Vitamin D, long known to prevent rickets the softening and weakening of bones in children and widely added to milk for that purpose, has been touted in many venues as the latest miracle vitamin. Like vitamin C, D has been hailed as having a myriad of health effects other than its basic function of enabling the absorption of calcium from food.
Blood levels of vitamin D both elevated and diminished have been associated with non-skeletal chronic conditions such as cancer and cardiovascular disease, metabolic disorders, autoimmune diseases, among other conditions. Because vitamin D deficiency has often been found in people living in the far northern latitudes, and thus deriving less sunlight-stimulated vitamin D production in the skin, it could be an important public health issue if the associations with various chronic conditions were found to be causal.
Epidemiologist John Ioannidis from Stanford University and colleagues from Greece and the UK, recently published the results of an extensive review they conducted encompassing the vast literature on vitamin D, in BMJ. Their goal was To evaluate the breadth, validity, and presence of biases of the associations of vitamin D with diverse outcomes.
These researchers examined both observational associations between blood concentrations of vitamin D and health outcomes, as well as randomized trials of vitamin D supplementation. Their review, called an umbrella review, examined both systematic reviews and meta-analyses of such studies. In sum, 107 systematic literature reviews, 74 meta-analyses of observational studies, and 87 meta-analyses were included in the umbrella review.
The authors concluded that there was no convincing evidence from either randomized nor from observational trials, for a clear role of vitamin D as a causal factor for any condition or disease. While there was probable evidence for some outcomes, for example decreased dental cavities in children, and an increased need for high vitamin D concentration in pregnant women at term, there was only suggestive evidence for a correlation between vitamin D concentration and lower risk of colorectal cancer, cardiovascular disease, high blood pressure, stroke, depression, or type 2 diabetes.
ACSH s Dr. Ruth Kava commented This very extensive review does not support the concept of high levels of vitamin D being necessary to prevent or treat a wide variety of ills, in spite of what one might see in the popular media. It s also important to recognize that very high levels of vitamin D intake over the long term can be toxic, and must be taken into account.