Having a high level of circulating vitamin D was associated with significantly lower risk of colorectal cancer, a recent study found.
Researchers pooled participant-level data from 17 cohorts, nine of them from previously published prospective studies on associations between circulating 25-hydroxyvitamin D concentrations and colorectal cancer. In total, there were 5,706 patients who developed colorectal cancer and 7,107 control participants. For 30.1% of participants, vitamin D level was newly measured. Measurements taken previously were calibrated to a newer assay. Results were published by the Journal of the National Cancer Institute on June 14.
Participants who had circulating vitamin D levels under 30 nmol/L had a 31% higher risk of colorectal cancer compared to those with levels between 50 and 62.5 nmol/L (relative risk [RR], 1.31; 95% CI, 1.05 to 1.62). Study authors noted the latter category is considered in the lower range of sufficiency for bone health. Patients with even higher levels of circulating vitamin D had even lower risk of colorectal cancer: Compared to levels of 50 to 62.5 nmol/L, risk of colorectal cancer was reduced by 19% (RR, 0.81; 95% CI, 0.67 to 0.99) and 27% (RR, 0.73; 95% CI, 0.59 to 0.91) with levels of 75 to 87.5 nmol/L and 87.5 to 100 nmol/L, respectively. The pattern of increasing reduction in risk for colorectal cancer stopped at 100 nmol/L, although few patients in the study had a level this high. The associations remained after adjustment for body mass index, physical activity, or other risk factors and were significant in all studied subgroups.
The association of lower colorectal cancer risk with higher vitamin D levels was more significant in women than in men, the study also found. For each 25-nmol/L increment in circulating vitamin D, colorectal cancer risk was 19% lower in women (RR, 0.81; 95% CI, 0.75 to 0.87) and 7% lower in men (RR, 0.93; 95% CI, 0.86 to 1.00). The biological explanation for this finding is unclear, although previous research has suggested that estrogen might influence vitamin D activity, the authors said. Explaining the study's overall findings, they noted that there are potential effects of vitamin D activity specific to the large bowel.
The results suggest that, for colorectal cancer prevention, the optimal level of circulating 25-hydroxyvitamin D is 75 to 100 nmol/L, higher than currently recommended for bone health by the Institute of Medicine, the authors said. They noted that strengths of the study are inclusion of the majority of participant-level data in the world on the topic and calibration of results from different vitamin D assays. Limitations include relatively few patients with very high vitamin D levels and from certain racial and ethnic groups.