The double-blind clinical study, conducted over four years, tested healthy women over the age of 55 living in rural Nebraska. Critics of the study cautioned that a larger study would have yielded more reliable and conclusive results.

But Reinhold Vieth, a nutritional scientist at the University of Toronto, said the new study is the last piece of evidence for which many in the field have been waiting. Vieth said that many cells in the body use vitamin D to produce a signaling molecule that allows the cells to communicate with each other.

"Those signals do things like helping cells to differentiate to recognize what kind of cell they should be becoming or they can signal cells to stop proliferating and those are good things in terms of cancer, you want differentiation so they become good well-behaved cells and you don't want them to keep replicating all the time," he said.

Other researchers have begun studying how the sunshine vitamin affects other forms of cancer. Researchers at the University of California, San Diego, suggested in the March 2007 issue of the American Journal of Preventive Medicine that taking 2,000 IU of vitamin D daily along with 10 to 15 minutes in the sun and a healthy diet could reduce the incidence of colorectal cancer by two-thirds.

The same authors found that breast cancer rates were 50 per cent lower in people with high levels of vitamin D in their blood, and suggested that the average person could maintain those levels by taking 2,000 IU of vitamin D daily and spending 10 to 15 minutes in the sun.

Similarly, a December 2006 study in more than seven million people found that white members of the U.S. military who had high blood levels of vitamin D were 62 per cent less likely to develop multiple sclerosis than those with the lowest levels of the vitamin. Researchers noted the findings were still too preliminary to suggest that a lack of vitamin D could trigger the nerve disorder.

A study published in the Archives of Internal Medicine on May 28, 2007, suggested that women who consume higher amounts of calcium and vitamin D may have a lower risk of developing breast cancer before menopause. The study followed more than 31,000 women aged 45 and older for 10 years. It found that intake of calcium and vitamin D was moderately associated with a lower risk of breast cancer before — but not after — menopause.

Bolstered by the benefits the sunshine vitamin offers, public health officials are encouraging people to include vitamin D in their diets as researchers continue to investigate how it helps the body.

Vitamin D helps the body absorb calcium thereby making bones stronger. Certain foods such as cow's milk and margarine are fortified with vitamin D and inexpensive supplements can help boost the body's vitamin D levels.

A deficiency in children can trigger rickets, a bone disease that leaves children with soft bones and skeletal deformities. As breast milk doesn’t contain sufficient levels of vitamin D, public health officials recommend that infants who are exclusively breastfed should take a supplement to prevent vitamin D deficiency.

In adults, low levels of vitamin D can cause osteoporosis, a disease that decreases bone mass and bone tissue, putting patients at risk of fractures. People with darker skin tones are also often advised to take a supplement as they have more difficulty generating natural vitamin D from the sun's ultraviolet rays.

As well, patients with a reduced ability to absorb dietary fat, including people suffering from Crohn's disease, cystic fibrosis or liver disease, often have low vitamin D levels.

The Canada Food Guide says men and women over the age of 50 are advised to consume three servings of milk and alternatives along with a supplement equal to 10 micrograms or 400 IU of Vitamin D every day. One cup of milk has 100 IU of vitamin D. All other age groups are encouraged to have two cups of milk to ensure adequate vitamin-D levels.

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

The Canadian Cancer Society, though, does not recommend that people rely solely on increasing their exposure to the sun to boost their levels of vitamin D. The society points out that there are other sources of vitamin D, including vitamin supplements, oily fish and fortified foods.

It also warns that — for some people — increasing exposure to the sun by even a few minutes a day could increase the risk of skin cancer. The society recommends that people consider a balance of vitamin D supplements and small amounts of sun exposure to maintain proper levels of the vitamin while keeping risks of skin cancer to a minimum.

As researchers continue to explore how the vitamin helps the body, Health Canada warns overenthusiastic consumers that exceptionally high levels can be damaging. The U.S.-based Institute of Medicine of the National Academies has set 2,000 IU of vitamin D as the daily maximum tolerable amount. Health Canada warns exceeding this limit could lead to an overdose that can cause kidney stones as well as damage to the heart, lungs and blood vessels.

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