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: 17 kJ (4 kcal) per gram; in contrast, lipids provide 37 kJ (9 kcal) per gram. The most important aspect and defining characteristic of protein from a nutritional standpoint is its amino acid composition.
