Wikipedia
Nutrient for the human body
Summary
Proteins are essential nutrients for the human body. They are one of the constituents of body tissue and also serve as a fuel source. As fuel, proteins have the same energy density as carbohydrates: 17 kJ per gram. From a nutritional perspective, the defining characteristic of protein is its amino acid composition. Protein content in foods is commonly measured based on nitrogen content, as nitrogen is a fundamental component of amino acids.