Islanders can't get enough sunshine to make vitamin D in winter. (Kevin Yarr/CBC)

Increased testing of Prince Edward Islanders for vitamin D shows most of them need to be taking a supplement.

A few years ago, medical labs on the Island were testing just 35 people a year for levels of vitamin D . Recent studies have shown that during the winter, Canadians have difficulty producing enough of the vitamin, which is made by the skin on exposure to sunlight. That research has increased interest in the vitamin, and last year about 400 people were tested.

So many of the tests are coming back with low levels of vitamin D that Dr. Humaira Khanam, acting director of medical laboratories at Charlottetown's Queen Elizabeth Hospital, is recommending that most of the testing be stopped.

"Basically, people who are less than 50 years of age and have no symptoms of bone loss, they shouldn't be tested," she said. "They should just take their recommended doses of vitamin D, which is between 800 and 2,000 international unit for adults and 400 international units for children."

She said about 70 per cent of tests were coming back with low vitamin D levels. The vitamin plays an important role in preventing a number of health problems.

"It helps in decreasing the chances of cancers, like breast cancer or prostate cancer or colon cancer, which are very common," said Khanam.

"Lack of it also leads to different diseases like autoimmune diseases like rheumatoid arthritis."

The Canadian Cancer Society recommends adults increase their daily dose of vitamin D to 1,000 international units daily, especially during the fall and winter. The society says it may update its advice as new studies are released.