Dark winters and vitamin D

The long dark winters in Canada have been blamed for a shortage of vitamin D and consequently a higher risk of diseases such as osteoporosis or cancer.

A report in Canada's National Post​ newspaper cites a study from the University of Calgary which showed that 97 per cent of the healthy people interviewed by researchers had insufficient levels of the vitamin sometime during the year. The researchers suggested that all Canadians should increase their vitamin D intake through supplementation as the vitamin helps the body absorb calcium and therefore strengthen bones - vital to protect against osteoporosis and other bone-thinning maladies.

The body synthesises dietary vitamin D in the skin through exposure to the sun's ultraviolet rays, but food sources of the vitamin are few and far between. The best source of vitamin D is the sun itself, but the long dark winters in Canada mean that for at least six months of the year, even this source is insufficient to meet the body's needs.

The research, which was published in the latest edition of the Canadian Medical Association Journal, focused on 188 healthy individuals aged 27 to 89. The team from the University of Calgary led by Dr David Hanley followed the participants over the course of a year, testing vitamin D levels every three months. During the summer their levels were fine, but the winter months between October and March led to a sharp decline in their vitamin D levels.

Dr Hanley told the paper that since Calgary's elevation was higher than other major cities in Canada, its citizens would usually expect to have greater exposure to the sun's UV rays. So if vitamin D levels there are still at dangerously low levels, then those in the rest of Canada are likely to be worse still.

Just 15 to 30 minutes of exposure to the sun for the face, hands and forearms should be enough to ensure sufficiently high levels of vitamin D, although Dr Hanley added that a good diet and additional supplementation would do no harm.