<P> Proteins are essential nutrients for the human body . They are one of the building blocks of body tissue and can also serve as a fuel source . As a fuel, proteins provide as much energy density as carbohydrates: 4 kcal (17 kJ) per gram; in contrast, lipids provide 9 kcal (37 kJ) per gram . The most important aspect and defining characteristic of protein from a nutritional standpoint is its amino acid composition . </P> <P> Proteins are polymer chains made of amino acids linked together by peptide bonds . During human digestion, proteins are broken down in the stomach to smaller polypeptide chains via hydrochloric acid and protease actions . This is crucial for the absorption of the essential amino acids that cannot be biosynthesized by the body . </P> <P> There are nine essential amino acids which humans must obtain from their diet in order to prevent protein - energy malnutrition and resulting death . They are phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine, and histidine . There has been debate as to whether there are 8 or 9 essential amino acids . The consensus seems to lean towards 9 since histidine is not synthesized in adults . There are five amino acids which humans are able to synthesize in the body . These five are alanine, aspartic acid, asparagine, glutamic acid and serine . There are six conditionally essential amino acids whose synthesis can be limited under special pathophysiological conditions, such as prematurity in the infant or individuals in severe catabolic distress . These six are arginine, cysteine, glycine, glutamine, proline and tyrosine . </P>

Where is protein broken down into amino acids