Research published in June 2018 found that blood levels of vitamin D higher than those recommended for bone health are associated with a lower risk of colorectal cancer. Some earlier studies had suggested that higher levels of D might be cancer protective, but the results were inconsistent.
For this latest report, investigators from the American Cancer Society, the Harvard T.H. Chan School of Public Health, the National Cancer Institute and 20 other medical centers worldwide analyzed data collected from the U.S., Europe and Asia on more than 5,706 persons with colorectal cancer and 7,107 people of similar age and race who didn’t have the disease. They found that compared to participants whose circulating concentrations of vitamin D were considered sufficient for bone health, those with lower concentrations had a 31 percent higher risk of colorectal cancer over the 5.5 years they were followed.
Levels of D above the amount deemed sufficient for bone health were linked to a 22 percent lower cancer risk. The risk wasn’t any lower than that, however, in those with the highest levels. This indicates that the ideal concentration (or “sweet spot” as one researcher put it) for lower risk has yet to be determined.
The investigation did find that the association with vitamin D persisted even after the researchers adjusted for other colorectal cancer risk factors, including being overweight or obese, physical inactivity, a diet high in red and processed meats, smoking, heavy alcohol use, or a personal or family history of colorectal polyps. The risk also increases with age.
Based on the research team’s findings, the association between vitamin D levels and lower colorectal cancer risk was stronger in women than men. The lifetime risk of the disease is one in 24 for women compared to one in 22 for men.
Some background on this subject: in 2010 results from the Women’s Health Initiative (WHI), a large government-sponsored study including more than 36,000 women, showed that daily supplements of 1000 mg of calcium and 400 IU of vitamin D didn’t protect against colon cancer. At the time, I wrote on this site that doses of vitamin D higher than 400 IUs may be necessary to protect against both colon cancer and osteoporosis. (The WHI also found that calcium and vitamin D didn’t protect against that condition either.) In another study published five years later, participants were randomly assigned to take either 1,200 mg of calcium carbonate, 1,000 IUs of vitamin D, the combination of both, or a placebo. Here, too, taking either or both supplements didn’t help at all. One of the authors suggested at the time that results might have been different if higher doses had been used and if the participants had been followed for a longer stretch.
We may know much more about the effect of vitamin D on the risk of colorectal and other cancers later this year when an ongoing study at Boston’s Brigham and Women’s Hospital is completed. It has enrolled 25,874 men and women throughout the U.S. in an effort to learn whether taking daily supplements of 2000 IU of vitamin D3 or omega-3 fatty acids can reduce the risk of developing cancer, heart disease and stroke in people who have no history of these diseases.
Andrew Weil, M.D.
Marjorie L. McCullough et al, “Circulating Vitamin D and Colorectal Cancer Risk: An International Pooling Project of 17 Cohorts.” Journal of the National Cancer Institute, June 14, 2018, doi.org/10.1093/jnci/djy087