Weak winter sun jeopardizes health
Vitamin D critical to human biology
Can we get enough without UV rays?
Feb. 24, 2006. 01:00 AM
BARBARA TURNBULL
LIFE WRITER
The long, dark days of winter may be jeopardizing our health more than we know, and scientists are only beginning to understand why.
That's because the less sunlight we get, the less vitamin D we make in our bodies. A deficiency of the so-called sunshine vitamin has been linked to an increased risk of osteoporosis, cancers of the prostate, colon, breast and ovaries, multiple sclerosis and diabetes ? all in scientific papers published in the past two years.
Researchers now believe everyone who lives north of Florida is deprived of vitamin D in winter. Some believe we're even in danger in the summer, since skin-cancer campaigns have been so effective in getting us to cover up or wear sunblock.
And in Toronto, it wouldn't be far-fetched to assume that in winter every single person is vitamin D deficient, says Reinhold Vieth, a clinical biochemist who has been studying the nutrient ? actually a hormone ? for the past 30 years.
Dark-skinned people are particularly at risk because they synthesize the hormone at a much lower rate than light-skinned people.
"A dark skinned person is not designed to live in Canada," he notes. "They have too little vitamin D, so their health is not optimal as a result. The easy solution is to take more vitamin D orally."
Vieth says the absolute minimum acceptable level of vitamin D is 75 nanomoles per litre of blood. Other researchers recommend 100 nmol/L or more. Most light-skinned Toronto residents hover around 40 or 50 in winter. Dark-skinned people would have even less, he says, some with undetectable levels.
"What happens with dark skin is it functions as a sun protection factor and it reduces the amount of ultraviolet light that penetrates down into the skin. So it takes them longer to make the vitamin D," Vieth explains. Since some dark-skinned people avoid sun exposure, their deficiency is even greater.
A study released yesterday by the Harvard School of Public Health and the University of California, Los Angeles found adequate levels of vitamin D helped minorities and foreign-born residents fight tuberculosis, for which they have significantly higher rates than the average American. Researchers identified a vitamin-D dependent pathway used to stave off the infection.
And young people also need a vitamin D boost, says Susan Sullivan, a University of Maine researcher who studied vitamin D levels in girls 10 to 13. She found that in peak sun, 17 per cent fell below standard levels.
"You would expect teenaged girls to be less deficient than older people, because these girls play in the summer and they get plenty of sun," she says. "So if we're seeing it in kids, it's a problem across all age groups."
Natural solar radiation is the most important source of vitamin D, which is made in the skin when it is exposed to direct sunlight. The liver and kidney activate the hormone, which enters the bloodstream and travels to the intestine to regulate calcium absorption, which is important for bone mineralization.
The vitamin works differently in other tissues, including the colon, prostate and breast, which activate the hormone to regulate cellular growth.
The body requires vitamin D to absorb dietary calcium, which is deposited in the bones, reducing incidences of osteoporosis and bone fractures. But it's now thought to do more; risks of breast, colon and prostate cancer, multiple sclerosis and diabetes are cut with adequate levels. It may even help combat Seasonal Affective Disorder and chronic musculoskeletal pain.
"The last 10 years there's been a piling up of evidence ... that higher levels of vitamin D are protective against disease," says Vieth, director of the bone and mineral lab at Mount Sinai Hospital and a nutrition professor at the University of Toronto.
It's not all clear-cut, though. Last week researchers at the New Jersey Medical School suggested vitamin D and calcium supplements offered only limited protection against broken bones in older women. But many scientists, Boston University's Michael Holick among them, criticized the results.
The professor of medicine, physiology and biophysics, widely considered one of the world's top authorities on vitamin D, says the women in that study were deficient in vitamin D to begin with, and then were given sub-optimal doses. What's more significant, he says, is that the women who were most deficient in vitamin D were 150 per cent more likely to develop colon cancer than the others.
"That's consistent with all the observations that have been made in terms of the effective latitude ? increased risk of colon cancer and many other cancers ? as well as the observation that we recently had with our mouse model, showing that simple vitamin D sufficiency decreases colon tumour growth by more than 50 per cent," Holick says about the findings his research group published last year in the Journal of Nutrition. After sun, the only other natural sources of vitamin D are salmon, sardines, liver and cod liver oil. Milk, some juices and most breakfast cereals are fortified with it and it's available as an over-the-counter supplement. But it's difficult to get an adequate amount from food alone.
The question is how much do we need to get that 75 nmol/L in our blood? Right now, most doctors rely on numbers from the Food and Nutrition Board, a joint Canada-U.S. non-governmental group that determines daily vitamin recommendations.
The board's guidelines call for 200 IU daily from infancy until age 50, 400 IU daily for adults over 50 and 600 IU for those over 70, but Vieth argues they are woefully inadequate.
"Dietary guidelines were designed in the 1960s by a panel of white pediatricians to prevent rickets in white babies," Vieth says. "Those dietary guidelines, at 200 IU of vitamin D per day for the newborn baby, apply the same for a 200-kilogram black man," he notes.
There is still no geriatric multivitamin on the market that contains 600 IU of vitamin D, not to mention that the Osteoporosis Society of Canada advises all adults over the age of 50 to take 800 IUs a day, he says.
Vieth predicts that 10 years from now the daily recommendations will be increased to 4,000 IU for nursing mothers, 1,000 IU for all other adults and 400 IU for children.
Late last year a group of medical professionals brought together by the pharmaceutical giant Merck & Co., Inc. said older people need more vitamin D to help strengthen bones than the current guidelines recommend. The group recommended that everyone over 50 should take 800 IU to 1000 IU daily. The problem is that, according to Holick, our ability to activate the vitamin D we absorb in the skin decreases as we age.
The Merck group only looked at people over 50, but recommendations will likely change even for children, says Dr. Kimberly Templeton, associate professor of orthopedic surgery at the University of Kansas Medical Centre and a spokesperson for the group, which met in New York last fall to explore the current state of knowledge on the nutrient.
"The best way for preventing osteoporosis is to start with the kids and help them develop their bone mass. We probably aren't doing a good job of that yet, but that's something to take a look at further on down the road."
Vieth will be among a group of North American experts on cancer, dermatology, nutrition, autoimmune diseases and bone health who will meet in Toronto at the beginning of March to evaluate current research at a workshop organized by the Canadian Cancer Society.
The goals are to develop consistent public health messages, open communication between experts on the issue and identify research gaps and opportunities.
"The potential benefit of vitamin D is on the magnitude of the potential benefit for telling a group of smokers to stop smoking," Vieth says. "It reduces mortality by a similar magnitude, whether you are looking at osteoporosis or various cancers."
He admits the evidence is largely based on epidemiologic studies ? surveys of population groups ? rather than randomized, double-blind studies that use a control group.
And although the work is promising, getting the scientific establishment to embrace it and build on that body of knowledge could take a long time, says the researcher.
Full-scale clinical trials have not been funded as yet, despite the impact it could have on improving health.
But that might be changing, according to Sullivan at the University of Maine, who cited an "explosion" on the topic of vitamin D and health in the last five years.
The fact that it plays a role in preventing several chronic diseases that don't relate to bone health, coupled with the low levels in our blood, is prompting a closer look.
For Vieth, there's ample evidence to change the guidelines, but he realizes it's difficult for the public to know what to do with conflicting information.
"I feel very strongly that the message I'm telling you is absolutely real and should be implemented widely. This particular nutrient, you'll see, will make a big difference." |
