Amino acid
Amino acids are the building blocks proteins are made from; nine of the twenty used by the human body are essential, meaning they must come from food because the body cannot make them.
When you eat protein, digestion breaks it into amino acids, which are absorbed and reassembled into whatever the body currently needs — muscle fibre, enzymes, antibodies, hair. The word "protein" on a label is shorthand for the amino acids it carries.
Essential and non-essential
Nine are essential: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. The rest can be synthesised from other compounds. A food's protein quality is largely a question of whether it supplies all nine in useful proportions and how well it is digested.
The limiting amino acid
A protein source is constrained by whichever essential amino acid it contains least of relative to requirement. Cereals — wheat, rice, maize — are typically limited by lysine. Legumes are typically limited by methionine. Which is why the two together make a better protein than either alone, and why dal with roti or rice is a genuinely good pairing rather than a folk belief.
Why you do not need to combine at every meal
The old advice to pair complementary proteins within a single meal has not held up. The body maintains a pool of free amino acids, so variety across the day is sufficient. Eat legumes, grains, dairy and whatever else you eat, spread through the day, and the profile takes care of itself.
Sources
- 1.Protein and amino acid requirements in human nutrition (WHO Technical Report 935) — WHO, FAO and UNU
- 2.Office of Dietary Supplements — National Institutes of Health
Related reading
- Vegetarian protein worth the calories it costs
Ranked by calories per gram of protein, not by grams per 100 g. Which vegetarian sources are efficient, which are not, and how to build a 130 g day.
- Protein in dal, chana, rajma and the rest
A katori of dal is 7 to 9 g of protein. Useful, and less than its reputation. What each pulse supplies, and where legumes fit in a protein target.
- How much protein do you actually need per day?
The RDA is a floor, not a target. What the evidence supports for people who train, how to reach it on Indian food, and why it is the one macro worth counting.
- What macros are, and which ones are worth counting
Protein, carbohydrate, fat and alcohol, what each supplies, and why calories plus protein is the two-number system most people should actually use.