Essential nutrients are defined in nutrition as those required by the body, but which cannot be synthesised endogenously. There is therefore a requirement to obtain such nutrients from food. The essential nutrients include the vitamins and minerals, as well as the eight essential amino acids and the two essential fatty acids. Without adequate amounts of any of these essential nutrients in the diet a deficiency disease will develop, followed by death. As well as essential nutrients, there are a number of other compounds that are characterised as conditionally essential. Under normal circumstances these nutrients can be synthesised endogenously. However, under some physiological conditions a dietary source is required, usually because the cellular requirement increases beyond the ability to synthesise the required quantity. A number of conditionally essential amino acids are known, although the exact definition is controversial and open to interpretation.
It is known that the essential fatty acid α-linolenic acid (ALA, C18:3 (n-3)) is metabolised in humans to the longer more unsaturated fatty acids eicosapentanoic acid (EPA, C20:5 (n-3)) and docosahexanoic acid (DHA, C22:6 (n-3)). Both EPA and DHA are required for the formation of important cellular autocrines (eicosanoids and docosanoids) that play an important role in cellular regulation. It is thought that ALA itself does not have a function other than as a precursor for EPA and DHA. Under ideal circumstances, ALA should provide adequate amounts of EPA and DHA through endogenous conversion. However, most humans have a genetic deficiency of the delta 5- desaturase enzyme that is the rate limiting step in the omega 3 pathway. Therefore dietary sources of EPA and DHA are required by most individuals. This suggests that both EPA and DHA should be reclassified as conditionally essential nutrients in human nutrition.