Macronutrient (large molecule) made of peptides, which are made of amino acids. The body synthesizes a lot of types of proteins, to use them in various ways and functions. This macronutrient needs to be consumed in the diet, because the lack of it can result in alterations in the metabolism, in organs, in energy and a lot of other homeostatic functions. It also is a very important source of calories (4kcal per gram).