 In animals , amino acids are obtained through the consumption of foods containing protein . Ingested proteins are then broken down into amino acids through digestion , which typically involves denaturation of the protein through exposure to acid and hydrolysis by enzymes called proteases . Some ingested amino acids are used for protein biosynthesis , while others are converted to glucose through gluconeogenesis , or fed into the citric acid cycle . This use of protein as a fuel is particularly important under starvation conditions as it allows the body 's own proteins to be used to support life , particularly those found in muscle . Amino acids are also an important dietary source of nitrogen . 
