Cysteine

Cysteine is a conditionally essential sulfur-containing amino acid. The body makes it from methionine (which provides the sulfur) and serine (which provides the carbon skeleton), and food provides more. Its best-known role is as the limiting building block of glutathione, the main antioxidant made inside cells. Cysteine’s sulfur atom also lets it form disulfide bonds — chemical “staples” that hold many proteins in shape, including keratin in hair and nails. Its supplement form, N-acetylcysteine (NAC), is a well-established medicine: it’s the standard antidote for paracetamol (acetaminophen) overdose and is used to loosen mucus in lung conditions. For athletes, though, the picture is more nuanced than “more antioxidants means better recovery” — in some studies, high-dose NAC actually interfered with recovery signalling.

Classification

• Type: Conditionally essential amino acid — made from methionine and serine; it becomes essential when that conversion doesn’t work, such as in premature infants or the inherited disorder homocystinuria
• Sulfur: Contains a sulfur atom in a thiol (-SH) group — one of the two sulfur-containing amino acids used in proteins, along with methionine
• Disulfide bonds: Two cysteine units can link through their sulfur atoms, forming a disulfide bond that holds parts of a protein together. Two cysteines joined this way form cystine, which is how much of it is found in food

What Cysteine Does

Glutathione:

Cells make glutathione from three amino acids: cysteine, glutamate, and glycine. Cysteine is usually the limiting one — when it runs short, glutathione production slows, regardless of how much glutamate and glycine are available. (In older adults, glycine may also become limiting; see our glycine page.)

Glutathione neutralizes hydrogen peroxide and other reactive molecules, helps recycle vitamin C, and helps the liver process medicines and toxins. Exercise increases the production of reactive molecules, and glutathione is one of the systems that keeps them in check. Importantly, some of these reactive molecules also act as signals that drive training adaptations — so the goal is a well-functioning antioxidant system, not the maximum possible antioxidant intake.

Liver protection — and paracetamol overdose:

The liver uses glutathione to neutralize a toxic by-product of paracetamol (acetaminophen). In an overdose, glutathione is used up and the liver can fail. NAC, given in hospital, rapidly restores cysteine for glutathione production and is the standard antidote — highly effective when given early. A suspected paracetamol overdose is a medical emergency: get help immediately, even if the person feels fine.

NAC has also been studied in other liver conditions, with mixed results: in severe alcohol-related hepatitis, adding NAC to standard treatment improved survival at one month but not at six months. NAC isn’t a way to make heavy drinking or medication overuse safe, and it shouldn’t be used to self-treat liver problems.

Keratin — hair, nails, and skin:

Keratin, the protein of hair, nails, and the outer layer of skin, is unusually rich in cysteine — hair is roughly 10–18% cystine by weight. The many disulfide bonds between cysteine units give hair its strength and shape. Perms work by breaking these bonds chemically and then reforming them in a new shape, and chemical straighteners also act on them.

Hair thinning during periods of undereating is usually caused by the overall shortage of energy and protein, which pushes more hair follicles into their resting phase, rather than a specific lack of cysteine.

Taurine production:

The body converts cysteine into taurine through several steps. Taurine is concentrated in heart and skeletal muscle, where it helps regulate cell volume, electrolyte movement, and muscle function. Meat, fish, and shellfish also provide taurine directly. Taurine is a common ingredient in energy drinks and pre-workout supplements, though its effects on performance are modest and inconsistent.

Disulfide bonds in proteins:

Many proteins, especially those that work outside cells, depend on disulfide bonds to hold their shape:

• Insulin — two disulfide bonds hold its A and B chains together
• Antibodies and many other immune proteins
• Keratin in hair, nails, and skin
• Many hormones, receptors, and digestive enzymes

Cysteine’s reactive thiol group also does chemical work in the active sites of many enzymes.

Recommended Intake

There’s no separate requirement for cysteine — it’s combined with methionine as the “sulfur amino acids.” The US recommendation (RDA) for methionine plus cysteine is 19mg per kg of body weight per day (about 1.3g for a 70kg person), and the World Health Organization estimate is 15mg per kg. Diets with enough protein easily cover this, and there’s no evidence that training raises cysteine needs beyond what the higher protein intakes recommended for athletes provide.

NAC studies typically use 600–1,800mg a day.

Found In (per 100g)

USDA reports cysteine as “cystine,” which covers both forms.

As a share of protein, some plant foods are the richest cysteine sources: oats (3.5%), sunflower seeds and Brazil nuts (about 2%), and rice (2%) all beat meat and fish, which are about 1%. Eggs are also rich, at 2.3%. Legumes are about 1.1–1.3%. Milk and cheese are low, because casein, their main protein, contains little cysteine — although whey protein is rich in it.

Whey’s advantage for building muscle comes mainly from its high leucine content and fast digestion. Its cysteine content has led to its use in studies on raising glutathione, with some positive results, but whether this translates into better recovery isn’t established.

Plant-based diets that include grains, seeds, and legumes provide plenty of cysteine and methionine combined (see our methionine page), so vegans and vegetarians eating varied protein don’t need NAC supplements.

Helps With

• Glutathione production — the main antioxidant made inside cells
• The liver’s processing of medicines and toxins, through glutathione
• The structure of hair, nails, and skin, through keratin
• Taurine production
• Protein structure — disulfide bonds in insulin, antibodies, and many other proteins
• As NAC: treating paracetamol overdose, and loosening mucus in lung conditions

Cysteine for Athletes and Active People

Oxidative stress and recovery: hard training increases the production of reactive oxygen species, and glutathione helps manage them. But these reactive molecules are also part of the signal that tells muscles to adapt — building mitochondria, increasing antioxidant defenses, and repairing damage. For healthy athletes eating enough protein, there’s no evidence that cysteine supply limits recovery.

NAC and training — mixed evidence: some studies found that NAC reduced markers of muscle damage or fatigue, and in one 2018 study, NAC improved performance only in people who started with low glutathione levels — while those with normal levels didn’t benefit. On the other hand, a 2013 study found that high-dose NAC taken after muscle-damaging exercise interfered with the inflammatory and repair signals that drive muscle recovery, delaying it. Like high-dose vitamins C and E, NAC may blunt some training adaptations, so it isn’t recommended for routine use around training.

Whey protein: whey is a good-quality protein that also happens to be rich in cysteine. Its main benefits for athletes come from its leucine content and convenience; any glutathione-related benefit is a possible bonus rather than a proven effect.

Alcohol and medicines: the safest way to protect your liver is to limit alcohol and use medicines as directed. Taking NAC doesn’t make heavy drinking or high medication use safe, and you should talk to a doctor before using it for liver concerns.

Hair during weight cuts: hair shedding during aggressive dieting is mainly a sign of too little energy and protein. Eating enough is the fix — not cysteine supplements.

Cysteine Deficiency

Cysteine deficiency is rare in people eating enough protein. Low intake happens mainly with:

• Severe protein-energy malnutrition
• Very low-calorie or very low-protein diets
• Premature infants, who can’t yet make enough cysteine from methionine
• Homocystinuria — an inherited disorder in which methionine can’t be converted into cysteine, so cysteine becomes essential

Symptoms such as slow recovery, muscle soreness, brittle hair, frequent illness, or slow wound healing have many causes and aren’t reliable signs of low cysteine.

Inherited cystine disorders: in cystinuria, a faulty transporter in the kidneys lets cystine build up in the urine, where it forms kidney stones. It’s managed with plenty of fluids, making the urine less acidic, and sometimes medicines. In cystinosis, cystine builds up inside cells, damaging the kidneys, eyes, and other organs; it’s treated with the drug cysteamine.

Excessive Cysteine

No Tolerable Upper Intake Level has been set for cysteine or NAC.

NAC: doses of 600–1,800mg a day are usually well tolerated. Higher doses more often cause nausea, vomiting, diarrhea, and headache. NAC has a sulfurous smell.

Free L-cysteine: in laboratory studies, high concentrations of free cysteine can react with metals to generate reactive molecules, so NAC is the preferred supplement form.

Medicines: NAC can strengthen the effects of nitroglycerin and other nitrate medicines for angina, causing severe headaches and low blood pressure. It may also affect blood clotting, so people taking anticoagulants should check with their doctor. Inhaled NAC can trigger airway narrowing in people with asthma.

Types and Forms

• N-Acetylcysteine (NAC): the most common supplement form, and a medicine in its own right. Only a small fraction of an oral dose reaches the bloodstream intact, but it reliably raises cysteine and glutathione levels. Used medically for paracetamol overdose and to loosen mucus; in COPD, high doses reduced flare-ups in some trials, while in idiopathic pulmonary fibrosis, a large trial found no benefit.
• L-Cysteine (free form): less stable than NAC and not generally recommended as a supplement. It’s also used as a food additive (E920) to condition bread dough.
• L-Cystine: two cysteines joined by a disulfide bond, as found in food; the body converts it back into cysteine.
• Glutathione supplements: much of standard oral glutathione is broken down in the gut; evidence that liposomal forms raise body levels meaningfully comes from small studies.
• Whey protein: a cysteine-rich food source, alongside its other benefits as a protein.

Which form for your goal:

How to Take Cysteine

From food: eggs, oats and other whole grains, seeds, meat, fish, legumes, and whey all provide cysteine. A varied diet with enough protein covers your needs, whether or not you eat animal foods.

If you take NAC for a specific reason: typical doses are 600mg once or twice a day. Taking it with food reduces stomach upset. Avoid high doses around training, given the evidence that NAC can interfere with recovery signalling.

Check your medicines: talk to your doctor before taking NAC if you use nitrate medicines for angina, take anticoagulants, or have asthma.

Availability: NAC was first approved as a medicine, and in some countries it’s sold mainly as an over-the-counter or prescription medicine rather than a supplement. In the US, the FDA questioned its status as a supplement in 2020–2022 for this reason, though it has remained on sale. Check the rules where you live.

Interactions

Methionine provides the sulfur for making cysteine, via homocysteine, and serine provides the carbon skeleton. Vitamin B6 is needed by both enzymes in this conversion. Glutathione recycles vitamin C back into its active form, and used glutathione is itself recycled by the enzyme glutathione reductase, which depends on riboflavin (vitamin B2). Selenium is needed by glutathione peroxidases, the enzymes that use glutathione to neutralize peroxides. NAC can strengthen the effects of nitrate medicines.

Synergy

• Works with glutamate and glycine to make glutathione — cysteine is usually the limiting building block.
• Works with selenium, which powers the glutathione peroxidases that use glutathione.
• Works with riboflavin, needed to recycle used glutathione, and with vitamin C, which glutathione helps keep in its active form.
• Works with methionine and serine, which the body uses to make cysteine.

Antagonistic Interactions

• High-dose NAC around training may blunt recovery and adaptation signals.
• NAC combined with nitrate medicines can cause severe headache and low blood pressure.
• High-dose NAC and anticoagulants may interact — check with a doctor.
• NAC and free cysteine bind metals such as zinc and copper, which could affect their balance at high doses — another reason to avoid unnecessary high-dose use.

Food values: USDA FoodData Central, SR Legacy, reported as cystine — oats, dry (FDC ID 173904); sunflower seed kernels, dried (170562)*; chicken breast, roasted (171477); pumpkin and squash seed kernels, dried (170556)*; peanuts, raw (172430)*; pork top loin chop, broiled (167841); Brazil nuts, dried, unblanched (170569)*; yellowfin tuna, cooked (172006); egg, hard-boiled (173424); turkey breast, roasted (174516); ground beef, 80% lean, cooked (171797); sardines, canned in oil (175139); Atlantic salmon, farmed, cooked (175168); parmesan, hard (170848); almonds (170567)*; chickpeas, cooked (173757); lentils, cooked (172421); black beans, cooked (173735); white rice, cooked (168878). * SR Legacy values used as fill-ins for the Foundation Foods entries for sunflower seeds (2515381), pumpkin seeds (2515380), peanuts (2515376), Brazil nuts (2515373), and almonds (2346393), whose amino acid data aren’t included in our tracker. Cysteine as % of protein is calculated from the same USDA records.