<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>

Where is protein stored in the human body