Vitamin D has gotten unending amounts of press lately. It's been touted for a whole number of health conditions, including cancer, immune conditions, heart disease, and diabetes. That said, research into the therapeutic effects of vitamin D supplementation have been inconsistent, and there's been some discussion of the importance of blood levels of vitamin D as a test. The version of vitamin D tested by health care professionals is not the active form of vitamin D, but is a 'pre-hormone' that acts as a reservoir and is converted into vitamin D for use by the body. Researchers are asking themselves what exactly this test measures, and whether raising those levels has a therapeutic effect.
Enter into this a paper that was recently published which suggests that serum vitamin D levels are the result of chronic disease, rather than the cause of it. The authors postulate that during chronic disease, vitamin D levels fall, rather than inadequate vitamin D levels being the cause of the chronic disease. The paper is a review of hundreds of individual studies of vitamin D, some of which looked at serum levels of vitamin D, while others looked at the effect of vitamin D supplementation on disease. While I think it's of limited clinical use to assess the effect of vitamin D on health generally, rather than on specific diseases (consider the results we'd get if we were to assess the benefits of a statin cholesterol drug on human health over a variety of conditions), the study nonetheless deserves enormous plaudits for the sheer scope of the study. With 290 studies to draw from, the information they generated should have some weight.
In summary, the findings were this: Low blood levels of vitamin D appear to be the result of chronic diseases, rather than their cause. The researchers noted many, many associations of low vitamin D with conditions such as cardiovascular disease, multiple sclerosis, mood disorders, and weight gain, but did not find strong evidence that vitamin D supplementation resolved these issues. As stated previously, there are some issues with applying this information - reconciling the broad conclusions of this study (that vitamin D supplementation does not affect disease generally) with the specific conclusions of others will be a challenge for clinicians.
The primary concession the authors make is that among the elderly, vitamin D supplementation has a clear beneficial effect on reducing overall mortality. They were hesitant to discuss why this was, but agreed that vitamin D supplementation was an important intervention for the elderly, who are susceptible to a number of ailments.
Nonetheless, the idea that the body responds to chronic disease by lowering serum levels of vitamin D is not without precedent - it does similar things with iron. However, the reason that the body appears to do this is still unclear. Stay tuned as we continue to learn more about vitamin D.
Vitamins have risen and fallen in the past, be they vitamin C or vitamin E. This study will likely not prove to be a nail in the coffin of vitamin D, but it will certain shape our understanding of vitamin D in the future. Consumers should still note the fact that this study will not change recommended supplementation guidelines. The Institute of Medicine increased the recommended amount a few years ago from 400IU to 800IU for healthy adults, and that number still stands. Supplementing with vitamin D, despite the findings of this study, is still strongly recommended.