By David Liu, PHD

Friday July 20, 2012 (foodconsumer.org) -- Taking vitamin D supplements may help prevent colorectal cancer and breast cancer, according to a new study published recently in the American Journal of Epidemiology.  But the study was meant to demonstrate that selecting and adjusting for apparent relevant risk factors may affect the study outcome.

The study led by Marian L. Neuhouser of Fred Hutchinson Cancer Research Center in Seattle, Washington and colleagues  showed that those who had lowest levels of vitamin D were more than twice as likely as those who had highest levels to develop colorectal cancer and about 33 percent more likely than those who had highest levels to develop breast cancer.

The  researchers came to these findings after analyzing data from one case-control study (310 cases and 310 control for colorectal cancer and data from the Women’s Health Initiative Calcium and Vitamin D Clinical Trial which involved 1,080 cases and 1,080 controls.  Case-control matching factors considered in these studies include age, latitude, race/ethnicity and blood collection date, all of which may have an impact on the vitamin D level.

The association of vitamin D with colorectal cancer and the association of vitamin D with breast cancer were derived from models considering only matching factors. In multivariate adjusted models, the association of vitamin d with colorectal cancer was even more significant, that is, those with lowest levels were more than four times as likely as those who had the highest levels to be diagnosed with colorectal cancer.  However, the association of vitamin D with breast cancer was not significant in the multivariate models.

The authors concluded "Adjusting for health and lifestyle characteristics has differential effects depending on the cancer site; when modeling such relations, investigators should take these factors into account."

Breast cancer is diagnosed in an estimated 210,000 American women each year and the disease kills about 50,000 each year in the U.S., according to the National Cancer Institute.  

Colorectal cancer refers to colon cancer and rectal cancer combined.  In 2012, 103,170 new cases of colon cancers and 40,290 new cases of rectal cancer are expected and the cancers are expected to kill 51,690 people in the U.S.