Protein (nutrient)

Lore & Background

Proteins are polymer chains composed 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 the 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: 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. The body can synthesize five amino acids: 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 such as prematurity or severe catabolic distress.

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 such as enzymes and antibodies are constructed from proteins, as are cell membrane components like glycoproteins, G proteins, and ion channels. The types of amino acids and their sequence determine the unique three-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 of protein 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% of protein sources. 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, and more than 2 billion people eating insects daily. Protein powders such as casein, whey, egg, rice, soy, and cricket flour are processed sources, but people eating a balanced diet do not need protein 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: phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine, and histidine.
• On a worldwide basis, plant protein foods contribute over 60% of the per capita supply of protein.
• The classic assays for protein concentration in food are the Kjeldahl method and the Dumas method, which determine total nitrogen in a sample.