The RDA for Vitamin C intake is 40mg per day – although, as with many health guidelines, the numbers suggested only really take into account the amount necessary to avoid health problems relating to a vitamin deficiency. In the fitness industry we’re far more focused on optimising health. I take vitamin C year round to avoid minor colds and flus, and if I do feel something creeping up on me I’ll take a high dose of 2000-3000mg, which I find often turns the tide in my favour and allows me to avoid getting sick. In my opinion, anyone with a strenuous and active lifestyle should consider supplementing Vitamin C as a general boost to the immune system.
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.