Let us treat those who are deficient, not everybody!
The authors of the extensive review on observational and interventional vitamin D studies have to be congratulated for doing the important and challenging work to sum up the available vitamin D research in humans. They can however, by definition not overcome the surprising problem that is clearly evident from many of the primary intervention studies: often vitamin D deficiency was not a necessary inclusion criterion for entry in these studies.
We all know how important antibiotics, insulin, thyroxin, aspirin etc. are - but what if we performed studies on these very powerful medications in patients who do not actually need them, say aspirin for everybody, antibiotics without bacterial infection, insulin without diabetes, thyroxin with normal thyroid function. What would we conclude? That these are inefficient treatments.
This is what is currently happening to vitamin D: without deficiency, no treatment is necessary, and obviously no advantage is possible.
Besides, many of the available RCTs are fraught with other problems as well: (very) small sample size (let the statisticians see this!), usually without prior power calculation, (too) small doses, (too) short follow-up, no baseline (and post-intervention) vitamin D level measured, etc.
Unfortunately, some of the ongoing vitamin D trials continue to make these mistakes, and I will not surprised if they too fail to find a benefit.
So let us stay rational and perform properly powered and designed RCTs to finally show that vitamin D is powerful as well, but only in those who are vitamin D deficient!
Competing interests: I have received speaker honoraria from Fresenius Kabi and I am a lead investigator in the VITdAL@ICU study - ClinicalTrials.gov identifier: NCT01130181