But even though Ms. Logan says the Canadian Cancer Society agrees that all the science on vitamin D may not be in yet, evidence to date strongly suggests that not acting on the implications of the research is risky. Cancers affected include such big killers as breast, prostate and colon, which will claim more than 10,000 Canadians this year.
"You don't need to wait for every scientific question to be answered before you take action," Ms. Logan says. "Where there is evidence of harm, even in the face of scientific uncertainty, you should so something about it."
Martin Mittelstaedt is The Globe and Mail's environment reporter.
JUST D FACTS
Vitamin D is measured by levels in blood. Many Canadians have 40 nanomoles/litre or less, particularly in winter. Many researchers believe levels need to be at least twice that high to reduce chronic disease risk.
Vitamin D is produced when exposed skin has a photochemical reaction to ultraviolet light rays from the sun. Nearly all the vitamin D circulating in our bodies is made this way, with a typical white-skinned person in a bathing suit under a noonday summer sun in Canada producing about 10,000 international units in 15 to 20 minutes. Non-whites need about five times longer to make the same amount, because the melanin in their skin acts as a sunscreen against UVB rays. During the fall and winter, sunlight at Canadian latitudes is too weak to cause any vitamin D production.
Vitamin D synthesis in skin occurs only when the UV index is three or higher, roughly the period around noon from March to October in southern parts of the country. A rule of thumb is that if your shadow is longer than you are, the sunlight is not intense enough.
Some of the very few foods that contain vitamin D are: cod liver oil (1,300 IU per tablespoon); wild salmon (1,000 IU per serving); farmed salmon (250 IU); sardines (600 IU); fortified milk or orange juice (100 IU); egg yolk (25 IU); fresh shiitake mushrooms and some organ meats (traces in both). Most multivitamins contain 400 IU. Over-the-counter pills and drops contain up to 1,000 IU.
Health Canada's daily intake recommendations, based primarily on a 1997 study, are: newborns to 12 months, 400 IU; age 1 to 50, 200 IU; 51 to 70, 400 IU; over 70, 600 IU; with an overall upper limit of 2,000 IU.
Many vitamin D advocates say Health Canada is too conservative. The Canadian Cancer Society, for example, recommends that non-white adults take 1,000 IU daily year-round and whites take that amount in fall and winter. The Canadian Pediatric Society recommends 2,000 IU daily for pregnant and breastfeeding women.
Toxicity has occurred after long-term exposure to massive amounts, ranging from 50,000 IU to 150,000 IU daily. Effects such as bone demineralization may occur with chronic daily doses exceeding 10,000 IU. No illnesses have been reported for doses under 3,800 IU daily.
A U.S. study in 2007 found that overall risk of cancer in women was cut by 60 per cent when they were given 1,100 IU of vitamin D per day, plus a calcium supplement.
Another study estimated the dose to cut colon-cancer risk in half: 1,000 IU daily. The amount estimated to cut breast-cancer risk in half: 4,000 IU daily. Researchers say women could stay within Health Canada guidelines and still reach 4,000 IU daily by getting 2,000 IU from diet and supplements and the rest from modest sun exposure.
There is some evidence that girls can cut their future risk of breast cancer by taking high levels of vitamin D during their teens.
U.S. researchers estimate that vitamin D insufficiency causes up to 60,000 premature cancer deaths a year in the country, or nearly 10 per cent of total mortality from the disease. If the same percentage applies to Canada, low vitamin D status leads to about 7,000 premature cancer deaths here annually.
While there is a risk of skin cancer from overexposure to ultraviolet light, researchers say, the benefits of modest sun exposure in preventing serious, hard-to-treat cancers outweighs that risk. Furthermore, they say, skin cancer is relatively easy to treat.
A 2001 Finnish study found that children given 2,000 IU daily cut their risk of getting juvenile diabetes by 80 per cent.
The strong correlation between latitude and the incidence of multiple sclerosis has led researchers to suspect the trend is related to vitamin D status. In the U.S., for example, MS rates are four times higher in northern states, along the Canadian border, than in the southern parts of the country. Similarly, Australian research shows the incidence of MS increases the farther people live from the equator. The highest incidence rates in the world are found in Northern Europe and Canada.