If vitamin D level in the blood is merely a biomarker, a consequence of good or bad health, then adding vitamin D to the diet will not necessarily improve your health. To really know whether vitamin D supplementation is beneficial, we need to look at interventional studies, where supplements are provided, and the outcomes are compared with those of control subjects who don’t get the supplement. In fact the two above-cited studies on the effects of supplementation on bone fractures in older women, and prostrate cancer in older men are two such interventional studies. And they showed that vitamin D supplementation was harmful in both cases. And note that the positive Utah study I cited above–showing a correlation between low vitamin D levels and elevated incidence of cardiovascular disease and stroke–was an observational study, not an interventional one. The men in that study with the higher vitamin D blood levels and lower incidence of heart disease were not given supplements.
Furthermore, low levels of 25-D are frequently noted in patients with autoimmune disease, leading to a current consensus that a deficiency of the secosteroid may contribute to the autoimmune disease process. However, Marshall and team explain that these low levels of 25-D are a result, rather than a cause, of the disease process. Indeed, Marshall's research shows that in autoimmune disease, 25-D levels are naturally down-regulated in response to VDR dysregulation by chronic pathogens. Under such circumstances, supplementation with extra vitamin D is not only counterproductive but harmful, as it slows the ability of the immune system to deal with such bacteria.