Nutrition research never fails to amaze and delight me. Over the many years (and believe me it's been a long time!) that I've worked in the field of nutrition and dietetics, researchers are constantly coming up with new and exciting information about nutrients and foods that are generally regarded as ‘old hat’.
The idea that we know everything about nutrients and food has been proved wrong over and over again. Vitamin D is such a nutrient. Until quite recently it was generally regarded as mainly important for bone health and growth and the recommended daily allowance (RDA) was specified as 5 microgram per day (or 200 IU/day). It seemed that we knew all there is to know about vitamin D.
But one can never become complacent in this field. Lately researchers have been concentrating their efforts on this vitamin and have come up with a number of startling new findings.
Are we getting enough vitamin D?
Part of the complacency associated with vitamin D is due to the fact that humans are able to produce their own vitamin D when they expose their skins to sunlight. However, there are many factors that will influence how efficiently we produce vitamin D, for example:
- Seasonal variations – people are more likely to expose their skin to the sun in summer than in winter, when we're all muffled up against the cold.
- Populations that are at risk because of less regular skin exposure to sunlight are young children, the aged, anyone who doesn't spend some time in the sun, populations with dark skins, and individuals who wear thick protective clothing for climatic or religious reasons.
- In a European study, up to 36% of elderly men and 47% of elderly women had vitamin D deficiencies (Thurnham, 2008).
- Other studies showed that up to 50% of the American population may be at risk of a vitamin D deficiency (Solomons, 2008).
At the US National Institutes of Health (NIH) conference on vitamin D and health in the 21st Century, held in September 2007 in Maryland, USA, researchers posed the following important question that still has to be answered: “Is there a level of sunlight exposure that is sufficient to maintain adequate vitamin D levels, but does not increase the risk of non-melanoma or melanoma skin cancer?”(Solomons, 2008)
This is indeed an important question that requires a well-researched answer. On the one hand, we have nutritionists advising us to expose our skin to sunlight to produce enough vitamin D, while on the other hand dermatologists are warning us not to expose ourselves to sunlight to avoid developing skin cancer. A conundrum indeed!
What does vitamin D do?
Scientists have known for a long time that there are so-called ‘vitamin D receptors’ in bone cells, the intestine, the kidney and the parathyroid gland. Now, however, new sites in practically every type of body tissue have been identified, for example in adipose or fat tissue, as well as in the brain, breasts, heart, liver, lungs, immune system, reproductive organs, skin and thyroid, to name but a few. (Solomons, 2008)
It would, therefore, seem evident that vitamin D has a role to play in many more body processes than was once believed.
Dietary sources of vitamin D
Dairy products (milk, yoghurt, cottage cheese and other cheeses), eggs and fatty fish contain natural vitamin D, but only in small amounts. A recent move to add vitamin D to certain foods such as margarine also contributes to our intake of this vitamin.
Delegates at the NIH Conference pointed out that many foods have as yet not been analysed for their vitamin D content, which makes it difficult to pinpoint exactly how much vitamin D we obtain from our food. (Solomons, 2008)
Vitamin D and heart health
Evidence is accumulating that a vitamin D deficiency may have a negative effect on heart health. Because the above-mentioned vitamin D receptors have been identified in the muscles and the lining of the heart, researchers are now investigating how low vitamin D levels in the blood may impact on heart disease.
One study reported that older individuals had a 60% higher risk of developing cardiovascular disease when their vitamin D blood levels were lower than 37.5 nmol/L. The subjects who had blood levels lower than 25 nmol/L were 80% more likely to develop heart disease (Thurnham, 2008).
This research suggests that vitamin D status may play an important role in the development of heart disease and that it would be desirable to maintain blood levels above the 37.5 nmol/l level (Thurnham, 2008).
Vitamin D and cancer
As early as 1981 it was found that vitamin D inhibited cancer cell growth in laboratory studies. Other studies showed that the incidence of colon, breast and prostate cancer are higher in those areas of the world where exposure to sunlight is low and vitamin D production by the body is hampered.
It was concluded that the risk of colon cancer was three times lower in people with vitamin D blood levels of 50 nmol/L or higher. Similar findings apply to women exposed to the risk of breast cancer. (Thurnham, 2008)
Should we use vitamin D supplements?
The question whether we should use vitamin D supplements has also not been answered. It would seem that the type of vitamin D (vitamin D2) used in standard supplements, may not be as effective as naturally occurring vitamin D (vitamin D3). However, people who never expose their skin to the sun may need to use a vitamin D supplement to boost their intake.
So where do we stand with regards to vitamin D at the moment? We now know that it's much more important than was previously believed and that this vitamin may protect against diseases such as cancer and heart disease in addition to its traditional role of ensuring strong bones and teeth.
What we don’t know yet is how we're to achieve adequate blood levels of vitamin D, given the fact that excessive exposure of the skin to sunlight is associated with an increase in skin cancer. Many questions still need to be answered.
(Solomons NW (2008). NIH Conference on Vitamin D & Health in the 21st Century. Sight & Life Magazine, Issue 1/2008, pp 34-39; Thurnham D (2008). Multiple Micronutrients - the way forward. Sight & Life Magazine, Issue 1/2008, pp 43-49.)
Image: Vitamin D, Shutterstock