I have reviewed the complex and inconsistent evidence concerning the potential risks and benefits of vitamin supplements here, and concluded that most of the common claims for health benefits are unjustified. I recently discussed an example of this sort of unjustified claims, a veterinarian promoting screening tests for blood levels of Vitamin D and recommending supplements for nearly all patients. Fortuitously, a new comprehensive review of the evidence regarding Vitamin D and Calcium supplementation has just been published, which illustrates the lack of real evidence for many of these sorts of claims.
Agency for Healthcare research & Quality, U.S. Department of Health & Human Services. Vitamin D and Calcium: A Systematic Review of Health Outcomes (Update). Evidence Report/Technology Assessment 217. AHRQ Publication No. 14-E004-EF. September 2014
Not surprisingly, the review concludes that the evidence is insufficient to support most of the claims made for supplements of these substances.
In solid agreement with the findings of the original report, the majority of the findings concerning vitamin D, alone or in combination with calcium, on the health outcomes of interest were inconsistent. Associations observed in prospective cohort and nested case-control studies were inconsistent, or when consistent, were rarely supported by the results of randomized controlled trials. Clear dose-response relationships between intakes of vitamin D and health outcomes were rarely observed. Although a large number of new studies (and longer followups to older studies) were identified, particularly for cardiovascular outcomes, all-cause mortality, several types of cancer, and intermediate outcomes for bone health, no firm conclusions can be drawn…it is difficult to make any substantive statements on the basis of the available evidence concerning the association of either serum 25(OH)D concentration, vitamin D supplementation, calcium intake, or the combination of both nutrients, with the various health outcomes because most of the findings were inconsistent.
It is particularly important to note that the evidence is often inconsistent, and that uncontrolled study designs often lead to conclusions that don’t turn out to be true when examined in controlled trials. The value of any single study is low, and the balance of the evidence must be evaluated to draw meaningful conclusions.
Regarding cancer specifically, the review did not find consistent evidence that low Vitamin D levels predispose to cancer nor that supplementation is protective.
No qualified systematic reviews have evaluated relationships between vitamin D and total cancer incidence or mortality. No new RCTs were identified for the current report that addressed the effect of vitamin D or vitamin D combined with calcium on the risk for total cancer or cancer mortality. One cohort study found no association between total (all-cause) cancer and 25(OH)D concentrations (rated A), whereas a second cohort study observed an association in men but not in women (rated B). Ten cohort studies and one nested case control study addressed the association of serum 25(OH)D concentrations and cancer mortality. Five of the cohort studies (1 rated A, 4 rated B) observed no association of serum 25(OH)D concentration with total cancer mortality. Three cohort studies and the nested case control study observed a trend toward increased risk with decreased serum 25(OH)D (all rated B). One analysis using updated NHANES III data (rated B) observed a trend toward increasing risk for death with increasing serum 25(OH)D among men at higher latitudes whose blood was drawn in summer but the reverse in women. One cohort study observed a U-shaped association of increasing mortality with both low and high serum 25(OH)D.
One RCT in the original report showed no effect of combined vitamin D3 (1000 IU/d) and calcium (~1500 mg/d) supplementation versus calcium supplementation (~1500 mg/d) alone on the risk of total cancer in healthy postmenopausal women (>55 years old) living in Nebraska (latitude 41°N). Another RCT also found no difference in total cancer mortality or incidence between supplemental vitamin D3 (100,000 IU every 4 months) and placebo in elderly (71+ years old) men and women living in the United Kingdom (latitude 52° N). Both RCTs were rated B quality.
Analyses using NHANES III data (general adult populations living in the United States) showed no significant association between baseline 25(OH)D concentrations and total cancer mortality.
As always, this review doesn’t preclude the possibility that Vitamin D and Calcium may be associated with the risk for some cancers in some populations. But it does expose the lack of real support for the often-heard claims about these vitamins and cancer.