Studies published over the past six months have linked vitamin D deficiency to a wide range of problems, including obesity, Alzheimer’s disease, cancer, infections, multiple sclerosis, diabetes, inflammation and heart trouble.
This lack of Vitamin D provides a glaring illustration of the impact that modern industrial lifestyles can have on health. The vitamin’s main natural source is a photochemical reaction in the skin: ultraviolet (UVB) radiation from sunlight converts 7-dehydrocholesterol, a plentiful bio-molecule, into a precursor of vitamin D (which then undergoes further reactions to produce the active vitamin).
Our ancestors were out in the daylight for long enough to make plenty of vitamin D. Those who lived in high latitudes, where the winter sunlight is too weak to trigger the photochemical synthesis, could make sufficient vitamin in their body to last for months. In addition, many northern peoples traditionally ate large amounts of fish – and oily fish are one of the few foods that are naturally rich in vitamin D.
Today, most people in the developed world live and work indoors; when they go out, they wear too many clothes to make much vitamin D in the sunshine. Some make an exception on holiday, sunning themselves for a couple of weeks, but that is not long enough to build up an adequate reserve of the vitamin.
Of course people have been bombarded in recent years with health messages about the dangers of over-exposure to the sun, which can cause skin cancer. In an attempt to resolve any public confusion, two UK charities, National Osteoporosis Society and the British Association of Dermatologists have teamed up “to set the record straight about safe sun and vitamin D”. They recommend short bursts of unprotected sun exposure on the face, arms and legs to minimise the chance of sunburn.
“In the UK, people should try to get outdoors for ten minutes once or twice a day without sunscreen, which blocks the manufacture of vitamin D,” the organisations say. “If you are at high risk of skin cancer, you should obtain your vitamin D from diet and supplements rather than by further increasing your skin cancer risk.”
For many people – even those who are not at unusual risk of skin cancer – getting enough regular sunshine may be impractical. And a very fishy diet may not appeal. For them, supplements are the only solution.
Over the past couple of years growing awareness of the dangers of vitamin D deficiency has prompted rapid growth in the market for supplements and testing. US sales of vitamin D supplements more than doubled in 2008.
Immunodiagnostic Systems, a UK-based company with a strong position in the market for vitamin D diagnostics, reported an increase of 80-90 per cent in the demand for tests in the US last year. “Vitamin D is a remarkable market at the moment and has been for the past couple of years,” says Roger Duggan, chief executive of Immunodiagnostic Systems.
But how much vitamin D do we really need? Recommended blood levels and dietary intakes of the vitamin vary around the world – and many countries have no official guidelines.
In the US, which has long taken vitamin deficiency more seriously than most of Europe, vitamin D has been added to milk since the 1930s and many other foods such as breakfast cereals are also fortified. However, levels of fortification were designed to prevent rickets and bone disease, and they may not be sufficient to boost vitamin D levels in people who have little exposure to sunlight.
The US Institute of Medicine last set official recommendations for vitamin D consumption in 1997. Its “dietary reference intakes” are 200 international units (IU) per day for children and adults up to the age of 50, 400 IU for 51-70 year olds and 600 IU for people over 70. The guidelines set a safe upper limit of 2,000 IU.
Under pressure from experts who say these recommendations are too low, the Institute of Medicine is reviewing the levels in light of up-to-date clinical evidence.
Although the medical literature is full of relatively small clinical studies showing a link between vitamin D deficiency and a myriad of health problems, what has been missing is a rigorous longitudinal trial, in which researchers monitor the health of participants taking vitamin D supplements and placebos. Now the US National Institutes of Health (NIH) is planning a $20m (£12m) study involving 20,000 volunteers who will take large doses of vitamin D (2,000 IU) and/or fish oil for five years.
The NIH wants a quarter of the participants in the trial to be African- Americans, because the melanin pigment in black skin reduces the efficiency of vitamin D biosynthesis in sunshine. African-Americans are more prone to vitamin D deficiency than their white counterparts – and some medical experts suspect that this may contribute to health disparities between races in the US.
For example, African-Americans have a greater frequency of diabetes, hypertension, and certain types of cancer. “We are excited about the potential of vitamin D to reduce this health gap, but it is important to get answers from clinical trials before recommending mega doses of this supplement,” says JoAnn Manson of Boston’s Brigham and Women’s Hospital, co-director of the study.
Dr Manson cautions against unrestrained enthusiasm. “We tend to forget the lessons of other nutrients: many had high hopes for vitamin E, vitamin C, beta carotene, folic acid, selenium, and other supplements as preventive tools for many diseases, but large-scale trials didn’t confirm the hoped-for benefits and even found some risks,” she warns.
A report released this summer by the International Osteoporosis Foundation shows that vitamin D deficiency occurs worldwide. “Although there is ongoing debate as to what constitutes the optimal level of vitamin D, the report shows that regardless of [the exact definition], vitamin D status is seriously inadequate in large proportions of the population across the globe,” says the foundation.
Some of the worst cases of deficiency occur in the Middle East and south Asia, particularly among women who live and work indoors and, for cultural and religious reasons, cover their skin in traditional clothing when they go out.
Children are especially vulnerable to vitamin D deficiency. According to a study by researchers at the Albert Einstein College of Medicine in New York, published in August, seven out of 10 American children have suboptimal levels of the vitamin. “The striking findings suggest that vitamin D deficiency could place millions of children at risk of high blood pressure and other risk factors for heart disease,” the researchers say.
“We know from earlier data that vitamin D levels have declined over the past 20 years,” says Michal Melamed, the study leader. “Kids have more sedentary lifestyles today and are not spending as much time outdoors. The widespread use of sunscreens, which block UVB rays, has only compounded the problem.”
Her advice to parents? “It would be good for them to turn off the TV and send their kids outside. Just 15-20 minutes a day should be enough.”