For the study, the researchers assessed data from 170,000 participants of two long-term research projects - the Nurses' Health Study and the Health Professional Follow-up Study.
The team hypothesized that if the two phenomena were related, colorectal tumors developing in participants with high levels of vitamin D would likely be more resistant to the cells of the immune system than those developing in participants with lower levels of the vitamin.
From the data pool, the researchers selected 942 participants - 318 with colorectal cancer and 624 who were cancer free. Each participant had had a blood sample taken in the 1990s, at a time before any of the participants had developed cancer. These samples were then tested for a substance produced in the liver from vitamin D.
Participants with high amounts of the substance - 25-hydroxyvitamin D (25(OH)D) - were found by the researchers to be less likely to develop colorectal tumors permeated with large numbers of immune system cells, suggesting that their hypothesis was correct.
Findings 'vindicate basic laboratory discoveries'
"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," states Dr. Ogino.
Unmeasured variables may have impacted on the study's findings, acknowledge the authors. They write that the findings need to be replicated in other research, "given the uniqueness of the current study."
Further evidence for vitamin D's cancer-fighting qualities is provided in additional research from scientists at the Dana-Farber Cancer Institute. A team, led by medical oncologist Dr. Kimmie Ng, observed that patients with metastatic colorectal cancer and high levels of vitamin D prior to treatment survived longer than patients with lower levels of the vitamin in their bloodstream.
"This is the largest study that has been undertaken of metastatic colorectal cancer patients and vitamin D," says Dr. Ng. "It's further supportive of the potential benefits of maintaining sufficient levels of vitamin D in improving patient survival times."
"In the future," concludes Ogino, "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."
Medical News Today previously reported on a study published in the BMJ stating that genetically low vitamin D levels are linked to all-cause mortality.