MedicalResearch.com: What are the main findings of the study?
Answer: We identified 268 studies either meta-analyses or systematic reviews of plasma vitamin D concentrations or meta-analyses of randomised trials of vitamin D supplementation.
The relation between vitamin D and 137 outcomes has been explored, covering a wide range of skeletal, malignant, cardiovascular, autoimmune, infectious, metabolic, and other diseases.
Ten outcomes were examined by both meta-analyses of observational studies and meta-analyses of randomised trials, but the direction of the effect and level of statistical significance was concordant only for birth weight.
Despite a few hundred systematic reviews and meta-analyses, highly convincing evidence of a clear role of vitamin D does not exist for any outcome, but associations with a selection of outcomes are probable: birth weight, dental caries in children, maternal vitamin D concentrations at term, and parathyroid hormone concentrations in chronic kidney disease patients requiring dialysis.
Finally, despite the vast volume of published studies, this review highlights the absence of meta-analyses in relation to autoimmune disease (either of observational studies or randomised clinical trials), and the absence of meta-analyses of randomised clinical trials of vitamin D supplementation in respect to cancer, cognitive and infectious disease outcomes.
MedicalResearch.com: Were any of the findings unexpected?
Answer: In contrast to previous reports, evidence does not support the notion that vitamin D only supplementation increases bone mineral density or reduces the risk of fractures or falls in older people.
MedicalResearch.com: What should clinicians and patients take away from your report?
Answer: The lack of convincing associations and the relative dearth of probable associations suggest that evidence for benefits that may be reaped from population-wide vitamin D supplementation is weak at least for supplementation levels considered by studies included in this review.
Probable associations, for which highly significant effects appear in randomised trials, hold the most promise for clinical translation, but they pertain to specific populations (children, pregnant women, patients with chronic kidney disease).
MedicalResearch.com: What recommendations do you have for future research as a result of this study?
Answer: As universal conclusions about vitamin D benefits cannot be drawn, further studies and better designed trials (in relation to dose and vitamin D only supplementation) especially for cancer and autoimmune related outcomes are needed.