For each 25 nmol/L increment of circulating vitamin D, colorectal cancer risk was 19% lower in women and 7% lower in men.
Higher levels of vitamin D-measured as circulating 25(OH)D-were associated with a significantly lowered colorectal cancer risk, according to the results of a study published in the Journal of the National Cancer Institute.
According to lead study author Marji L. McCullough, ScD, and colleagues, colorectal cancer is the third most commonly diagnosed cancer in the United States, with almost 150,000 new diagnoses expected in 2018. Previous studies have suggested that vitamin D in the blood may lower colorectal cancer risk, but results were inconsistent.
“Laboratory studies have identified potential mechanisms for vitamin D in inhibiting cancer development,” explained McCullough, an epidemiologist with the American Cancer Society. “Our findings strengthen the evidence that vitamin D may lower the risk of colorectal cancer.”
In the study, McCullough and colleagues pooled data from 17 cohorts that included 5,706 patients with colorectal cancer and 7,107 patients who were colorectal cancer–free. About one-third of patients had newly measured 25(OH)D concentrations, and the remainder had previously measured 25(OH)D figures calibrated to the same assay, to permit estimating risk by absolute concentrations.
Vitamin D levels considered sufficient for bone health were in the range of 50–62.5 nmol/L; vitamin D deficiency was designated at levels below 30 nmol/L. Compared with participants who had circulating vitamin D concentrations considered sufficient for bone health, participants with deficient concentrations had a 31% increased risk of colorectal cancer during follow-up (relative risk [RR] = 1.31; 95% CI, 1.05–1.62).
Vitamin D levels above sufficiency (75 nmol/L to < 87.5 nmol/L; and 87.5 nmol/L to < 100 nmol/L) were associated with a 19% lower risk (RR = 0.81; 95% CI, 0.67–0.99) and a 27% lower risk for colorectal cancer (RR = 0.73; 95% CI, 0.59–0.91), respectively. Above the level of 100 nmol/L, risk no longer continued to decline.
When the researchers compared results between men and women, they found a noticeably stronger association for women than for men. For each 25 nmol/L increment of circulating vitamin D, colorectal cancer risk was 19% lower in women and 7% lower in men.
According to McCullough, this study contributes to the body of evidence reviewed by scientific committees that evaluate all of the evidence on vitamin D and health. “These findings are not yet ready for public health policy,” she said. “However, it is prudent to make sure you are at least meeting the RDA for vitamin D, which is 600 IU for most adults and 800 IU for those over age 70.”
People who do not eat foods that are high in vitamin D (eg, fatty fish such as tuna, mackerel, and salmon; beef liver; cheese; egg yolks; and certain fortified foods, including dairy products and cereals), who live in northern latitudes, and who are never exposed to sunlight tend to have lower levels.
McCullough also warned that extremely high consumption of vitamin D can be toxic. The National Academy of Medicine recommends that intake not exceed 4,000 IU of vitamin D per day.