Protein

About Protein

Protein is a macronutrient that provides 4 calories per gram and is made of chains of amino acids. Proteins build and repair tissues and serve as enzymes, hormones, antibodies, transport molecules such as hemoglobin, and structural components such as collagen and keratin. Of the 20 amino acids used to build human proteins, 9 are essential and must come from food: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. Official protein recommendations are based on body weight, and needs vary with age, activity and illness. Protein quality depends on amino acid profile and digestibility, measured by scores such as PDCAAS and DIAAS, with animal proteins generally scoring higher. Leucine is a key signal for muscle protein synthesis through the mTOR pathway. Plant proteins can supply all essential amino acids when varied sources are combined, since grains tend to be low in lysine and legumes low in methionine. Protein restriction is part of managing chronic kidney disease.