An essential nutrient is a nutrient required for normal body functioning that either cannot be synthesized by the body at all, or cannot be synthesized in amounts adequate for good health (e.g. niacin, choline), and thus must be obtained from a dietary source. Essential nutrients are also defined by the collective physiological evidence for their importance in the diet, as represented in government approved tables for Dietary Reference Intake.[1]
Some categories of essential nutrients include vitamins, dietary minerals, essential fatty acids, and essential amino acids. Water and oxygen are also essential for human health and life, as oxygen cannot be synthesized by the body, and water, while a biochemical reaction product of metabolism, is not created in sufficient amounts. Both are necessary as biochemical reactants in some processes, and water is used in various ways such as a solvent, carrier, coolant, and integral polar structural member, but both are often not included as nutrients.
Different species have very different essential nutrients. For example, most mammals synthesize their own ascorbic acid, and it is therefore not considered an essential nutrient for such species. It is, however, an essential nutrient for human beings, who require external sources of ascorbic acid (known as Vitamin C in the context of nutrition).
Many essential nutrients are toxic in large doses (see hypervitaminosis or the nutrient pages themselves below). Some can be taken in amounts larger than required in a typical diet, with no apparent ill effects. Linus Pauling said of vitamin B3, (either niacin or niacinamide), "What astonished me was the very low toxicity of a substance that has such very great physiological power. A little pinch, 5 mg, every day, is enough to keep a person from dying of pellagra, but it is so lacking in toxicity that ten thousand times as much can [sometimes] be taken without harm."[2]
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The body's requirements vary widely. At one extreme, a 70 kg human contains 1.0 kg of calcium, but only 3 mg of cobalt.
Many elements have been implicated at various times to have a role in human health. For none of these elements, however, has a specific protein, complex or dietary reference intake been established:
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