Amino Acids

Amino acids are the building blocks of protein. The body uses 20 of them: nine are essential and must come from food, and the rest the body can make. They also serve as raw material for neurotransmitters, hormones and other molecules.

What is it?

The nine essential amino acids are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. The others (such as glutamine, glycine, alanine and tyrosine) can be made by the body, though some become "conditionally essential" during illness, stress or rapid growth. Leucine, isoleucine and valine are called branched-chain amino acids (BCAAs).

A "complete" protein provides all nine essential amino acids in useful amounts. Animal foods and soya are complete; grains are lower in lysine, and legumes are lower in methionine, so eating both over the day covers both.

What it does

1. Are linked together to make every protein in the body
2. Tryptophan is used to make serotonin and niacin; tyrosine to make dopamine, adrenaline and thyroid hormone
3. Provide nitrogen for making DNA and other molecules
4. Can be burned for energy or turned into glucose when needed

Food sources

Complete proteins: meat, poultry, fish, eggs, dairy, soya (tofu, tempeh, edamame), quinoa. Combine grains with legumes (rice with dhal, bread with peanut butter) for a complete set. Leucine is plentiful in dairy, meat, eggs and soya; lysine in meat, fish, legumes and dairy; tryptophan in turkey, eggs, cheese, nuts and seeds.

Singapore food examples

1. Rice with dhal, beans or tofu — a classic complementary pair
2. Eggs, fish, chicken and other meat
3. Tofu, tempeh and soya milk
4. Yoghurt and cheese
5. Peanuts and nuts with wholegrain bread

How much do I need?

Singapore's RDA table does not list amino acids. The US National Academies give a daily requirement for each essential amino acid per kilogram of body weight, for example 42 mg of leucine, 38 mg of lysine, 24 mg of valine, 19 mg of isoleucine, 20 mg of threonine, 14 mg of histidine, 5 mg of tryptophan, 19 mg of methionine plus cysteine and 33 mg of phenylalanine plus tyrosine. Eating enough varied protein meets these. See Protein and Singapore's age- and sex-specific Recommended Dietary Allowances (HealthHub).

Low intake and deficiency

A shortage of one essential amino acid limits how much protein can be made, as in poor diets based on a single staple. It can cause slow growth, muscle loss, weakness and poor immunity. Vegetarians and vegans avoid this by eating a variety of plant proteins.

Too much and safety

Amino acids from food are safe. High-dose single amino acid supplements (for example, BCAAs, arginine, tryptophan, glutamine) are not needed by most people and can cause stomach upset, imbalance with other amino acids and, with tryptophan or 5-HTP, serious interactions with antidepressants. People with certain inherited conditions, such as phenylketonuria (PKU, restrict phenylalanine) or maple syrup urine disease, need medical diets.

Absorption and bioavailability

Dietary protein is broken into amino acids and short peptides, absorbed in the small intestine through specific transporters, and carried to the liver. Amino acids compete for transporters, which is one reason that large doses of one can reduce absorption of others. Protein from animal foods is more digestible; cooking generally improves digestibility, though very high heat can damage lysine.

Connection to daily life

1. Exercise and recovery: leucine-rich protein after training supports muscle repair — see Exercise
2. Mood and sleep: the brain needs tryptophan and tyrosine from protein foods, but extra supplements are not proven to improve mood or sleep — see Sleep
3. Plant-based eating: eat a variety of legumes, soya, grains, nuts and seeds

Sources: 1. National Academies of Sciences, Engineering, and Medicine — Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (2005) 2. HealthHub Singapore — Recommended Dietary Allowances: https://www.healthhub.sg/well-being-and-lifestyle/food-diet-and-nutrition/recommended_dietary_allowances 3. Health Promotion Board — Singapore Food Insights Database (SG FoodID): https://www.hpb.gov.sg/healthy-living/food-and-beverage/sgfoodid/ 4. FAO/INFOODS — Food composition tables and databases: https://www.fao.org/food-composition/tables-and-databases/en/