1.3 Vitamin D
The main role of vitamin D is to facilitate the uptake of calcium from food, through the lining of the small intestine into the blood. It also controls the deposition of calcium in the bones during growth and maintains adult bone structure. If vitamin D is deficient, with less calcium available, the skeleton fails to develop normally. The most obvious symptom is the bowing of the leg bones in children, producing the condition called rickets ( see Figure 2 ). Children with vitamin D deficiency grow more slowly and may become smaller adults which, in women, has serious consequences because the pelvis may end up so small that giving birth normally is impossible. Vitamin D deficiency in adults is called osteomalacia. Rickets and osteomalacia were relatively common in Europe during the 19th century, especially in urban slums.
Given that vitamin D is a fat-soluble vitamin, predict which foods are likely to contain it.
You would expect to find vitamin D in those foods that contain significant amounts of fat, such as dairy products and, by analogy with vitamin A, probably liver too.
If you look back at Table 1 you will find oily fish on the list and you may know that a generation or two ago, a regular dose of cod liver oil was given to children to prevent rickets. Levels of vitamin D in dairy products vary throughout the year but it is now added to margarine and many low-fat spreads, providing an all-year-round supply. Breakfast cereals, yoghurts and food for babies and infants are often also ‘fortified’ with vitamin D.
However, there is some debate as to whether vitamin D should actually be classified as a vitamin, since it does not fit completely with the usual definition, which is that vitamins are obtained from the diet. It is not essential to obtain vitamin D in the diet. It can be synthesised below the surface of the skin in the presence of ultraviolet (UV-B) light. However, in the UK, there is insufficient UV-B in sunlight between October and March for synthesis to occur, although most people probably make and store enough in the body during the summer months to last through the winter.
Which groups of people would be most at risk if their diet contained insufficient vitamin D?
You might have thought of: people who are housebound during the summer; people who live in areas of high air pollution; those who never expose their skin to sunlight or only do so when wearing high factor (UV-B blocking) sunscreens; and people with dark skin which prevents the UV light from penetrating far enough into the skin for vitamin D synthesis.
In fact, the beneficial effect of sunlight, in playing a part in vitamin D synthesis, has to be balanced with the detrimental effect, its role in causing skin cancers. It appears that an exposure to sunlight of about 30 minutes per day (avoiding the part of the day when sunlight is strongest) is an appropriate balance between the harmful and beneficial effects. Although rickets has largely been eliminated in the UK, due to the addition of vitamin D to food especially for infants and children, the condition has reappeared in Asian communities in the UK, especially in the more northern parts of the country.
List the factors, relating to both dietary intake and production of vitamin D, that may be the cause of vitamin D deficiency in Asian communities.
Some of the factors are:
darker skin colour, so that more exposure to sunlight is required to stimulate vitamin D production
less sunlight in northern parts of the UK than in the south, or than would be found in the Asian countries from where this population group originated
for cultural reasons, spending less time out of doors than white UK residents, (such habits are especially common among Asian women)
the wearing of clothes that cover more of the body surface than those of white UK residents, so exposing less skin to sunlight
a diet containing less dairy products than a typical UK diet; a strict vegetarian diet is particularly low in vitamin D.