By Jeremy Appleton, ND, CNS

Healthnotes Newswire (May 10, 2007)—A new study has concluded that vitamin D, mostly known for its beneficial relationship to calcium in promoting bone strength, may also help prevent colon cancer. Previous research had begun to uncover the mechanisms of this protective effect, but studies in humans have produced unclear results.

“The Women’s Health Initiative demonstrated that [supplementing with] a low dose of vitamin D did not protect against colorectal cancer within seven years of follow-up,” wrote Edward D. Gorham, PhD, of the Moores Cancer Center at the University of San Diego and lead author of the new study. But the analysis pooled data from five studies and showed something different: that a higher intake of vitamin D is associated with a reduced incidence of colorectal cancer.

In 2005, there were more than 145,000 new cases of colorectal cancer in the United States, and 56,300 deaths from the disease. Previous studies showed that higher rates occurred in the northern parts of the United States (where there is less sunlight) than in the southwest, Hawaii, and Florida. This led researchers to a theory that vitamin D, which the body can make in the skin after sun exposure, might reduce colorectal cancer risk.

The median intake of vitamin D in the United States is about 250 to 300 IU per day, which is lower than the National Academy of Sciences’ recommendation of 400 IU for day for mature adults. Gorham and colleagues suggested that a higher amount might be beneficial.

“Increasing [vitamin D] intake to 1,000 to 2,000 IU per day…would be associated with substantially lower incidence rates of colorectal cancer,” Gorham concluded, “with only minimal risks.”

The National Academy of Sciences has established that 2,000 IU per day of vitamin D produces no observed negative effects in healthy people. However, Alan R. Gaby, MD, chief science editor for Healthnotes, cautions against giving too much weight to this study, which had limitations in its design. “While the results of this study are consistent with other research suggesting that increasing vitamin D intake can prevent cancers of the prostate, breast, and pancreas, they should be considered preliminary, because it is not clear whether there is a cause–effect relationship between vitamin D status and colorectal cancer risk.”

Gaby goes on to point out that in observational studies that measure the relationship between blood vitamin D levels and cancer risk, certain factors might skew the results. For example, in study participants who exercise regularly, especially outdoors where they get sun exposure that the body can turn into vitamin D, it’s difficult to know whether the exercise or higher vitamin D levels reduced their risk of colorectal cancer. By the same token, obesity is associated with both higher cancer risk and impaired vitamin D production in response to sun exposure. Perhaps the obesity, rather than the lower vitamin D levels, was responsible for the higher cancer risk.

“A low blood concentration of vitamin D may be merely a marker of increased risk, rather than a cause, of colorectal cancer,” Gaby concludes. Further studies in which vitamin D is administered under controlled circumstances are needed to shed light on the relationship between this nutrient and colon cancer.

(Am J Prev Med 2007;32:210–16)

Jeremy Appleton, ND, CNS, is a licensed naturopathic physician, certified nutrition specialist, and published author. Dr. Appleton was the Nutrition Department Chair at the National College of Naturopathic Medicine, has served on the faculty at Bastyr University of Natural Health Sciences, and is a former Healthnotes Senior Science Editor and a founding contributor to Healthnotes Newswire. He has worked extensively in scientific and regulatory affairs in the supplement industry and is now a consultant through his company Praxis Natural Products Consulting and Wellness Services.

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