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.
Structural relationship of vitamin D 3 (cholecalciferol) and vitamin D 2 (ergocalciferol) with their respective provitamins, cholesterol, and a classic steroid hormone, cortisol (see inset box). The two structural representations presented at the bottom for both vitamin D 3 and vitamin D .2 are equivalent; these are simply different ways of drawing the same molecule. It is to be emphasized that vitamin D 3 is the naturally occurring form of the vitamin; it is produced from 7-dehydrocholesterol, which is present in the skin, by the action of sunlight (see Figure 2). Vitamin D 2 (which is equivalently potent to vitamin D 3 in humans and many mammals, but not birds) is produced commercially by the irradiation of the plant sterol ergosterol with ultraviolet light.