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 e.g. US government approved tables for Dietary Reference Intake.
Some categories of essential nutrients include vitamins, dietary minerals, essential fatty acids, and essential amino acids. 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."
For humans 
Fatty acids 
Essential fatty acids cannot be synthesized by humans, as humans lack the desaturase enzymes required for their production.
α-Linolenic acid is not used by the body in its original form. It is broken down by the body into the required long-chain polyunsaturated fatty acids eicosapentaenoic acid (EPA, 20:5) and docosahexaenoic acid (DHA, 22:6). EPA and DHA can also be consumed from a direct source by consuming fish or fish oil.
Linoleic acid is not used by the body in its original form either. It is broken down by the body into the required long-chain polyunsaturated fatty acids gamma-linolenic acid (GLA, 18:3), dihomo-gamma-linolenic acid (DGLA, 20:3) and arachidonic acid (AA, 20:4).
Omega-9 fatty acids are not essential in humans, because humans generally possess all the enzymes required for their synthesis.
Amino acids 
- Essential amino acids necessary for preterm children but not healthy individuals:
- Vitamin A (retinol)
- Vitamin Bp (choline)
- Vitamin B1 (thiamin)
- Vitamin B2 (riboflavin, vitamin G)
- Vitamin B3 (niacin, vitamin P, vitamin PP)
- Vitamin B5 (pantothenic acid)
- Vitamin B6 (pyridoxine, pyridoxamine, or pyridoxal)
- Vitamin B7 (biotin, vitamin H)
- Vitamin B9 (folic acid, folate, vitamin M)
- Vitamin B12 (cobalamin)
- Vitamin C (ascorbic acid)
- Vitamin D (ergocalciferol, or cholecalciferol)
- Vitamin E (tocopherol)
- Vitamin K (naphthoquinoids)
Dietary minerals 
- Calcium (Ca)
- Chloride (Cl−)
- Chromium (Cr)
- Cobalt (Co) (as part of Vitamin B12)
- Copper (Cu)
- Iodine (I)
- Iron (Fe)
- Magnesium (Mg)
- Manganese (Mn)
- Molybdenum (Mo)
- Phosphorus (P)
- Potassium (K)
- Selenium (Se)
- Sodium (Na)
- Zinc (Zn)
The required quantity varies widely between nutrients. At one extreme, a 70 kg human contains 1.0 kg of calcium, but only 3 mg of cobalt.
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, however, has a specific protein, complex or dietary reference intake been established.
- "National Academy of Sciences. Institute of Medicine. Food and Nutrition Board. Dietary Guidance: DRI Tables". US Department of Agriculture, National Agricultural Library and National Academy of Sciences, Institute of Medicine, Food and Nutrition Board. October 2009.
- Pauling, L. (1986). How to Live Longer and Feel Better. New York NY 10019: Avon Books Inc. ISBN 0-380-70289-4. Page 24.
- J D Kopple and M E Swendseid (May 1975). "Evidence that histidine is an essential amino acid in normal and chronically uremic man.". J Clin Invest. 55 (5): 881–891. doi:10.1172/JCI108016. PMC 301830. PMID 1123426.
- Westman, EC (2002). "Is dietary carbohydrate essential for human nutrition?". The American journal of clinical nutrition 75 (5): 951–3; author reply 953–4. PMID 11976176.
- "National Academy of Sciences. Institute of Medicine. Food and Nutrition Board. Dietary Reference Intakes: Elements". US Department of Agriculture, National Agricultural Library and National Academy of Sciences, Institute of Medicine, Food and Nutrition Board. October 2009.
- R. Bruce Martin "Metal Ion Toxicity" in Encyclopedia of Inorganic Chemistry, Robert H. Crabtree (Ed), John Wiley & Sons, 2006. doi:10.1002/0470862106.ia136
Further reading 
- Hausman, P, 1987, The Right Dose. Rodale Press, Emaus, Pennsylvania. ISBN 0-87857-678-9