Vitamins: to “D” or not to “D”

by 21 Jul 2009

With recent skin cancer prevention cam paigns resulting in greater awareness and, therefore, more people sensibly limiting their sun exposure, there is some debate now turning to what benefits we may now be miss ing out on by not getting enough sun. By this people are generally referring to vitamin D, be cause studies continue to show that vitamin D deficiency is surprisingly a common prob lem in Australia.

Vitamin D is, in fact, more correctly a pre- hormone rather than a vitamin. Humans have two ways of getting vitamin D. One is by the ingestion of vitamin D-rich foods such as fatty fish and their oils (such as cod liver oil), eggs, liver, and vitamin D-fortified foods such as mar garine. This, however, accounts for only a small proportion of our vitamin D requirements.

More important is that which is produced in our skin itself in response to sun exposure. A derivative of cholesterol that is found in the outer layers of the skin is converted by ultra violet B rays from the sun (the same rays that also cause the majority of skin cancers) into vitamin D3, which eventually gets converted first by the liver, followed by the kidneys, into the active form of vitamin D required through out our bodies.

Two factors influence this production. One is how much sun exposure the skin receives – the result of not only how much skin is un covered and the amount of time spent in the sun, but also what the season is, the distance from the equator, and even how much cloud cover or air pollution there is on any given day.

The other important factor is how dark the skin is. This is because darker skinned peo ple require more sun exposure for the ultravi olet B rays to get in and do their job. It follows, then, that certain people are at higher risk of vitamin D deficiency: dark-skinned people (es pecially from Africa and Asia), those who cover their skin for cultural and religious reasons, breastfed babies of vitamin D-deficient moth ers (there have been a number of recent cases of infant rickets throughout Australia), the housebound or institutionalised, and the eld erly (more information on this group below).

There is a limit, however, because the more your skin is exposed doesn’t necessarily re sult in more vitamin D production. In fact after an adequate amount of skin exposure, any ex cess vitamin D3 produced is simply immedi ately degraded.

This means that once the required amount of vitamin D has been produced, any extra ex posure to ultraviolet B is simply contributing to increasing the risk of skin cancer. So it is, therefore, difficult to determine exactly how much sun exposure is required for a given in dividual simply because of the number of different factors to take into account.

Guidelines, however, suggest that fair- skinned people in the southern parts of Aus tralia generally require about 15 minutes of sun exposure outside of peak UV times three times a week in summer, and about two to three hours of exposure a week in winter. Those with dark skin, however, may require up to six times that amount!

But why exactly is it a problem? What does vitamin D actually do? The most widely known function of vitamin D is its role in bone miner alisation. Because it increases the intestinal absorption of calcium, as well as the transport of calcium into bone cells, deficiency can re sult in bone-softening diseases such as rick ets in children, and osteoporosis in adults. As sociated with this is its role in the function of the large muscle groups controlling the limbs, such that a deficiency leads not just to mus cle aches and weakness, but poorer balance and more falls.

So checking vitamin D levels, particularly as people age and in those at high risk, is like ly to become a more common part of people’s routine blood testing (just as it is for choles terol and blood sugar). It is, however, impor tant to realise that the use of sun protection measures (such as “slip, slop, slap”) during peak UV times does not impact on getting enough sun exposure in order to maintain ad equate vitamin D levels.

As such, consistent and deliberate sun ex posure without any form of sun protection is not recommended, even in those whose lev els are sub-optimal ... the problem being that too much sun may result in skin cancer, or a person may not get enough vitamin D anyway because of a lack of skin production.

Instead, an oral vitamin D3 supplement, in doses advised by a doctor based on the severity of the deficiency, are both cheap and effective, as well as carrying no risk of skin cancer at all.

By Dr Benson Riddle, head of department, Peak Health Medical Services.