Diet alone unlikely to supply enough vitamin D for winter, some experts say
Toronto — Confused about how to get enough vitamin D in your diet or whether you should pop a daily supplement?
Don’t worry, you’re not alone.
A report released this week by the Institute of Medicine (IOM) that was meant to clarify just how much Canadians and Americans need of the so-called sunshine vitamin seems to have clouded the issue even more.
Following a two-year review of vitamin D research, an expert panel tripled the recommended daily dietary intake to 600 international units for people up to age 70, and hiked the optimal amount for those 70-plus to 800 IUs.
But the committee said its recommendations, which also set calcium intake levels, are aimed only at maintaining good bone health. The IOM said it could not weigh in on vitamin D’s possible role in reducing the risk of some cancers and other diseases because it believes the scientific evidence isn’t conclusive.
What has many confused is the panel’s assertion that people can get their daily quotient of vitamin D through diet alone and that most have adequate levels of the nutrient circulating in their blood.
“I don’t know what diet they’re talking about,” said an exasperated Susan Whiting, a professor of nutrition at the University of Saskatchewan. “Canadians don’t have access to many sources in the diet, and the message that everyone’s fine the way they are is not true.”
Granted, there’s more than enough sunlight in the spring-summer half of the year for most of Canada’s population to produce vitamin D through skin exposure.
But during the roughly six months of fall and winter, the sun’s rays are too weak in northern climes to make the nutrient naturally — even if Canadians weren’t bundled up head to toe, Whiting explained from Saskatoon. “For us in Canada, October to April is zero, unless you’ve taken a vacation.”
To reach 600 IUs from the diet, a person would have to consume two glasses of milk — one of the few dairy products fortified with vitamin D — an egg or two, a half-can of salmon or a can of sardines, and a large serving of meat, Whiting calculated.
Not only would such daily meal choices be unpalatable for many people, but “I don’t think very many nutritionists would be promoting that kind of diet,” she said, noting that Canada’s Food Guide advises limiting weekly servings of fish, eggs and meat.
“I really think that the immediate message is that we’ve promoted supplements before and I think that it really is a way for people to get through the winter.”
The Canadian Cancer Society, for instance, is sticking with its advice that adults consider taking a 1,000-IU vitamin D supplement during the fall-winter months, and that those with darker skin, the elderly and people who get little time outdoors consider taking that amount year-round.
The society’s advice represents a bit of a balancing act: it wants to encourage people to get optimal levels of vitamin D for overall health while limiting sun exposure to prevent melanoma and other skin cancers.
At the same time, the organization is aware of a growing body of scientific evidence that suggests the nutrient could play a pivotal role in reducing the risk of some malignancies, among them colorectal and breast cancer and certain types of leukemia.
“The evidence is particularly strong for colorectal cancer,” said John White, a professor of physiology and medicine at McGill University, whose lab is involved in vitamin D research.
For instance, epidemiological studies that compare populations have shown that the farther away from the equator people live, the higher their rates of colon cancer, with some of the highest levels in the Far North.
“The epidemiological data does not stand alone,” White said from Montreal. “It is bolstered by a substantial body of basic research at the cell and animal level that is largely supportive of vitamin D being a cancer chemo-preventive agent.”
Yet in its report, the IOM panel said hundreds of studies it reviewed on the possible beneficial effects of vitamin D beyond bone health — including cancer-prevention — “provided often mixed and inconclusive results and could not be considered reliable.”
While he believes the panel was too conservative in its assessment, White said he was heartened to see the IOM raise the amount of vitamin D it considers safe to 4,000 IUs, double the upper tolerable limit set in its previous report in 1997.
“That’s exactly what I do for six months of the year is I take 4,000 international units a day. I do it over the winter,” dropping to 1,000 to 2,000 IUs in the summer, he said.
“I do it also for personal reasons. There’s colon cancer in my family. And I am not going to wait around for how many years until the level of rigour (in research) is obtained to satisfy the IOM to make my own personal decision.”
Dr. Gerry Schwalfenberg said vitamin D is an immune system booster and numerous studies worldwide suggest it lowers the risk of infections like colds and flu and may prevent the onset of autoimmune diseases such as multiple sclerosis and Type 1 diabetes.
The Edmonton family physician, who supplements his diet daily with 4,000 IUs of vitamin D year-round, has also had success using vitamin D supplements to treat chronic back pain in patients.
Schwalfenberg, an assistant professor at the University of Alberta, advises people to take at least 1,000 IUs of vitamin D a day, winter and summer.
“I usually use 2,000 in people who have darker skin, who are obese, who don’t spend much time out in the sun,” he said. “Some people need more than that.”
Whiting, who is also a member of the Canadian Nutrition Society, agreed that it’s reasonable for people to augment their diet with 1,000 to 2,000 units of vitamin D from a bottle — especially during the cold, often sunless days of fall and winter.
“But if you have any kind of medical condition and you are talking to your physician, you should think about whether it should be more than that,” she said.