TORONTO - A new U.S. study contradicts the notion that high blood levels of vitamin D might help protect men from developing prostate cancer -- a finding that is likely to add to the public's confusion over the possible benefits of the sunshine nutrient.

Laboratory studies of cells have suggested that high doses of vitamin D may reduce prostate cancer from developing, but international research that has looked at the disease and the effect of vitamin D in different groups of men has shown inconsistent results.

In the study by the National Cancer Institute, researchers compared vitamin D concentrations in the blood of 749 men diagnosed with prostate cancer with those of 781 men without the disease.

Vitamin D concentration was measured in a blood sample taken at enrolment in the study and participants were followed for up to eight years.

"Our main findings are higher levels of vitamin D were not associated with a decreased risk of prostate cancer,'' lead author Jiyoung Ahn said Tuesday from Bethesda, Md. "However, we did see some evidence of an increased risk of aggressive disease associated with the higher concentration of vitamin D.''

But Ahn stressed that the latter finding was not statistically strong and more research is needed to determine whether too much vitamin D actually increases the risk of a prostate cancer being more aggressive.

The study was published online Tuesday in the Journal of the National Cancer Institute.

Commenting on the study, Reinhold Vieth of Toronto's Mount Sinai Hospital said he is not surprised by the NCI researchers' results, but he suggests the link between prostate cancer risk and vitamin D is related to seasonal changes in the amount of the nutrient in the body.

"What I think happens is that this phenomenon of higher risk relates not just to a flat-out how high is your vitamin D,'' said Vieth, who specializes in vitamin D research. "The thing that causes risk with regards to cancer ... is fluctuation.''

Vitamin D, required for proper cell growth and regulation, is made when the skin is exposed to sunlight. But people in northern countries such as Canada become deficient in the nutrient during winter months when the sun's rays are at their weakest.

"Higher vitamin D is good because it seems to make cells behave properly,'' said Vieth.

But when levels drop as fall and winter set in, the body's cells have to adjust to having less of the nutrient -- and it is this continual adjustment that may upset the way cells proliferate, he said.

The bigger the fall in vitamin D levels, the harder it is for cells in the body -- in this case, prostate cells -- to adjust, he said, noting that tumours can arise when regulation of cell growth goes awry.

Vieth said the public has good reason to be confused about the role of vitamin D in cancer prevention, particularly when it comes to prostate cancer, because there has been so much conflicting research.

Until more is known, he said, there is no simple answer.

The best way to avoid the seasonal vitamin D yo-yo is to maintain a stable level in the blood by taking dietary supplements year-round, he said, suggesting that 1,000 IUs daily for adults would be an adequate amount.

Last June, the Canadian Cancer Society recommended that adults consider taking a vitamin D supplement of 1,000 IUs daily during fall and winter, while darker-skinned and older people should think about taking the little white pills year-round.

Health Canada currently recommends 200 IUs for adults 19 to 50 years of age. Last September, it released a statement saying recommendations by various organizations to boost intake were premature, and a comprehensive review was needed before it would revise its own recommendations.