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One of the major food groups (others being carbohydrate and fat). Protein is used in building body tissues. Protein is found in animal (e.g. meat, dairy, eggs) and plant (e.g. legumes, nuts, soy) foods. Protein is essential for all people but may be especially important for athletes & active people (to build & repair muscle). There is increasing interest in high-protein low-carbohydrate diets for weight-loss but further research is needed before this can be widely advocated.

Protein_(nutrient) (Wikipedia)
This article is about protein as a nutrient. For protein as class of molecule, see protein. For other uses, see bodybuilding supplement.
Amino acids are the building blocks of protein.
Amino acids are necessary nutrients. Present in every cell, they are also precursors to nucleic acids, co-enzymes, hormones, immune response, repair and other molecules essential for life.

Proteins are essential nutrients for the human body. They are one of the building blocks of body tissue, and can also serve as a fuel source. As a fuel, proteins provide as much energy density as carbohydrates: 4 kcal (17 kJ) per gram; in contrast, lipids provide 9 kcal (37 kJ) per gram. The most important aspect and defining characteristic of protein from a nutritional standpoint is its amino acid composition.

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

There are nine essential amino acids which humans must obtain from their diet in order to prevent protein-energy malnutrition and resulting death. They are phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine, and histidine. There are five dispensable amino acids which humans are able to synthesize in the body. These five are 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. These six are arginine, cysteine, glycine, glutamine, proline and tyrosine.

Humans need the essential amino acids in certain ratios. Some protein sources contain amino acids in a more or less 'complete' sense. This has given rise to various ranking systems for protein sources, as described in the article.

Dietary sources of protein include both animals and plants: meats, dairy products, fish and eggs as well as grains, legumes and nuts. Vegetarians and vegans can get enough essential amino acids by eating a variety of plant proteins. It is commonly believed that athletes should consume a higher-than-normal protein intake to maintain optimal physical performance.

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