Protein (nutrient)

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.