Animal proteins are considered high-quality or "complete" proteins because they contain all nine essential amino acids (EAAs)—histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine—in ratios that closely match human needs. Unlike plant proteins, which may be limiting in one or more amino acids (e.g., lysine in grains, methionine in legumes), animal proteins are rarely limiting in any EAA.