Why is Vitamin D special?
Sunlight (ultraviolet B) is used to convert a precursor into vitamin D in the skin and two further reactions are required to generate the active form of the vitamin. Although just 15 minutes of bright sunshine is necessary to make enough vitamin D for a day, there is evidently a major problem in obtaining enough to meet the demands of the body in some parts of the world. Regular exposure to sunshine is not easily attained in high latitudes in the winter and heavily pigmented skin makes the light-dependent reaction much less efficient. Fortunately, the vitamin can be stored so that the benefits of the summer sun can be eked out in winter to some extent. The surprising part of this story is that there are so few sources of vitamin D in food. The liver of oily fish is by far the richest; unfortified milk, eggs and meat contain only a tiny proportion of humans needs for the vitamin. In the absence of adequate sunlight or a regular intake of oily fish, artificial supplements are the only way of obtaining enough of the vitamin.
These peculiar circumstances make vitamin D insufficiency so widespread that half of the world’s population is affected using the criteria currently employed by health professionals. The precise intake required is a matter of debate but most experts now believe we need twice the current recommendation of 500 IU.
Startling statistics have emerged from America indicating a doubling of vitamin D insufficiency between 1994 and 2004 so that 90% of Black, Hispanic and Asian Americans and 75% of white Americans are affected. Contributing to the trend is reduced consumption of fortified milk and increased avoidance of bright sunshine. Vitamin D insufficiency in pregnant women may have serious consequences for the foetus.
One of the most startling outcomes of the human genome project was the recognition that vitamin D regulates about 800 genes and that most cells in the human body have receptors for the active form of the vitamin.
Earlier it was known that deficiency of vitamin D was the cause of the epidemic of rickets. This disease was characterised by bendy bones and bowed legs and blighted the physical development of city children in 19th century Europe. We know now that a variety of symptoms of insufficiency are evident at serum levels of the vitamin less than half of that associated with rickets. Vitamin D has important roles in the immune system, regulation of blood glucose, blood pressure, the cardiovascular system, the skeleton and skeletal muscle and in brain physiology. Vitamin D also has a role in preventing colon and breast cancer.