Protein (nutrient)

This article is about protein as a nutrient. For protein as a class of molecule, see Protein. For other uses, see Bodybuilding supplement.

Proteins are essential nutrients for the human body.[1][2][3] They are one of the constituents of body tissue and also serve as a fuel source.[1] As fuel, proteins have the same energy density as carbohydrates: 17 kJ (4 kcal) per gram. The defining characteristic of protein from a nutritional standpoint is its amino acid composition.[1][4]

Proteins are polymer chains made of amino acids linked by peptide bonds.[1] During human digestion, proteins are broken down in the stomach into smaller polypeptide chains via hydrochloric acid and protease actions.[1] This is crucial for the absorption of the essential amino acids that cannot be biosynthesized by the body.[2][5]

There are nine essential amino acids that humans must obtain from their diet to prevent protein-energy malnutrition and resulting death. They are phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine, and histidine.[4][6] There has been debate as to whether there are eight or nine essential amino acids.[7] The consensus seems to lean toward nine since histidine is not synthesized in adults.[8] There are five amino acids that the human body can synthesize: alanine, aspartic acid, asparagine, glutamic acid and serine. There are six conditionally essential amino acids whose synthesis can be limited under special pathophysiological conditions, such as prematurity in the infant or individuals in severe catabolic distress: arginine, cysteine, glycine, glutamine, proline and tyrosine.[4] Dietary sources of protein include grains, legumes, nuts, seeds, meats, dairy products, fish, and eggs.[2]

  1. ^ a b c d e "What are proteins and what do they do?". MedlinePlus, National Library of Medicine, US National Institutes of Health. 26 March 2021. Retrieved 31 March 2025.
  2. ^ a b c "Protein in diet". MedlinePlus, National Library of Medicine, US National Institutes of Health. 23 April 2023. Retrieved 31 March 2025.
  3. ^ Cite error: The named reference proteinbody was invoked but never defined (see the help page).
  4. ^ a b c Cite error: The named reference DRIProteinChpt was invoked but never defined (see the help page).
  5. ^ Genton L, Melzer K, Pichard C (August 2010). "Energy and macronutrient requirements for physical fitness in exercising subjects". Clinical Nutrition. 29 (4): 413–23. doi:10.1016/j.clnu.2010.02.002. PMID 20189694.
  6. ^ Young VR (August 1994). "Adult amino acid requirements: the case for a major revision in current recommendations" (PDF). The Journal of Nutrition. 124 (8 Suppl): 1517S – 1523S. doi:10.1093/jn/124.suppl_8.1517S. PMID 8064412.
  7. ^ Rosane Oliveira, "The Essentials–Part One", UC Davis Integrative Medicine, Feb 4, 2016. July 12, 2017.
  8. ^ Kopple JD, Swendseid ME (May 1975). "Evidence that histidine is an essential amino acid in normal and chronically uremic man". The Journal of Clinical Investigation. 55 (5): 881–91. doi:10.1172/jci108016. PMC 301830. PMID 1123426.
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