In the largest study of its kind, low levels of vitamin D are linked with a significant increase in colorectal cancer risk. Conversely, higher levels appear to offer protection.
Vitamin D is produced in the skin after contact with sunlight, as well as absorbed in our guts from several dietary sources — including fortified foods and fatty fish.
Its primary role was long considered to be bone maintenance. But, as researchers dig deeper, vitamin D's sphere of influence widens.
Vitamin D and bowel cancer
Recently, researchers from a host of organizations, including the American Cancer Society (ACS) in Atlanta, GA, the Harvard T.H. Chan School of Public Health in Boston, MA, and the United States National Cancer Institute in Rockville, MD, combined forces to investigate vitamin D's role in colorectal cancer risk.
Understanding what factors play a role in its development is crucial. And, if vitamin D is involved, it might form the basis of a simple and cost-effective intervention.
Some previous studies have found a link between vitamin D deficiency and colorectal cancer, but others have not. This new, large-scale effort was designed to iron out the creases and present more concrete evidence.
The researchers' findings were published recently in the Journal of the National Cancer Institute.
Co-senior study author Stephanie Smith-Warner, Ph.D. — an epidemiologist at the Harvard T.H. Chan School of Public Health — says, "To address inconsistencies in prior studies on vitamin D and to investigate associations in population subgroups, we analyzed participant-level data, collected before colorectal cancer diagnosis, from 17 prospective cohorts and used standardized criteria across the studies."
In all, the team used data from studies conducted on three continents that included 5,700 cases of colorectal cancer and 7,100 controls.
Previously, researchers found it difficult to pool data from different studies because of the variety of ways that vitamin D was measured. These researchers calibrated the existing measurements so that a direct comparison could be made between multiple trials in a meaningful way.
Vitamin D's influence on cancer
The researchers compared each individual's vitamin D levels with the current National Academy of Medicine recommendations for bone health.
People who had vitamin D levels below the current guidelines had a 31 percent increased risk of colorectal cancer during the follow-up — an average of 5.5 years. Those with vitamin D above the recommended levels had a 22 percent reduction in risk. The link was stronger in women than in men.
These relationships remained significant even once the team had adjusted the data to account for other factors that are known to increase colorectal cancer risk.
But, it is worth noting that the reduced risk did not become more pronounced in the people with the highest levels of vitamin D in their system.
"Currently," notes co-first study author Marji L. McCullough, "health agencies do not recommend vitamin D for the prevention of colorectal cancer."
"This study adds new information that agencies can use when reviewing evidence for vitamin D guidance and suggests that the concentrations recommended for bone health may be lower than would be optimal for colorectal cancer prevention."
Marji L. McCullough
This study adds to the evidence that vitamin D offers protection against bowel cancer. Guidelines on vitamin D intake may need to be changed in light of these findings.