Talker, take an epidemiology and stats class. Since the internet can't always convey the attitudes, I am going to clarify that I am recommending this in a sincere and non-condescending way. You're young and you want to be in charge of your health for the rest of your life - it would benefit you greatly if you'd get the principles of medical literature appraisal down. The quoted study was a case-control study, it is an observational epidemiologic study. It did what it was intended to do - observe that in studied subsets lower vit D was associated with higher cancer rates. Even if the second comparison group had normal vit D and lowest cancer rates, the conclusion wouldn't have been that normal vit D (or supplementation thereof] protect from cancer; the conclusion would've been just the same - low vit D is associated with higher rates of cancer, and that's it. There is a very long distance between correlation and causation; not that it never happens - in fact, the idea of those studies is to find such strong associations that causation becomes very likely. The distance from observation to causation to proving the effectiveness of intervention is much longer though. We (as in medicine) go through this stuff all the time. What you see is what has been brought up in a mainstream media - vit D, vit E, whatever. What you don't see is a myriad of measurable substances, chemicals and particles that come up in research all the time. CRP, Mycoplasma antibodies, metalloproteinases, can't keep up with all of that. And then the same discussion happens all over again: is it a causative offending substance, or is it just a marker, and would intervention directed at that substance be effective.