Editing Human nutrition (section)

=== Amino acids ===
[[File:Myoglobin.png|thumb|right|Proteins are chains of amino acids found in many nutritious foods. Pictured above is a computer rendering of [[myoglobin]], a protein found in muscles.]]
{{further|Protein (nutrient)|Protein quality|Amino acid score}}

Proteins are the basis of many animal body structures (e.g. muscles, skin, and hair) and form the [[enzyme]]s that control chemical reactions throughout the body. Each protein molecule is composed of [[amino acids]] which contain nitrogen and sometimes sulphur (these components are responsible for the distinctive smell of burning protein, such as the keratin in hair). The body requires amino acids to produce new proteins (protein retention) and to replace damaged proteins (maintenance). Amino acids are soluble in the digestive juices within the small intestine, where they are absorbed into the blood. Once absorbed, they cannot be stored in the body, so they are either metabolized as required or excreted in the urine.{{medical citation needed|date=July 2015}} Proteins consist of amino acids in different proportions. The most important aspect and defining characteristic of protein from a nutritional standpoint is its amino acid composition.<ref name="DRI">Dietary Reference Intakes: The Essential Guide to Nutrient Requirements, published by the Institute of Medicine's Food and Nutrition Board, currently available online at {{cite web | url=http://fnic.nal.usda.gov/dietary-guidance/dietary-reference-intakes/dri-reports | title=DRI Reports {{pipe}} Food and Nutrition Information Center | access-date=14 July 2014 | url-status=dead | archive-url=https://web.archive.org/web/20140705140516/http://fnic.nal.usda.gov/dietary-guidance/dietary-reference-intakes/dri-reports | archive-date=5 July 2014}}</ref>

For all animals, some amino acids are ''[[essential amino acid|essential]]'' (an animal cannot produce them internally so they must be eaten) and some are ''[[non-essential amino acid|non-essential]]'' (the animal can produce them from other nitrogen-containing compounds). About twenty amino acids are found in the human body, and about ten of these are essential. The synthesis of some amino acids can be limited under special pathophysiological conditions, such as prematurity in the infant or individuals in severe catabolic distress, and those are called conditionally essential.<ref name="DRI" />

A diet that contains adequate amounts of amino acids (especially those that are essential) is particularly important in some situations: during early development and maturation, pregnancy, lactation, or injury (a burn, for instance). A [[complete protein]] source contains all the essential amino acids; an incomplete protein source lacks one or more of the essential amino acids. It is possible with [[protein combination]]s of two incomplete protein sources (e.g., rice and beans) to make a complete protein source, and characteristic combinations are the basis of distinct cultural cooking traditions. However, complementary sources of protein do not need to be eaten at the same meal to be used together by the body.<ref>{{cite journal | author=American Dietetic Association | title=Position of the American Dietetic Association and Dietitians of Canada: Vegetarian diets | journal=Journal of the American Dental Association | year=2003 | volume=103 | issue=6 | pages=748–65 | pmid=12778049 | doi=10.1053/jada.2003.50142 | last2=Dietitians Of | first2=Canada}}</ref> Excess amino acids from protein can be converted into glucose and used for fuel through a process called [[gluconeogenesis]].

There is an ongoing debate about the differences in nutritional quality and adequacy of protein from [[Veganism|vegan]], [[Vegetarianism|vegetarian]] and animal sources, though many studies and institutions have found that a well-planned vegan or vegetarian diet contains enough high-quality protein to support the protein requirements of both sedentary and active people at all stages of life.<ref>{{cite journal | vauthors=Alexandrov NV, Eelderink C, Singh-Povel CM, Navis GJ, Bakker SJ, Corpeleijn E | title=Dietary Protein Sources and Muscle Mass over the Life Course: The Lifelines Cohort Study | journal=Nutrients | volume=10 | issue=10 | pages=1471 | date=October 2018 | pmid=30308987 | pmc=6212815 | doi=10.3390/nu10101471 | doi-access=free}}</ref><ref>{{cite web | url=https://www.hsph.harvard.edu/nutritionsource/what-should-you-eat/protein/ | title=Protein | date=18 September 2012 | website=The Nutrition Source | language=en-us | access-date=31 October 2019 | archive-date=27 October 2019 | archive-url=https://web.archive.org/web/20191027201530/https://www.hsph.harvard.edu/nutritionsource/what-should-you-eat/protein/ | url-status=live}}</ref><ref>{{cite journal | vauthors=Rogerson D | title=Vegan diets: practical advice for athletes and exercisers | journal=Journal of the International Society of Sports Nutrition | volume=14 | pages=36 | date=13 September 2017 | issue=1 | pmid=28924423 | pmc=5598028 | doi=10.1186/s12970-017-0192-9 | doi-access=free}}</ref><ref name="dinu">{{cite journal | vauthors=Dinu M, Abbate R, Gensini GF, Casini A, Sofi F | title=Vegetarian, vegan diets and multiple health outcomes: A systematic review with meta-analysis of observational studies | journal=Critical Reviews in Food Science and Nutrition | volume=57 | issue=17 | pages=3640–3649 | date=November 2017 | pmid=26853923 | doi=10.1080/10408398.2016.1138447 | url=https://flore.unifi.it/bitstream/2158/1079985/4/Vegetarian%2c%20vegan%20diets%20and%20multiple%20he...meta-analysis%20of%20observational%20studies.pdf | hdl=2158/1079985 | s2cid=10073754 | access-date=2018-08-02 | archive-date=2023-01-24 | archive-url=https://web.archive.org/web/20230124005332/https://flore.unifi.it/rm-password-login | url-status=live}}</ref>