Low levels of vitamin D are associated with a number of disorders and diseases . . . cardiovascular diseases, diabetes, weight gain, infectious diseases, multiple sclerosis, mood disorders, cognitive impairment, and even all-cause mortality. Scientifically, we simply do not know whether this is causative or correlated. Stated another way, do people who have certain health problems produce less vitamin D such that lower levels are a marker of poor health? Or do low levels of vitamin D cause bad health? If the latter, does taking vitamin D supplementation help to prevent disease?
Recently, Phillipe Autier published an article in The Lancet Diabetes & Endocrinology looking at the totality of evidence amassed to date. His findings are that vitamin D is a marker, not a cause. Stated another way, taking vitamin D to boost low levels does not seem to help prevent disease.
To reach this conclusion, Dr Autier scoured the scientific literature and evaluated 462 clinical trials. This is a large data base upon which to come up with an educated conclusion. The evidence indicates that taking vitamin D does not produce a benefit.
Even taking vitamin D for bone health may be questionable (vitamin D is important in regulating calcium and is known to be important for bone health). Another meta-analysis in The Lancet evaluated 23 studies looking specifically at the benefits of vitamin D supplementation to treat osteoporosis. The finding was that most bone areas evaluated showed no increase in bone density [aka BMD] at all. There was about 0.8% increase in BMD at the femoral neck, but that amount (if it is real) is probably not clinically meaningful. In total, if the goal is to prevent osteoporosis, vitamin D supplements are probably of limited use, though treating low levels of vitamin D is still probably warranted in the face of documented osteoporosis.
Despite the negative evidence available to date, it is worth pursuing the question further given the observed correlation between low vitamin D levels and disease. Several large studies are ongoing. One of these is the VITAL study, being conducted in the UK, which will evaluate 20,000 people. In several years, this should provide a definitive answer. Until then, indiscriminate testing of vitamin D levels (and taking vitamin D to treat low levels identified from a lab test) are not clinically warranted and do not seem likely to improve health outcomes.
sources: Vitamin D status and ill health: a systematic review (The Lancet Diabetes & Endocrinology); Effects of vitamin D supplements on bone mineral density: a systematic review and meta-analysis (The Lancet)