Though observational studies have linked low blood levels of vitamin D to a wide variety of diseases and conditions, it is very possible that low vitamin D is not a cause of these conditions but rather an effect of them (this is called reverse causality)—or it may also be just a marker of poor health in general. That is, disorders such as heart disease, depression, and diabetes, even in their early stages, may result in vitamin D deficiency because they keep people indoors and thus reduce sun exposure. Healthy, active people may simply spend more time outdoors, so they have higher blood levels of D.
Moreover, many factors that tend to reduce vitamin D levels—such as aging, obesity, lack of physical activity, and an unhealthy diet—also increase the risk of many diseases. In good studies, researchers adjust the data statistically to account for such factors, but “residual confounders” remain a concern. When such statistical adjustments are made, the connection between low levels of vitamin D and deteriorating health weakens or sometimes even disappears.