Adults living in Canada should consider increasing their vitamin D intake during the fall and winter, the Canadian Cancer Society says in new supplement recommendations released Friday aimed at reducing the risk for colorectal, breast and prostate cancers.

The recommendation comes on the heels of the most rigorous study to date that underscores mounting evidence on how people with lower levels of vitamin D in the blood have a greater risk of developing cancer, and how supplements seem to reduce cancer risk.

In consultation with a health-care provider, the society recommends that:

• Adults living in Canada should consider taking 1,000 international units (IU) of vitamin D supplements a day in the fall and winter, when sun exposure in the country is not high enough to produce adequate vitamin D naturally.
• Adults at higher risk of having vitamin D deficiency should consider taking the 1,000 IU supplement year round.

Factors that increase the risk for vitamin D deficiency include:

• Getting older, since the skin is less capable of producing vitamin D from exposure to the sun with age. No precise age cutoff has been determined.
• Darker skin, since the darker pigment acts as a natural sunscreen that makes it harder to produce the vitamin. 
• Not going outside often.
• Wearing clothing that covers most skin.

"We're recommending 1,000 IUs daily because the current evidence suggests this amount will help reduce cancer risk with the least potential for harm," said Heather Logan, the society's director of cancer control policy.

As more evidence on vitamin D becomes available, the society plans to update its recommendations.

There has not yet been enough research to say how much vitamin D is safe or beneficial for children, the group said. 

Also Friday, a study published in the American Journal of Clinical Nutrition showed a reduction in cancer rates among postmenopausal women who took vitamin D combined with calcium compared to a placebo.

The study looked at almost 1,200 postmenopausal women from rural eastern Nebraska.

Cancer risk halved

One group was given 1,400 to 1,500 mg of supplemental calcium plus 1,100 IU of vitamin D3, also called cholecalciferol, the calcium supplement alone, or a placebo.

In terms of real numbers, there were about 37 cancers in just over 1,000 women. When calcium and vitamin D were given, the risk was reduced by about half, to about 18 cancers per 1,000 women, compared to a placebo, said Dr. Pam Goodwin, an oncologist with Mount Sinai Hospital in Toronto.

"This is a safe, inexpensive way of preventing cancer," said Joan Lappe, a professor at the Creighton University School of Medicine in Omaha, who headed the study. "And you know, it just seems like it's worth trying."

Many cells throughout the body seem to use vitamin D to produce a signalling molecule. The signals help cells recognize what kinds of cells they should become or when they should stop proliferating, which may explain the vitamin's anti-cancer properties, said Reinhold Vieth, a professor of nutritional sciences at the University of Toronto and an expert on the vitamin.

Balancing sun risks

"As Canadians, we get virtually no vitamin D from sun exposure in the winter," said Goodwin. "Even if we're out in the sun, the rays of the sun are too weak for us to efficiently make vitamin D."

The cancer society continues to recommend that people avoid the sun when ultraviolet rays are strongest.

"Our sun-sense messaging is not changing," said Heather Chappell, the group's senior manager of cancer control policy. "When the UV index is three or more, continue to protect yourself from the sun."

The society is also calling for a longer, larger-scale clinical trial that is more representative of Canada's diverse population to help determine what amount of vitamin D maximizes health benefits and identify any long-term risks of taking large doses.

Taking more than 2,000 IU a day of vitamin D is probably not safe because it may affect liver function, Goodwin said.

Some companies have fortified their products with vitamin D, but Health Canada notes that fatty fish and egg yolks are the only natural sources.

Isabelle Neiderer, director of nutrition for the Dairy Farmers of Canada, said her group met with Health Canada officials earlier this year to discuss the possibility of adding vitamin D fortification to cheese and yogurt, as is done in the U.S.