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The striking observation that people with low levels of vitamin D are at an elevated risk of cancer, premature death and chronic diseases should be tested in a large-scale, drug-style clinical trial, says the World Health Organization's agency specializing in cancer research.

The International Agency for Research on Cancer made the call after conducting a major review into the state of the evidence on vitamin D and health.

The agency concluded that the data linking inadequate amounts of the nutrient to colorectal cancer was "consistent and persuasive" and said there was some weaker evidence connecting insufficiency to breast cancer. However, it said studies on prostate cancer have indicated the vitamin has "little or no effect."

But the IARC cautioned that much of the evidence pointing to beneficial effects comes from epidemiological studies, which have found that people with the lowest amounts of the vitamin in their blood have the highest risks of cancer and other diseases.

Although epidemiology has had noted successes, such as discovering the link between lung cancer and smoking, these type of studies are considered by many researchers to be a scientific version of circumstantial evidence because the findings don't prove cause and effect.

"Currently, the key question is to understand whether low vitamin D status causes an increased risk of cancer, other chronic health conditions and death, or is simply a consequence of poor health status," IARC said in its review.

"The only way to further address the cause-effect issue is to organize new randomized trials" looking at vitamin D and mortality from common conditions, including cancer, it said.

Pending these trials, which could take years, the agency said it is premature for people to start popping vitamin D pills, a position that puts it at odds with advice from the Canadian Cancer Society, which last year became the first major public health advocacy group in the world to recommend population-wide use of the nutrient. As a cancer-preventative step, the society said, white people should take supplements containing 1,000 international units a day in fall and winter. Non-white people should take that amount year-round since they make less of the nutrient the natural way - through naked skin exposed to strong sunlight - the CCS said.

Heather Chappell, a spokesperson for the society, also endorsed the call for clinical trials, but said it will probably cost $150-million to conduct testing that would settle the question of the potency of vitamin D as a cancer fighter.

In recent years, studies have noted that people with low levels of vitamin D are at higher risk of heart disease, cancer and diabetes. Other research has shown that people who take supplements containing 400 or 800 IU have a measurable reduction in their risk of premature death, while those with chronic kidney disease treated with the vitamin also have reduced mortality.

But the IARC said that none of theses studies could identify what it was about vitamin D that caused the reduced death risk. It also cautioned that initially promising findings with other nutrients, such as vitamin E, selenium and beta carotene, have been followed by disappointing results after more detailed clinical trials were conducted.

The Canadian and U.S. governments have asked the U.S. Institute of Medicine to set up an expert panel to review whether current health guidelines on taking the vitamin need to be changed. Currently, Health Canada recommends taking between 200 and 600 IU daily - depending on age - based on vitamin D's role in preventing bone diseases.