Essential amino acidsEAAs
The nine amino acids the body can't make and must get from food.
Amino acids are the twenty building blocks of protein, and nine of them are essential: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. Essential here means your body cannot make them, so food has to supply them.
They matter because muscle protein synthesis depends on having all of them available at once. A protein source short in one essential amino acid limits how much new tissue can be built from it, whatever the total grams say, which is the origin of the idea of a limiting amino acid.
Leucine has a special role beyond being a building block. It acts as a signal, triggering the machinery that starts protein synthesis, which is why the leucine content of a meal predicts the muscle building response better than total protein alone does.
Animal sources supply all nine in generous proportions. Plant sources are individually lower in one or another, typically lysine in grains and methionine in pulses, which is where the old advice about combining proteins at every meal came from. That advice was overstated: the body maintains a pool of amino acids, so what matters is the day's intake rather than the individual plate.
That said, the practical implication for plant based eaters is real. Total protein needs to be a little higher and variety matters more, because the margin for a limiting amino acid is smaller.
Free amino acid supplements, including BCAAs, are largely redundant when protein intake is adequate. BCAAs supply three of the nine, and building tissue from three of nine is not possible, which is why they underperform whole protein in trials.
In practice, eat enough total protein from varied sources and the amino acid arithmetic looks after itself. Our protein intake calculator sets the total.