Essential nutrient

An essential nutrient is a nutrient required for normal body functioning that cannot be synthesized by the body and must be obtained from a dietary source. Some categories of essential nutrient include vitamins, dietary minerals, essential fatty acids, and essential amino acids.

Different species have very different essential nutrients. Most essential nutrients are substances that are metabolically necessary but cannot be synthesized by the organism. Dietary minerals, for example, cannot be synthesized in biological systems, so (for example) a human must obtain the iron they need to build hemoglobin from their diet. Of course, this iron is recycled, but some is inevitably lost, for example during menstruation.

All 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." A similar statement can be made about vitamin C and some other vitamins.

List of essential nutrients
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 or 0.5 mg of bismuth.
 * Essential fatty acids:
 * Linolenic acid (the shortest chain omega-3 fatty acid)
 * Linoleic acid (the shortest chain omega-6 fatty acid)
 * Essential amino acids necessary for all humans:
 * Histidine
 * Isoleucine
 * Lysine
 * Leucine
 * Methionine
 * Phenylalanine
 * Threonine
 * Tryptophan
 * Valine
 * Essential amino acids necessary for human children and not adults:
 * Arginine
 * Vitamins:
 * Biotin (vitamin B7, vitamin H)
 * Choline (vitamin Bp)
 * Folate (folic acid, vitamin B9, vitamin M)
 * Niacin (vitamin B3, vitamin P, vitamin PP)
 * Pantothenic acid (vitamin B5)
 * Riboflavin (vitamin B2, vitamin G)
 * Thiamine (vitamin B1)
 * Vitamin A (retinol)
 * Vitamin B6 (pyridoxine, pyridoxamine, or pyridoxal)
 * Vitamin B12 (cobalamin)
 * Vitamin C (ascorbic acid)
 * Vitamin D (Cholecalciferol, Ergocalciferol, Calcitriol)
 * Vitamin E (tocopherol)
 * Vitamin K (naphthoquinoids)
 * Dietary minerals: Biochemical studies reported in 2006 indicate that the following elements (aside from H, C, N, and O) are required for human health:
 * Calcium (Ca)
 * Chloride (Cl-)
 * Cobalt (Co)
 * Copper (Cu)
 * Iodine (I)
 * Iron (Fe)
 * Magnesium (Mg)
 * Manganese (Mn)
 * Molybdenum (Mo)
 * Phosphorus (P)
 * Potassium (K)
 * Selenium (Se)
 * Sodium (Na)
 * Sulfur (S)
 * Zinc (Zn)

Elements with speculated role in human health
Many elements have been implicated at various times to have a role in human health. For none of these elements has the specific protein or complex been identified:
 * Bismuth (Bi) (suspect)
 * Boron (B)
 * Bromine (Br)
 * Chromium (Cr)
 * Fluorine (F) (necessity unknown in humans)
 * Germanium (Ge)
 * Rubidium (Rb) (suspect)
 * Nickel (Ni) common in unicellular organisms
 * Silicon (Si)
 * Strontium (Sr) (suspect)
 * Tellurium (Te) (suspect)
 * Tin (Sn) (may depend on definition of essential -- see reference)
 * Titanium (Ti) (suspect)
 * Tungsten (W) (some organisms use tungsten rather than molybdenum)