Vitamin D can be manufactured in the skin on exposure to sunlight, or obtained from the diet. It’s important as a protector against cancer, in ensuring a robust immune system, as a regulator of blood pressure and in enabling the proper secretion of insulin. But it is in its interaction with the essential mineral, calcium, and consequently vital role in the development and maintenance of strong, healthy bones, that vitamin D is best known.
Adequate blood calcium levels are essential for the proper functioning of the nervous system and a sufficient supply of vitamin D is vital for the maintenance of these levels. If blood calcium falls too low because of a deficiency of the vitamin, the body will respond by withdrawing calcium from the most immediately available alternative source, which is the large quantities stored in the bones.
In extreme cases, thankfully now rare, though by no means unheard of even in the developed world, the consequence of this process is the deficiency disease known as rickets, in which bones are stripped of minerals, become soft or brittle and in the load bearing limbs may be bowed. Since the demand for calcium is at its greatest in rapidly growing bones, the disease is tragically most common in infants and children.
Even fully formed adult bones, however, require a constant supply of calcium for continuing repair and regeneration, and deficiency may result in a progressive loss of bone density, or osteoporosis, which is a factor in many of the fractures commonly suffered by the older population. Vitamin D deficiency has also been identified as a cause of general muscular pain and weakness in both children and adults, with consequently increased incidence of the falls which so often lead to these fractures in already weakened bones in the elderly.
Traditionally it has been assumed that sufficient vitamin D for the avoidance of deficiency can be synthesised through the exposure of the skin to sunlight. But the problem is that a large proportion of the population in the affluent world lives in latitudes 40 or more degrees above or below the equator where there is very limited sunlight for six months of the year. They work indoors in towns and cities whose buildings in any case screen out a large proportion of the available sunlight before it reaches ground level. And when they do manage to get outdoors in the summer months, they are increasingly inclined to smother themselves in high factor sunblock.
In these circumstances the traditional view appears questionable at best, if not downright complacent. And the situation is even worse for the elderly, who may not only enjoy even less exposure to the sun than the young, but are less able to make use of that which they do obtain.
Unfortunately there are few good natural food sources of vitamin D, and the best source, oily fish such as sardines, mackerel and salmon is one which many people, and perhaps children in particular, often find unpalatable. The situation has improved somewhat with the fortification of milk, orange juice, bread and some cereal products, but it may still be difficult for individuals to ensure they obtain a sufficient intake of the vitamin, particularly because the amounts added to different foods and drinks are very variable.
No Recommended Dietary Allowance (RDA) has been established for vitamin D because the requirement will vary enormously depending on the amount of exposure to sunlight enjoyed by the individual. But the Food and Nutrition Board has suggested 200 IU (5mcg) for infants, children, and adults up to 50, rising to 400 IU for the 50-70s, and 600 IU for the over 70s, as an “Adequate Intake” of the vitamin from food on the assumption that none is being obtained from sunlight.
These figures, however, are generally regarded as conservative, and even for those individuals fortunate enough to enjoy regular exposure to good quality sunlight, a strong body of orthodox opinion recommends the intake of 400 IU (10 mcg) of vitamin D as part of a multi-vitamin and multi-mineral supplement. Higher doses still, however, are recommended for the over 65s, those with less than optimal liver or digestive health, those living in less advantageous climates and those who spend the majority of their time in indoor occupations. The recommended supplementary dose is a further 400 IU, for a total of 800 IU.
But some specialised nutritional practitioners and therapists, of course, are inclined to recommend even higher doses for optimum health. Although vitamin D is fat soluble and therefore has the potential to accumulate in the liver, this appears extremely unlikely to cause problems in practice. The upper safe limit suggested by the Food and Nutriton Board is 2,000 IU (50 mcg) but recent research suggests that this is a very cautious figure and that intakes of up to 10,000 units a day should present no risk to generally healthy individuals.
For maximum effect, however, vitamin D supplementation should be always be combined with adequate dietary calcium – supplemented if necessary to achieve an intake of 1,000 – 1,200 mg per day of this essential mineral.
