Phytochemicals are biologically active compounds, found in plants in small amounts. At present, most interest in phytochemicals is focused on carotenoids and polyphenolic flavonoids, which seem to contribute to protection against degenerative diseases, such as cancer and cardiovascular disease. Mechanistically, phytochemicals are thought to act in many ways, which include their activity as antioxidants, antibacterial/ viral agents, phytohormones and as inducers or inhibitors of a variety of key enzymes. Because of the large number of chemically different phytochemicals and the lack of a distinctive deficiency syndrome or inherent physiological role in many cases, allocating recommended dietary intakes to phytochemicals is problematic.

There is a substantial scientific evidence base to support important roles of omega-3 fatty acids in growth and development, in lowering the risk of many chronic human diseases and in contributing to the therapeutic regimen of those with advanced diseases or those requiring nutrition support. All the effects of omega-3 atty acids that have been described in studies are likely to be dose-dependent, but clear dose response data has not been identified in most cases. Eicosapentaenoic acid and docosahexaenoic acid are found in the greatest amounts in ‘fatty’ or ‘oily’ fish. However, in the UK, for example, less than 25 percent of adults eat oily fish on a regular basis.

The biggest changes in the nature of skin occurred during the course of human evolution in equatorial Africa, under regimes of high daytime temperatures and high ultraviolet radiation (UVR). Loss of most functional body hair was accompanied by the evolution of an epidermis with a permanent, protective, eumelanin pigmentation. The main reason for the evolution of dark pigmentation was to protect against folate deficiency caused by elevated demands for folate in cell division and DNA repair, and melanogenesis stimulated by UVR. Dispersal out of tropical Africa created new challenges for human physiology especially because of lower and more seasonal levels of UVR and ultraviolet B (UVB) outside of the tropics. Recent migrations and changes in lifestyle have brought many humans into different UVR regimes and exposed them to new disease risks, including skin cancer and vitamin D deficiency.