The amino acids that the human body cannot manufacture in adequate amounts and must therefore obtain from food. The nine indispensable amino acids are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine, with arginine additionally essential during childhood growth. Animal proteins such as eggs, dairy, meat, and fish supply all nine in good proportions, while most plant proteins are short of one or more, which is why varied plant diets combine sources like grains and legumes. Nutritionists assess protein quality against reference patterns of these amino acids, a cornerstone of infant formula design and public-health nutrition.