(Reuters Health) - A new analysis of earlier research finds that both higher vitamin D intake and higher blood levels of the vitamin's active form are linked to lower risk of colon and rectal cancers.
In 18 studies that included more than 10,000 people, colon cancer risk was as much as 33 percent lower in subjects with the highest blood levels of vitamin D compared to those with the lowest levels, researchers report in the Journal of Clinical Oncology. Those with the highest intake of vitamin D through supplements and food had 12 percent lower risk than those with the lowest intakes.
The total of studies available for analysis is still sparse, noted senior author Dr. Huanlong Qin and his colleagues from The Sixth People's Hospital affiliated with Shanghai Jiao Tong University in Shanghai. Additional studies would be "highly desirable to enable more precise estimates and a better understanding of the role of vitamin D in colorectal cancer carcinogenesis," they write.
Vitamin D has previously been linked to protection from various cancers, heart disease, diabetes and asthma, among other conditions. How the vitamin might exert a beneficial effect is still poorly understood, however. Some evidence suggests that to achieve a benefit people may need more than current recommended daily requirements.
Up to 58 percent of U.S. adults and adolescents may have vitamin D deficiency, which is "an important health problem in the industrial world," Qin's team said. While there are biologically plausible mechanisms through which low vitamin D levels could increase colon cancer risk, the researchers add, studies have had mixed results so far.
A recent review by the U.S. Institute of Medicine concluded there isn't enough information to justify increasing recommended intakes of vitamin D.
For their analysis, Qin's group looked only at the strongest studies that investigated the relationship between vitamin D intake or blood levels of 25-hydroxyvitamin D, the active form in the body, and colorectal cancer, colon cancer or rectal cancer.
They identified nine studies of vitamin D intake including 6,466 patients, four of which were conducted in the US, three in Europe, and two in Asia. Nine more studies, including 2,767 cases and 3,948 people in comparison groups, looked at blood levels of 25-hydroxyvitamin D, with six conducted in the US, two in Europe and one in the US. One study included both vitamin D intake and blood levels of 25-hydroxyvitamin D.
Limitations of the analysis, the researchers note, included the lack of uniform criteria for comparison groups across the various studies; also, not all of the studies had information on vitamin D intake or its blood levels for individuals, only ranges for groups. The "lowest" and "highest" categories in different studies also varied significantly.
Hence, the researchers' conclusion that their findings need to be confirmed in large, gold-standard randomized clinical trials of vitamin D supplements.
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