Vitamins are essential organic compounds needed in small amounts for normal metabolic regulation, usually as cofactors to enzymes. They are characterised in their own group because they cannot be synthesised in the body and so must be obtained from the diet. Vitamin research is ongoing, and in this respect the roles of vitamins in human nutrition has changed over time as more research has been undertaken. This has lead to the inclusion of certain substances in the category of vitamin, despite not really belonging to the group. The most obvious vitamin that is not a vitamin is cholecalciferol (and ergocalciferol), otherwise known as vitamin D. As vitamin D can be synthesised in the skin through the action of ultraviolet light on cholesterol, it is not a vitamin. In fact vitamin D is a steroid hormone belonging to the same category as testosterone, oestrogen, progesterone and cholesterol. Adequate sunlight exposure provides all the vitamin D that is required for correct metabolic regulation.
Likewise, vitamin A is not really a vitamin as it can be synthesised endogenously from other compounds. Beta carotene and other carotenoids present in plants can be converted to vitamin A in humans and as such are referred to as provitamin A. The ability of other compounds to provide substrates for vitamin A synthesis in humans has resulted in the use of retinol equivalents (RE) in the labelling of foods, a measure that ensures that the provitamin A content of foods are included. Of course carotenoids have a wide range of activities in human tissues and may also be effective antioxidants. Another vitamin with a dietary precursor is niacin. Niacin is also sometimes referred to as vitamin B3, however, dietary sources of niacin might not be required if adequate tryptophan is supplied in the diet. This is because dietary tryptophan is converted to niacin through the kynurenine pathway, and about 60 mg of tryptophan can be converted to about 1 mg of niacin. Some foods are low in niacin, but are paradoxically good sources of the vitamin because they contain high amounts of tryptophan.
Controversy also surrounds the production of certain substances by the intestinal microflora. From a holistic point of view the bacteria inhabiting the gut are part of the human host and the vitamins they synthesise in some cases may provide adequate amounts of the vitamins for normal metabolic regulation. The total absence of vitamin B12 from the diets of vegans is a case in point. Only animal foods contain vitamin B12, but vegans do not consume these products. While the vitamin B12 status of many vegans is poor compared to meat eaters, they do not by default develop the anaemia associated with deficiency of the vitamin. This may be due to the synthesis of the vitamin by small intestinal gut bacteria, which is absorbed in the normal way through bonding to intrinsic factor in the gut. Likewise vitamin K can be synthesised in the gut by bacteria, and many provide adequate amounts of the vitamin in healthy individuals. The symbiotic relationship between bacteria and humans is pivotal to health and these organisms should really be considered to be part of the human host.