Proteins are essential nutrients for the human body. 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 absorption 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. 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.
Retrieved 9 November WebMD Medical Reference. Nuts and seeds. Download as PDF Printable version. May Retrieved 27 April Aging Cell. Fad-diets can be tempting as they offer a quick-fix to a long-term problem. The condition gets its name from the distinctive sweet odor of affected infants’ urine. Eades, M.
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