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"Cancer is a very long process, and there is an even longer period between incidents of mortality," Davis said. "There was only a very small number of cancer deaths. If there is a relationship, this study is not large enough to show one."

Some other studies have given evidence that vitamin D may provide protection against colon cancer, but others have not, she noted.

In addition, Davis added, "We don't know what the optimum level of vitamin D is, we don't know whether genetics might affect the benefits, and we need to consider interactions with other dietary components. Some people may be put at risk. Evidence suggests an increase in kidney stones [with excess vitamin D], and while a kidney stone is not cancer, it is a problem."

The current recommendation for vitamin D is 200 International Units a day, Davis said, and "I believe that when possible, people are better off meeting their nutritional needs through diet rather than through supplements."

Some foods, such as milk, are fortified with vitamin D, and the nutrient can be found in some fish and fish oils, according to the U.S. National Institutes of Health. The skin also manufacturers vitamin D upon exposure to sunlight.

D. Michal Freedman, the NCI epidemiologist who led the study, said its main finding "was the lack of a relationship between total cancer deaths and vitamin D levels." Freedman downplayed the colorectal cancer data as "a secondary finding."

"The study doesn't address the issue of the effects of vitamin D in the blood," Freedman said. "The issue of what people should be taking in terms of vitamin D involves a lot of other factors."
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