For example, in this JNCI study, the numbers of certain cancer deaths may have been too small to reach an accurate conclusion. Or, perhaps the follow-up wasn't long enough, or perhaps the impact of vitamin D would have been greater if the blood test had been performed in younger people who were then followed for many more years.
The authors concluded with a statement that "additional studies with large numbers of samples of measured (vitamin D) levels, preferably at multiple time points, are needed to confirm the total cancer mortality findings of this paper and to obtain more accurate risk estimates for mortality from specific cancers."
An editorial in the same issue of the Journal agrees, and I particularly appreciated the conclusion of this editorial:
"Whether vitamin D reduces cancer risks and, if it does, whether these amounts suffice are actively being debated. Randomized clinical trials of the effects of vitamin D on the incidence of colonic polyps and invasive cancer are needed. While vitamin D may well have multiple benefits beyond bone, health professionals and the public should not in a rush to judgment assume that vitamin D is a magic bullet and consume high amounts of vitamin D. More definitive data on both benefits and potential adverse effects of high doses are urgently needed."
I couldn't agree more.
Since vitamin D has come into the spotlight, I have transitioned from being a skeptic to believing there may in fact be a role of vitamin D in reducing the risks of a variety of cancers. The evidence has been inferential, but I can't ignore the weight of that evidence suggesting that such a relationship exists.
But the American Cancer Society has resisted the temptation to draw the conclusion that we currently know enough to make a general recommendation to the more than 300 million people in this country (and throughout the world) that they should markedly and routinely increase their vitamin D intake.
We have consistently called for more research into this topic. This is especially important given our past experience with other vitamins, such as vitamin C and beta-carotene, where well-qualified experts touted the benefit of those vitamins in reducing cancer risk.
When the studies were actually done, we discovered that the vitamins had either no effect or, for some people, may have actually increased their risk of cancer.
So what is the bottom line? Is this study the end of the vitamin D debate (except perhaps for colorectal cancer and breast cancer)?
I don't think this study should end the discussion of the possible role of vitamin D in cancer prevention, any more than I think the other studies answer the question that we need to embark on a major nationwide effort to increase vitamin D intake to supernormal levels.
We need more research, we need more information and we need to answer the question definitively.
Until we get those research studies, it is essentially an individual decision what to do, perhaps in consultation with your medical professional.
We do not have all the answers about the possible benefits of vitamin D in preventing cancer (or lack thereof), and we don't have all the information about the risks.
Hopefully, as this debate continues, we will be able to move the research agenda forward and provide solid answers and guidance to replace the uncertainty that many of us currently face regarding the role of vitamin D in cancer prevention.
Len Lichtenfeld is deputy chief medical officer of the American Cancer Society. You can view the full blog by clicking here.