Leucine
Leucine is an essential branched-chain amino acid used as a building block for protein and as a nutrient signal involved in regulating protein synthesis. Because humans cannot make enough leucine, it must come from food. It is one of the nine EAAs and one of the three BCAAs.
Leucine helps activate signaling through the mechanistic target of rapamycin complex 1, or mTORC1, after a protein-containing meal. This contributes to the rise in muscle protein synthesis, but pathway activation is not identical to long-term muscle growth. New protein still requires the other essential amino acids, adequate energy, and a repeated training stimulus.
The popular "leucine threshold" is a useful teaching concept, not one fixed gram value for every meal and person. The response depends on total protein, the food matrix, age, body size, prior exercise, and the other amino acids available. A meal with sufficient high-quality protein normally supplies leucine along with the complete substrate set.
Dairy, meat, fish, eggs, soy, legumes, nuts, and grains all contribute leucine in different concentrations. Whey is particularly leucine-rich, which helps explain its strong acute amino-acid response. A higher concentration does not make every whey product or leucine-fortified food superior when daily protein is already adequate.
Isolated leucine has been studied in older adults and other groups with reduced anabolic responsiveness. Some acute studies show a larger protein-synthesis response when leucine is added to a smaller protein serving, but long-term results depend on population and background intake. The ISSN review of EAA evidence should not be read as proof that standalone leucine increases muscle or strength in protein-replete healthy lifters.
Leucine supplements may be sold alone or inside BCAA, EAA, and protein products. Count overlapping sources before adding them, and judge the full formula rather than the headline dose. More signaling is not endlessly better, and leucine cannot compensate for missing amino acids, inadequate calories, or poorly designed training. Medical disorders of amino-acid metabolism and prescribed protein restrictions require qualified advice.