What is L-cysteine, and which foods contain it?

L-cysteine is one of the amino acids the body uses to build protein, and one of the two common amino acids that contain sulfur; the other is methionine. The body can make cysteine from methionine, so it is called conditionally essential rather than essential. In food, cysteine comes with protein: soybeans, sunflower and sesame seeds, eggs, meat, fish and grains all contain it. It is also used in baking to treat flour. There is no Thai Reference Daily Intake (Thai RDI) for cysteine on its own; the Thai RDI for protein as a whole is 50 g a day.

In short:

• What it is: an amino acid that contains sulfur, formula C3H7NO2S
• Cystine: two cysteine molecules joined by a disulfide bridge
• Essential or not: conditionally essential; the body makes it from methionine
• In food: with protein in soybeans, seeds, eggs, meat, fish and grains
• In baking: used as a flour treatment agent
• Daily amount: no Thai RDI for cysteine; methionine and cysteine are assessed together internationally

This article explains what L-cysteine is, how the body makes it and where it is found in food. For the nine amino acids the body cannot make, see What are essential amino acids?

What is L-cysteine?

An amino acid that contains sulfur

PubChem, the chemical database of the US National Library of Medicine, gives L-cysteine the formula C3H7NO2S and the chemical name (2R)-2-amino-3-sulfanylpropanoic acid. The "sulfanyl" part is a thiol group, a sulfur atom bonded to a hydrogen atom. PubChem's description, taken from the ChEBI database, calls L-cysteine a proteinogenic amino acid, meaning one used to build proteins, and lists flour treatment agent among its roles. The "L" refers to the mirror-image form found in proteins; its opposite is D-cysteine.

Cysteine and cystine

The US National Library of Medicine's MeSH vocabulary describes cystine as formed by the oxidation of cysteine: two cysteine molecules are joined together by a disulfide bridge, a bond between their two sulfur atoms. A 1991 review by researchers at Australia's CSIRO describes the hard keratins of wool, hair, hooves, horns and nails as filaments set in a matrix that includes cystine-rich proteins, made insoluble by disulfide bonds.

Is cysteine an essential amino acid?

Made from methionine

A 2020 review by Stipanuk at Cornell University describes how the body handles methionine: through two linked pathways, transmethylation and transsulfuration, the sulfur atom of methionine is transferred to the amino acid serine to make cysteine. Cysteine that the body does not need for protein is broken down further, for example to taurine or sulfate.

A 2026 review by Lucaciu and colleagues at the Iuliu Hațieganu University of Medicine and Pharmacy in Romania puts it this way: methionine is nutritionally indispensable, whereas cysteine is considered conditionally indispensable because it can be made from methionine through transsulfuration. Cysteine in food therefore has a "methionine-sparing" effect: it reduces the amount of methionine that has to be diverted to making cysteine.

Which foods contain cysteine?

Cysteine comes with protein, so foods with more protein usually contain more of it. The figures below are per 100 g of raw food from the US Department of Agriculture's FoodData Central database (SR Legacy), which lists this amino acid as cystine. Methionine is shown alongside because the body can make cysteine from it.

These are figures for raw foods analysed in the United States. Cooked foods contain different amounts per 100 g because they gain or lose water.

Sesame and other plant proteins: research from Thailand

A 2025 review from Chiang Mai University looked at sesame seed meal, the protein-rich material left after oil is pressed from sesame seeds. The authors describe sesame proteins as rich in the sulfur amino acids, which are typically lacking in some plant protein sources such as legume seeds, including soybeans and peanuts. In the table above, soybeans have more cystine per 100 g than sesame seeds, but they also have more protein (36.49 g against 17.73 g); the review's point is about the share of sulfur amino acids within the protein.

A 2022 study led by the University of Hohenheim in Germany with Silpakorn University and the Rayong Field Crops Research Center analysed the leaves of four cassava varieties grown in Thailand, at two plant ages and three leaf positions. The leaf protein was rich in glutamine, aspartic acid and leucine but low in methionine and cysteine. The leaves of the variety Rayong 5, taken from the middle of 6-month-old plants, had the most protein and amino acids.

How is L-cysteine used in making food?

Apart from the cysteine that comes with protein, L-cysteine is also added to some foods as an ingredient.

Treating flour for bread

In the United States, 21 CFR 184.1271 lists L-cysteine for one specific use: as a dough strengthener in yeast-leavened baked goods and baking mixes, supplying up to 0.009 part of total L-cysteine per 100 parts of flour. The regulation states that it was issued before a general evaluation of the ingredient, and it covers only that use.

Researchers at Yıldız Technical University in Istanbul explain that L-cysteine is added to flour because it gives the dough low viscosity and more rise during baking, but that Turkey's food additive rules do not allow it in wheat flour. They developed a test that could detect L-cysteine and L-cystine in wheat flour at 0.125% by weight. A 2010 study from Tomas Bata University in Zlín, Czech Republic, found that mixtures containing L-cysteine reduced both the tenacity and the extensibility of wheat dough.

Rules differ between countries and change over time, so the table describes the rules as reported in these sources.

How much cysteine does a person need?

There is no Thai Reference Daily Intake (Thai RDI) for cysteine on its own. The Thai RDI covers protein as a whole, at 50 g a day, and cysteine arrives as part of that protein.

Methionine and cysteine are counted together

Because the body makes cysteine from methionine, international requirements treat the two together as the sulfur amino acids. The 2026 review by Lucaciu and colleagues reports that the FAO/WHO/UNU reference value for adults is about 15 mg of methionine and cysteine combined per kg of body weight a day as an average requirement. These are figures for the two amino acids together, from all foods, not amounts of L-cysteine to take as a supplement.

Who should talk to a doctor or pharmacist?

Pregnant or breastfeeding women and people who take regular medication

If you are pregnant or breastfeeding, or take medication regularly, check with a doctor or pharmacist before taking supplements, including amino acid supplements.

Dietary supplements are not intended to prevent, treat or cure any disease.