Vitamin D could protect some people with colorectal cancer by perking up the immune system's vigilance against tumor cells, according to a recent study.

Researchers from the Dana-Farber Cancer Institute found a link between vitamin D and the immune response to cancer has been shown in a large human population. The finding adds to a growing body of research showing that vitamin D - known as the "sunshine vitamin" because it is produced by the body in response to sunlight exposure - plays a key role in cancer prevention.

"People with high levels of vitamin D in their bloodstream have a lower overall risk of developing colorectal cancer," Shuji Ogino, the study's senior author said in a statement. "Laboratory research suggests that vitamin D boosts immune system function by activating T cells that recognize and attack cancer cells. In this study, we wanted to determine if these two phenomena are related: Does vitamin D's role in the immune system account for the lower rates of colorectal cancer in people with high circulating levels of the vitamin?"

For the study, Ogino and his colleagues collected and analyzed data from 170,000 participants in the Nurses' Health Study and Health Professionals Follow-up Study, two long-term health-tracking research projects. Within this population, researchers compared carefully selected groups of 318 colorectal cancer patients and 624 individuals who were free of cancer. All 942 of them had blood samples drawn in the 1990s, before any developed cancer. The investigators tested these samples for 25-hydroxyvitamin D, (abbreviated 25(OH)D), a substance produced in the liver from vitamin D.

They found that patients with high amounts of 25 (OH)D indeed had a lower-than-average risk of developing colorectal tumors that were enriched with immune system cells.

"This is the first study to show evidence of the effect of vitamin D on anti-cancer immune function in actual patients, and vindicates basic laboratory discoveries that vitamin D can interact with the immune system to raise the body's defenses against cancer," Ogino said. "In the future, we may be able to predict how increasing an individual's vitamin D intake and immune function can reduce his or her risk of colorectal cancer."

The findings are detailed in the journal Gut.