Vitamin D and calcium: a systematic review of health outcomes.

Background: Since the 1997 Dietary Reference Intake (DRI) values for vitamin D and calcium were established new data have become available on their relationship, both individually and combined, to a wide range of health outcomes. The Institute of Medicine/Food and Nutrition Board has constituted a DRI committee to undertake a review of the evidence and potential revision of the current DRI values for these nutrients. To support this review, several US and Canadian federal government agencies commissioned a systematic review of the scientific literature for use during the deliberations by the committee. The intent of providing a systematic review to the committee is to support transparency of the literature review process and provide a foundation for subsequent reviews of the nutrients.

Purpose: To systematically summarize the evidence on the relationship between vitamin D, calcium, and a combination of both nutrients on a wide range of health outcomes as identified by the IOM, AHRQ and technical expert panel convened to support the project.

Data sources: MEDLINE; Cochrane Central; Cochrane Database of Systematic Reviews; and the Health Technology Assessments; search limited to English-language articles in humans.

Study selection: Primary interventional or observational studies that reported outcomes of interest in human subjects in relation to vitamin D and/or calcium, as well as systematic reviews that met the inclusion and exclusion criteria. Cross sectional and retrospective case-control studies were excluded.

Data extraction: A standardized protocol with predefined criteria was used to extract details on study design, interventions, outcomes, and study quality.

Data synthesis: We summarized 165 primary articles and 11 systematic reviews that incorporated over 200 additional primary articles. Available evidence focused mainly on bone health, cardiovascular diseases or cancer outcomes. For many outcomes, it was difficult to draw firm conclusions on the basis of the available literature concerning the association of either serum 25(OH)D concentration or calcium intake, or the combination of both nutrients. Findings were inconsistent across studies for colorectal and prostate cancer, and pregnancy-related outcomes including preeclampsia. There were few studies for pancreatic cancer and immune function. Among trials of hypertensive adults, calcium supplementation lowered systolic, but not diastolic, blood pressure by 2-4 mm Hg. For body weight, the trials were consistent in finding no significant effect of increased calcium intake on weight. For growth rates, a meta-analysis did not find a significant effect on weight or height gain attributable to calcium supplement in children. For bone health, one systematic review found that vitamin D plus calcium supplementation resulted in small increases in BMD of the spine and other areas in postmenopausal women. For breast cancer, calcium intakes in premenopausal women were associated with a decreased risk. For prostate cancer, some studies reported that high calcium intakes were associated with an increased risk.

Limitations: Studies on vitamin D and calcium were not specifically targeted at life stages (except for pregnant and postmenopausal women) specified for the determination of DRI. There is large variation on the methodological quality of studies examined. Use of existing systematic reviews limits analyses that could be performed on this source of information.

Conclusions: The majority of the findings concerning vitamin D, calcium, or a combination of both nutrients on the different health outcomes were inconsistent. Synthesizing a dose-response relation between intake of either vitamin D, calcium, or both nutrients and health outcomes in this heterogeneous body of literature proved challenging.