Taking 1,000 to 2,000 international units (IU) of vitamin D each day might reduce the risk of colorectal cancer. Previous studies have found that vitamin D intake does not seem to protect against colorectal cancer. However, it is possible that the dose may simply have been too low to provide a benefit. Researchers from the Naval Health Research Center in San Diego, California included data from five studies that looked at the association between blood levels of 25-hydroxy-vitamin D, which provides a good estimate of vitamin D levels in the body, and colorectal cancer risk. It was found that as blood levels of vitamin D rose, the risk of colorectal cancer fell. Risk was reduced by 54 percent in groups with the highest vitamin D levels relative to those with the lowest levels. In the final analysis, a blood level of 33 nanograms per millilitre (ng/ml) or higher was associated with a 50 percent decreased risk of colorectal cancer relative to a level of 12 ng/ml or lower. The findings suggest that protective vitamin D blood levels could be achieved with daily intake of 1,000 to 2,000 IU of vitamin D . Moreover, the National Academy of Sciences has established that a dose of 2,000 IU/day is safe.
American Journal of Preventive Medicine,
February 2007