Protein in Human Nutrition

Protein is made of amino acids, of which nine are classed as essential because the body cannot synthesize them. Proteins build and repair tissues and also form enzymes, hormones, antibodies and transport molecules such as hemoglobin.

Protein quality depends on amino acid composition and digestibility, and scoring systems such as PDCAAS and DIAAS were developed to compare foods. Animal proteins generally contain all essential amino acids in useful proportions, while some plant proteins are limited in particular amino acids, such as lysine in cereals, a gap that combinations of grains and legumes can fill.

Official reference intakes for protein exist and vary with age, body size and life stage. Researchers continue to study how protein needs may differ in older adults, athletes and people who are ill, including the signaling role of the amino acid leucine in muscle protein synthesis..