Protein (nutrient)
Essential nutrient providing amino acids for body structure and function.
Protein is an essential nutrient for the human body, serving as a constituent of body tissue and a fuel source with an energy density of 17 kJ (4 kcal) per gram. Its defining nutritional characteristic is its amino acid composition, and it is commonly measured based on nitrogen content. Proteins are polymer chains of amino acids linked by peptide bonds, broken down during digestion into smaller polypeptides via hydrochloric acid and protease actions. This breakdown is critical for absorbing the nine essential amino acids—phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine, and histidine—which the human body cannot biosynthesize and must obtain from diet to prevent protein-energy malnutrition. Debate exists over whether there are eight or nine essential amino acids, but consensus leans toward nine because histidine is not synthesized in adults. The body can synthesize five amino acids: alanine, aspartic acid, asparagine, glutamic acid, and serine. Six others—arginine, cysteine, glycine, glutamine, proline, and tyrosine—are conditionally essential, meaning their synthesis can be limited under special conditions like prematurity or severe catabolic distress. After water, proteins account for more mass in an organism than any other molecule, present in every cell and structural in all body tissues, including hair, skin, blood, and bone, with particular abundance in muscle. Proteins also form cellular messengers, transport molecules, enzymes, antibodies, and cell membrane components. Amino acids from protein catabolism enable biosynthesis of essential non-protein molecules like nucleotides, neurotransmitters, and heme. Dietary sources include grains, legumes, nuts, seeds, meats, dairy, fish, eggs, and insects; plant proteins contribute over 60% of global per capita supply, while in North America animal-derived foods provide about 70%. Protein powders are processed sources, but a balanced diet does not require supplements. Protein content in food is measured via nitrogen-based methods like the Kjeldahl or Dumas tests, which determine total nitrogen, multiplied by a factor (commonly 6.25) to calculate crude protein, though this cannot distinguish true protein from non-protein nitrogen.
- field
- Nutrition
- known_for
- Essential nutrient; source of amino acids; structural component of body tissues
- energy_density
- 17 kJ (4 kcal) per gram
- essential_amino_acids
- 9 (phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine, histidine)
- common_measurement
- Nitrogen-based crude protein (Kjeldahl or Dumas method)
Lore & Background
Proteins are polymer chains made of amino acids linked by peptide bonds. During human digestion, proteins are broken down in the stomach into smaller polypeptide chains via hydrochloric acid and protease actions, which is crucial for absorption of essential amino acids that cannot be biosynthesized by the body. There are nine essential amino acids that humans must obtain from their diet to prevent protein-energy malnutrition and resulting death: phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine, and histidine. Debate exists as to whether there are eight or nine essential amino acids, with consensus leaning toward nine since histidine is not synthesized in adults. Five amino acids can be synthesized by the human body: alanine, aspartic acid, asparagine, glutamic acid, and serine. Six conditionally essential amino acids—arginine, cysteine, glycine, glutamine, proline, and tyrosine—have synthesis that can be limited under special pathophysiological conditions.
Reader's Guide
Protein is present in every cell and is a structural component of every body tissue and organ, including hair, skin, blood, and bone, and is especially abundant in muscle. Cellular messengers (hormones) and transport molecules are constructed from proteins, including enzymes and antibodies, as are cell membrane components such as glycoproteins, G proteins, and ion channels. The types of amino acids and their sequence determine the unique 3-dimensional structure and function of a protein. Amino acids obtained through protein catabolism also enable the biosynthesis of non-protein molecules essential for life, such as nucleotides, certain neurotransmitters, and heme. Dietary sources include grains, legumes, nuts, seeds, meats, dairy products, fish, and eggs. On a worldwide basis, plant protein foods contribute over 60% of the per capita supply of protein; in North America, animal-derived foods contribute about 70%. Insects are a source of protein in many parts of the world, with up to 50% of dietary protein deriving from insects in parts of Africa. Protein powders are processed sources, but people eating a balanced diet do not need supplements.
Did You Know?
- Proteins have the same energy density as carbohydrates: 17 kJ (4 kcal) per gram.
- There are nine essential amino acids that humans cannot biosynthesize and must obtain from diet.
- The classic assays for protein concentration in food are the Kjeldahl method and the Dumas method.
- In parts of Africa, up to 50% of dietary protein derives from insects.
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