-up-free period. This was NAC and other drugs as treatment, not a test of cysteine from food.
• ★★Glutathione supplements raised glutathione levelsIn 54 non-smoking adults, 250 or 1,000 mg of glutathione a day for 6 months raised levels in blood and immune cells, more with dose. Levels fell back to baseline after a 1-month break; this tested glutathione itself, not cysteine.
• ★Premature babies fed by vein kept more proteinAdding cysteine to intravenous feeding raised nitrogen retention, a measure of how much protein the body keeps (5 small trials, 95 infants). Growth did not clearly change in the one trial that measured it.
• ★No proven benefit for hair, nails or skinIn 2010 the EU’s food safety agency found no cause and effect link between L-cysteine and hair. Claims for nails and skin rested on animal or test-tube studies, and the EU Register lists these claims as non-authorized.

Foods high in cysteine

In our USDA ranking per serving, soybeans lead: 1 cup (172 g) holds 0.46 g of cysteine. A chicken drumstick with skin (0.28 g) and 4 oz of chicken breast (0.27 g) come next, then sardines and lentils.

Per 100 g, seeds lead, with hemp seeds at 0.67 g and sunflower seeds at 0.45 g. Per 100 kcal, cod and tuna lead at 0.23 g, and eggs rank sixth at 0.19 g. The ranking below shows the full lists.

The WHO figure of 4 mg per kg works out to 280 mg a day for a 70 kg adult, by our arithmetic. 4 oz of chicken breast (0.27 g) gives about 96% of that, and one can of sardines (0.24 g) about 86%. One cup of soybeans (0.46 g) gives about 1.6 times that.

The WHO gives that split inside a combined requirement for methionine plus cysteine, so it is a guide, not a separate daily need. US survey data cited by the National Academies put the average cysteine intake at 1.0 g a day. So an ordinary day of protein foods covers it.

Plant and animal proteins differ in how much sulfur amino acid they carry. The WHO report says methionine and cysteine are marginal in legume proteins and equally abundant in cereal and animal proteins.

The National Academies’ table, which gives the two together, shows egg protein at 56 mg per g of protein and whole wheat flour at 35. Milk has 34 and navy beans 19, against the WHO adult pattern of 22. Legumes are the weak spot per gram of protein. Still, a cup of soybeans or lentils carries 0.46 g or 0.23 g, because of how much protein they hold.

The FAO’s protein quality report scores foods by DIAAS (digestible indispensable amino acid score), which rates how well a protein meets amino acid needs. It uses the pattern for children aged 6 months to 3 years. Its examples are 40 for wheat, 64 for peas and 122 for whole milk powder.

In its worked table, pea protein had 25 mg of sulfur amino acids per g, against 35 in milk powder. Pea protein also had a lower digestibility (0.69 against 0.94).

Grains and legumes fill each other’s gaps. Lysine, not cysteine, is the weak spot in grains: 26 mg per g of protein in wheat flour, against 72 in navy beans. So beans with rice work both ways: legumes bring lysine and grains bring sulfur amino acids.

How cysteine works

Most of the biochemistry below is well established. But it comes from reviews, test-tube work and tracer studies, and the choices people make about supplements rest on a thinner set of trials.

Four ideas cover most of it. The body makes cysteine from methionine and can fall short, and cysteine becomes glutathione and other sulfur compounds. Two cysteines can join into a bridge that shapes proteins, and the kidney reclaims cystine through a carrier that can fail. Here is each one, and what has been seen in people.

It is made from methionine and serine, but not always fast enough

Cysteine is made from methionine and serine. A review of sulfur amino acid metabolism describes the route. Methionine is converted to homocysteine, and its sulfur is passed to serine to make cysteine, in a route started by the enzyme cystathionine beta-synthase.

The National Academies list methionine and serine as the precursors of cysteine. They note that in a premature baby, the rate at which cysteine can be made from methionine can be inadequate. The WHO adds that this onward conversion is regulated, and that it is limited or compromised in several circumstances. Examples are poor B-vitamin status, infancy and alcohol consumption.

In the EU, vitamin B6 has an authorized claim for contributing to normal cysteine synthesis. It applies to foods that are at least a source of vitamin B6.

It becomes glutathione, taurine and more

Cysteine is one of three amino acids in glutathione, the tripeptide gamma-glutamyl-cysteinyl-glycine. A review calls glutathione the most abundant non-protein thiol (sulfur compound) in mammal tissues, at 1 to 10 mM and highest in the liver. The same review says its synthesis depends mainly on the supply of cysteine and on the rate-limiting enzyme, glutamate cysteine ligase.

A 2006 review adds that the rat liver keeps its free cysteine between 20 and 100 nmol per g. It does so whether sulfur amino acid intake is low or high. The rat liver uses glutathione as a reservoir of cysteine, and may use cysteine for coenzyme A and hydrogen sulfide.

Excess cysteine is oxidized to taurine or sulfate, and some is turned into hydrogen sulfide, according to a 2020 review.

Glutathione synthesis can be measured with tracers. In 8 older adults, red cell glutathione was 1.12 mmol per L of cells, against 2.08 in 8 younger adults. Its absolute synthesis was about a third as fast (0.55 against 1.73).

After 2 weeks of cysteine (given as NAC) and glycine, both rose to the young range in that study, which had no placebo group. A larger 2022 trial of glycine plus NAC in 114 healthy older adults did not raise total glutathione, its main outcome.

Two cysteines can join into a bridge

Two cysteines can join through a sulfur-sulfur link called a disulfide bond, and the joined pair is called cystine. The National Academies treat L-cysteine and L-cystine together because they are interconvertible (each can turn into the other). A review says that by forming disulfide bonds, cysteine plays a crucial role in protein structure and in protein folding.

In human insulin, three disulfide bonds hold the molecule together. A7-B7 and A20-B19 join its two chains, and A6-A11 sits inside the A chain. Lab-made versions missing any one of them lost the ability to bind the insulin receptor.

A faulty kidney carrier causes cystine stones

Nearly every amino acid crosses the gut wall and the kidney by more than one transporter, a review of epithelial transport says. That gives the body backup. Cystinuria is one of the inherited disorders of amino acid transport the review names.

In cystinuria, mutations in the SLC3A1 or SLC7A9 gene disable a kidney carrier. It carries cystine and the dibasic amino acids, so cystine stones form. A review puts its prevalence at about 1 in 7,000 newborns. Cystine stones make up 1 to 2% of kidney stones in adults and 6 to 8% in children.

What has been seen in people

The firmest human facts are about supply. In premature babies fed by vein, adding cysteine raised nitrogen retention. In older adults and in malnourished children with low glutathione, cysteine given as NAC raised glutathione and its synthesis in small trials.

But in 114 healthy older adults, 2 weeks of glycine plus NAC did not raise total glutathione. And 6,000 mg of NAC for 28 days did not clearly raise brain glutathione in 8 people. A whey protein formula raised plasma glutathione in HIV-infected patients, though whey is a mixture.

Hair and skin products are mixtures with company-linked trials, and EFSA found no link. Links of cysteine with diabetes and body fat come from questionnaires and blood levels and cannot show cause. We found no trial showing a health benefit of extra cysteine in healthy adults. All of these are among the study cards.

How much cysteine do you need?

Cysteine has no daily requirement of its own, because the body makes it from methionine. The WHO and the US National Academies both set one value for methionine and cysteine together.

Adults need about 15 mg of methionine plus cysteine per kg of body weight a day, according to the WHO/FAO/UNU report of 2007. That is about 1,050 mg for a 70 kg adult. The US recommended dietary allowance (RDA, the daily amount that covers nearly all healthy people) is 19 mg per kg, about 1,330 mg (our arithmetic).

The WHO reached 15 mg per kg from obligatory oxidative losses, the amounts the body breaks down each day. Those were 10.4 mg of methionine and 4.1 mg of cysteine per kg, 14.5 in all, rounded. Its table shows the split as 10 and 4 mg per kg.

How far dietary cysteine can spare methionine (cut how much of it you need) is uncertain. One study found sparing at a 5 to 1 ratio of cysteine to methionine. Two others suggested none at ratios nearer to 1, which normal diets are more likely to have, so the WHO set one combined value.

In 6 young men fed no cysteine, the total sulfur amino acid requirement was 12.6 mg per kg. The population-safe intake was 21 mg per kg.

For children, the WHO values for sulfur amino acids fall with age. They are 31 mg per kg a day at 6 months, 22 at 1 to 2 years and 18 at 3 to 10 years. They then fall to 17 at 11 to 14, 16 at 15 to 18 and 15 above 18.

The US values for pregnancy are an estimated average requirement of 20 mg per kg and an allowance of 25. For lactation, the estimated average requirement is 21. The National Academies say there are essentially no direct data for either, so they scaled the values from protein needs.

A conditionally indispensable amino acid, like cysteine, is one whose dietary need appears only in some conditions. The National Academies name prematurity, when cysteine cannot be made fast enough from methionine. They say the amounts needed in such conditions have not been determined and presumably vary with the condition.

Cysteine deficiency: signs and who is at risk

We found no report of a cysteine deficiency syndrome in healthy people who eat enough protein. The average intake of 1.0 g a day is far above the roughly 280 mg that the WHO split suggests for a 70 kg adult. The risk groups are babies and people whose conversion from methionine is limited.

Premature babies on cysteine-free intravenous feeding are given cysteine, and Cochrane found that nitrogen retention rose with it. Red cell cysteine was lower in 8 older adults than in younger ones (19.8 against 26.2 µmol/L). Children with severe edematous malnutrition have low cysteine concentrations, and their low glutathione synthesis rate responded to NAC in a controlled trial.

In EPIC-Oxford men, cysteine is not among the six amino acids whose plasma level differed by diet group, according to the abstract. We found no study of cysteine deficiency in vegans.

Inherited disorders can limit cysteine production. In cystathionine beta-synthase (CBS) deficiency, homocysteine builds up because it is not converted to cysteine. Severely affected people usually need a low-methionine diet, betaine or both, according to international guidelines.

A cell study found that low cysteine reduced the deposit of fibrillin-1, a connective tissue protein. It suggested that keeping blood cysteine and cystine normal may be a therapeutic goal. A 1968 report in The Lancet described treating homocystinuria with a low-methionine diet, supplemental cystine and a methyl donor. These conditions need a metabolic specialist.

Forms of cysteine: absorption and effects

Cysteine reaches you in several ways. It comes bound in food protein, as free L-cysteine, as the joined pair cystine, as N-acetylcysteine (NAC), and in related compounds such as glutathione. NAC is a drug first and a supplement second. The FDA says it is excluded from the dietary supplement definition because it was approved as a drug before it was marketed as a supplement.

In a 2022 guidance, the FDA says it intends to use enforcement discretion for certain NAC products labeled as supplements. We found no later FDA decision.

The honest bottom line: trials of the forms measure glutathione and blood levels, not health outcomes. We found no trial comparing NAC with free cysteine in people.

Form	What it is	What studies show	Source
Free L-cysteine	The single amino acid in its natural L-form, in food and supplements, and in intravenous feeds for babies as cysteine hydrochloride	In premature babies fed by vein, adding cysteine raised plasma cysteine and nitrogen retention. In healthy adults, single oral doses of 5 and 10 g caused nausea and light-headedness, and the National Academies found no reports of long-term use. We found no trial of free L-cysteine for health in healthy adults.	1, 2
L-cystine	Two cysteines joined by a disulfide bond	The National Academies treat cysteine and cystine together because they are interconvertible. A skin trial tested 500 mg a day of cystine with glutathione, and the benefit came from the mix.	1, 18
N-acetylcysteine (NAC)	Cysteine with an acetyl group attached. A drug first, and also sold as a supplement	Oral bioavailability was 4.0% for reduced NAC and 9.1% for total NAC in 6 volunteers. As a drug it is the standard overdose antidote for paracetamol and a mucolytic in COPD. As a cysteine source it raised glutathione in malnourished children and did not raise brain glutathione at 6,000 mg a day. We found no trial comparing NAC with free L-cysteine in people.	3, 4, 5, 7, 6
Glycine plus NAC (GlyNAC)	A 1 to 1 mix of two amino acid compounds	A 16-week trial in 24 older adults reported better glutathione. A 2-week trial in 114 healthy older adults found no change in total glutathione.	13, 12
Glutathione	The tripeptide the body makes from cysteine, glutamate and glycine, sold as a supplement, and as a sublingual form	At 1,000 mg a day for 6 months, oral glutathione raised blood levels 30 to 35%. A 3-week crossover trial found a sublingual form raised glutathione more than oral glutathione or NAC, but its authors include company staff.	9, 10
Cysteine in whey protein	Protein-bound cysteine, in a mixture of proteins	45 g a day of cysteine-rich whey raised plasma glutathione in HIV-infected patients with one of two formulas.	11
Hair, skin and nail blends	Cysteine or cystine mixed with collagen, taurine, methionine, iron, selenium or glutathione	EFSA found no cause and effect link for L-cysteine and hair, nails or skin, and said no conclusion can be drawn from fixed combinations. Two later trials of blends were positive but had company authors and tested mixtures.	16, 17, 18
L-cysteine in food (E920)	A dough strengthener in baked goods, from protein hydrolysis, fermentation or synthesis	US rules allow up to 0.009 part of L-cysteine per 100 parts of flour. A 2006 review says cysteine has mainly been made by protein hydrolysis, with synthetic and biotechnological products preferred in the market. We found no source for the share made from each route today.	15, 14

The trials in the benefits section used NAC as a cysteine source (children with malnutrition, older adults, a Parkinson’s study). Others used glycine plus NAC, and whey protein. Here are the form-specific studies and the main drug uses of NAC, strongest evidence first, 6 in all.

Is cysteine safe? Supplements, dose and interactions

Cysteine from food is not a concern. The National Academies set no tolerable upper intake level (UL) for L-cysteine or L-cystine, which they treat together. A UL is the most you can take daily without likely harm. The data on adverse effects from supplements were not enough for a dose-response assessment.

We found no EFSA upper level either. For scale, the Academies cite US survey data with a mean intake of 1.0 g a day. The 99th percentile was 2.2 g a day in the highest group (men aged 51 to 70).

They report that single oral doses of 5 and 10 g of L-cysteine caused nausea and light-headedness in healthy people. They found no reports of long-term use in humans. They list animal studies in which large doses damaged brain and eye tissue in young mice and rats.

NAC is regulated differently from the amino acid. The FDA says NAC is excluded from the dietary supplement definition. That is because it was approved as a new drug before it was marketed as a supplement. The FDA also says it intends to use enforcement discretion for certain products labeled as supplements.

• NAC side effects. The FDA label for an oral NAC drug (Cetylev, for paracetamol overdose) lists the most common adverse reactions. They are nausea, vomiting, other gastrointestinal symptoms and rash with or without fever, and the label warns of hypersensitivity reactions such as hives. In the 28-day trial of 6,000 mg a day, 3 of 5 people with Parkinson’s disease reported symptomatic adverse events. In the 2-week trial of glycine plus NAC at up to 7.2 g a day, the authors called it safe and well tolerated. They work for the maker of such products.
• Asthma and inhaled NAC. MedlinePlus says inhaled acetylcysteine can cause wheezing and chest tightness, and tells people with asthma to ask their doctor first.
• Cystinuria and kidney stones. People with cystinuria form cystine stones. A consensus statement starts treatment with fluid intake and diet changes, then urine alkalinizing drugs (which make urine less acidic) and thiol drugs. A 2026 review says oral NAC lacks robust trial evidence in cystinuria. We found no study of cysteine supplements in it, so ask a kidney specialist.
• Pregnancy and breastfeeding. The FDA label says published reports of acetylcysteine in pregnancy are too few to judge the risk of birth defects or miscarriage. It also says nothing is known about its presence in human milk. We found no trial of cysteine supplements in pregnancy.
• Babies and inherited disorders. Cysteine for premature babies is given in hospital. There, the Cochrane review notes, metabolic acidosis was reported in the first 2 weeks of cysteine chloride. Do not give cysteine or NAC to a baby, or add them in an inherited disorder such as CBS deficiency, without a specialist.
• Medicines. We found no listed drug interactions for oral cysteine or NAC. Tell your doctor or pharmacist about any supplement you take.

How to get enough cysteine

• Eat enough protein from varied foods. The US average is 1.0 g a day, about 3.6 times the 280 mg the WHO split gives a 70 kg adult (our arithmetic). A 4 oz serving of chicken breast gives 0.27 g, a cup of soybeans 0.46 g and 3 tbsp of hemp seeds 0.2 g.
• Pair legumes with grains. Navy bean protein carries 19 mg of methionine plus cysteine per g, against 35 in wheat flour and 56 in egg. Wheat, in turn, is low in lysine (26 mg per g against 72 in beans), so each covers the other’s gap.
• Skip the pills unless a doctor suggests them. Glycine plus NAC did not raise total glutathione in 114 healthy older adults. Oral NAC reached the blood at only 4.0% to 9.1% in a small study.

History of cysteine

Cystine came first. In 1810 the English chemist William Hyde Wollaston described a new kind of bladder stone, which he had seen in only two cases. He suggested the name “cystic oxide” because both stones came from the bladder.

A consensus statement on cystine stones says Wollaston first described cystinuria but wrongly called it cystic oxide. Sir Archibald Garrod described it correctly in the early 1900s. The name cysteine also comes from the Greek word for bladder.

In 1884 the German chemist Eugen Baumann reduced cystine with zinc and obtained a new compound, which he named cysteine. That is the standard account of its history.

Medicine found its own use for the NAC form. In 1977 Prescott and colleagues reported 15 patients with paracetamol poisoning treated with intravenous N-acetylcysteine. In 1979 they reported 100 cases and concluded that it was the treatment of choice. A 2024 review says it soon became the standard for paracetamol overdose.

In food, the US regulation affirming L-cysteine as safe as a dough strengthener dates from 1977 (42 FR 14653).

Frequently asked questions about cysteine

What is the difference between cysteine and cystine?

Cystine is two cysteine molecules joined by a disulfide bond. The National Academies treat L-cysteine and L-cystine together because they are interconvertible. Cysteine’s ability to form disulfide bonds is what lets it shape proteins, as in the three bonds of human insulin. Cystine is also the compound that forms stones in the inherited disorder cystinuria.

Is NAC the same as cysteine?

No. N-acetylcysteine (NAC) is cysteine with an acetyl group attached, and it is a drug as well as a supplement. The FDA says it was approved as a drug before it was sold as a supplement. In 6 volunteers, 4.0% of an oral dose reached the blood as reduced NAC and 9.1% as total NAC. We found no trial in people comparing NAC with free L-cysteine. NAC can raise cysteine supply where it is low: in malnourished children, 0.5 mmol per kg a day raised glutathione synthesis.

Does cysteine or NAC raise glutathione?

Sometimes, mostly where levels are low. NAC raised glutathione and its synthesis in children with severe edematous malnutrition. Cysteine given as NAC with glycine did so in 8 older adults, and a whey protein formula raised plasma glutathione in HIV-infected patients. But in 114 healthy older adults, glycine plus NAC did not raise total glutathione. And 6,000 mg of NAC a day for 28 days did not clearly raise brain glutathione in 8 people.

Does cysteine help hair growth?

We found no trial showing that cysteine alone helps hair. EFSA’s 2010 opinion found no cause and effect link between L-cysteine and hair, and said no conclusion can be drawn from fixed combinations. The EU Register lists the hair claims as non-authorized. A 2023 trial of a tablet with collagen, taurine, cysteine, methionine, iron and selenium, added to drug treatment, improved hair-loss scores in 83 people. But it had no placebo tablet, tested a mixture and had company authors.

Which foods are highest in cysteine?

Per serving in our USDA ranking, soybeans lead with 0.46 g per cup. Chicken drumstick (0.28 g) and chicken breast (0.27 g in 4 oz) follow, then sardines (0.24 g per can) and lentils (0.23 g per cup). Per 100 g, hemp seeds come first at 0.67 g, then sunflower seeds at 0.45 g and oats at 0.41 g. Eggs rank sixth per 100 kcal at 0.19 g.

Do vegans get enough cysteine?

Probably, if they eat enough protein and mix foods, but we found no study of cysteine deficiency in vegans. The WHO says methionine and cysteine are marginal in legume proteins and abundant in cereals. Still, a cup of soybeans or lentils gives 0.46 g or 0.23 g. In EPIC-Oxford men, intakes of all 18 dietary amino acids were lower in vegans, by up to 47%. But cysteine was not among the six amino acids whose plasma levels differed by diet.

How much cysteine is too much?

No official upper limit exists. The National Academies found the data on supplements insufficient for a dose-response assessment, and we found none from EFSA. Single oral doses of 5 and 10 g of L-cysteine caused nausea and light-headedness. In a 28-day trial of 6,000 mg of NAC a day, 3 of 5 people with Parkinson’s disease reported symptomatic adverse events.

Is the L-cysteine in bread vegan?

Not necessarily, and labels rarely say. The US regulation allows L-cysteine as a dough strengthener up to 0.009 part per 100 parts of flour, and it does not state the source. A 2006 review says cysteine has mainly been made by protein hydrolysis (breaking proteins down), with synthetic and biotechnological products preferred in the market. Ask the maker, or look for a vegan label.