'''n?3 fatty acids (popularly referred to as ??3 fatty acids or omega-3 fatty acids') are a family of unsaturated fatty acids which have in common a carbon?carbon double bond in the n?3 position; that is, the third bond from the end of the fatty acid.

Important nutritionally essential n?3 fatty acids are: ?-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). The human body cannot synthesize n?3 fatty acids de novo, but it can form 20- and 22-carbon unsaturated n?3 fatty acids from the eighteen-carbon n?3 fatty acid, ?-linolenic acid. These conversions occur competitively with n?6 fatty acids, which are essential closely related chemical analogues that are derived from linoleic acid. Both the n?3 ?-linolenic acid and n?6 linoleic acid are essential nutrients which must be obtained from food. Synthesis of the longer n?3 fatty acids from linolenic acid within the body is competitively slowed by the n?6 analogues. Thus accumulation of long-chain n?3 fatty acids in tissues is more effective when they are obtained directly from food or when competing amounts of n?6 analogs do not greatly exceed the amounts of n''?3.

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.

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."

Page 24. A similar statement can be made about vitamin C and some other vitamins.