What is lysine, and which foods contain it?

Lysine is an amino acid, one of the building blocks of protein, and one of the nine essential amino acids that the body cannot make and must get from food. It comes with the protein in meat, fish, eggs, milk, beans and grains. In the protein of grains, including rice, the share of lysine is lower than the proportion human needs, while the protein of meat, fish and eggs has more. There is no Thai Reference Daily Intake (Thai RDI) for lysine itself; the Thai RDI for protein is 50 g a day.

In short:

• What it is: an essential amino acid; the form in protein is L-lysine
• Where it comes from: foods that contain protein
• Rice: has little lysine per 100 g, and lysine is low relative to rice's protein
• Cooking and processing: heating can make part of the lysine unavailable (the Maillard reaction)
• How the lysine in food additives is made: by bacterial fermentation
• Daily amount: no Thai RDI for lysine; Thai RDI for protein is 50 g

This article is about lysine alone. For all nine essential amino acids and how rice and beans complement each other, see What are essential amino acids?

What is lysine?

One of the nine essential amino acids

The Nutrition Source at the Harvard T.H. Chan School of Public Health explains that protein is made from twenty-plus building blocks called amino acids, and that nine of them, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine, are essential amino acids that must come from food. The body cannot make lysine from other amino acids.

Its chemical structure

These figures come from the US chemical database PubChem. "Diamino" means the molecule has two amino groups. One of them joins lysine to its neighbours in a protein chain; the other sits at the end of its side chain. That second amino group is the part that reacts during cooking, as explained below.

"L-" describes the shape of the molecule. Amino acids can exist in two mirror-image forms, and the proteins in food are built from the L-forms.

How is lysine connected to protein?

Lysine arrives in the body as part of the protein in food, and protein is a source of essential amino acids. For the other roles of protein, see Which plant foods have protein? How much do you need a day?

Which foods contain lysine?

Lysine in common foods

The figures below are per 100 g of food from the US Department of Agriculture's FoodData Central database (SR Legacy). They are averages of US samples; Thai foods of the same kind can differ with variety, breed and preparation.

Meat, fish, seafood and eggs have both more protein and more lysine per 100 g. Peanuts have about as much protein as pork per 100 g but much less lysine, which shows that the same amount of protein does not always carry the same amount of lysine.

Why is rice protein low in lysine?

A 2019 review by Mariotti and Gardner in the journal Nutrients says that all plant foods contain all the essential amino acids, but that lysine is present in grains in much lower than optimal proportions for human needs. In the table, 100 g of cooked white rice has 2.69 g of protein and 0.097 g of lysine, and glutinous rice has 2.02 g and 0.073 g.

Rice in the Thai diet

Rice still matters for protein because Thai people eat a lot of it. A 2000 study led by Miyagi University, working with Thai researchers, collected duplicates of everything eaten over 24 hours by 52 working women in Bangkok. They took in an average of 55 g of protein a day, 63% of it from animal foods, and 60% of their carbohydrate came from rice. The researchers concluded that protein intake was sufficient in most cases. The study was small, included only women in Bangkok, and is now more than 25 years old.

Does cooking or processing change lysine?

The browning reaction

When food is heated, amino acids can react with sugars in the Maillard reaction, the reaction that browns bread crusts and roasted foods. A 2007 review from the Institute of Human Nutrition and Food Science in Kiel, Germany, explains that in the early stage of this reaction, amino acids react with reducing sugars and lose their availability. This matters especially for lysine, because its side chain carries a free amino group. The review notes that lysine is also the limiting amino acid in many proteins, and that compounds such as furosine are measured as markers of how much lysine has reacted.

Milk and dairy foods

A 2016 review by researchers at Anand Agricultural University in India and the University of Queensland in Australia explains that lysine in milk proteins becomes "blocked" when it reacts with reducing sugars, especially lactose. Blocked lysine is less available to the body and also makes the protein harder for digestive enzymes to break down. The review names heating and storage of milk and milk products as the main drivers of this reaction.

Why the measured figure can be higher than the usable amount

Researchers at Massey University in New Zealand point out that in heat-processed foods, some lysine derivatives that the body cannot use turn back into lysine during the acid step of standard amino acid analysis. As a result, the usual analysis can overestimate the available lysine in processed foods, and a separate method is needed to measure it accurately.

How is the lysine used in food made?

By fermentation

Lysine added to foods and feed is made mainly by bacteria. A 2022 review from Jiangnan University in China describes industrial production of L-lysine with Corynebacterium glutamicum and says L-lysine is widely used in food processing, pharmaceutical preparations and feed additives. A 2024 review from Nanjing Tech University describes production with Escherichia coli and lists glucose, sucrose and corn steep liquor as common carbon sources. It reports that the global lysine market was valued at 6.96 billion US dollars in 2021.

Rules for adding lysine to food

In the United States, 21 CFR 172.320 allows L-lysine, in free form or as the hydrochloride, sodium or potassium salt, to be added to foods as a nutrient under conditions. It must be added to a food whose protein is a significant dietary source, it must improve the measured quality of that protein, and the added lysine plus the lysine already present must not exceed 6.4% of the total protein by weight.

How much lysine do you need a day?

No Thai RDI for lysine

The Thai Reference Daily Intakes (Thai RDIs) do not include individual amino acids. The Thai RDI for protein, set for the general population aged 3 and over, is 50 g a day, and lysine comes with that protein.

The international estimate

The 2007 report of the joint FAO/WHO/UNU expert consultation on protein and amino acid requirements sets an estimated average requirement of 30 mg of lysine per kilogram of body weight a day for adults; the 2019 review by Mariotti and Gardner uses this figure. It is an international estimate, not a Thai figure.

Who should ask a doctor or pharmacist about lysine?

Lysine comes with the protein in food.

Lysine is also available as an amino acid supplement. If you take one, tell your doctor or pharmacist about it and about any other supplements and medicines you use.

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.

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