The association of vitamin D supplementation with the risk of cancer in postmenopausal women.

Purpose: There is inconclusive evidence on whether vitamin D therapy reduces cancer risk. We investigated the effect of vitamin D (±calcium) supplementation on the risk of breast, ovarian, uterine, colorectal, and lung cancer in women.
Methods: We conducted a case-control study using the UK Clinical Practice Research Datalink (CPRD); cases were women aged ≥55 years with a first diagnosis of either breast, colorectal, lung, ovarian, or uterine cancer between 2002 and 2009, with at least 5 years of CPRD follow-up prior to the date of diagnosis, and controls were women without cancer, frequency-matched to cases by year of birth, date of study entry, length of follow-up, and general practice. The association of vitamin D supplementation with the odds of developing each cancer was determined using multivariable logistic regression, controlling for body mass index, smoking, alcohol, and deprivation.
Results: Ninety-seven percent of women took vitamin D with a calcium supplement. Exposure to three or more prescriptions of vitamin D was associated with a 17 % reduced odds (95 % CI 0.71-0.97) of breast cancer versus 1-2 prescriptions, but this effect disappeared when omitting women first exposed within a year of diagnosis (OR 1.0, 95 % CI 0.82-1.23). Having more than 10 prescriptions of vitamin D was associated with a 17 % lower odds (95 % CI 0.65-1.06) of colorectal cancer, but the estimates are imprecise. There was little evidence of associations of supplements with lung or gynecological cancers.
Conclusion: We found little evidence that vitamin D (largely with calcium) supplementation is associated with decreased breast, lung, ovarian, and uterine cancer risk. There is a possible protective association between having more than 10 prescriptions of vitamin D supplements and colorectal cancer, but it requires further investigation.