I should start by saying "I hate to say I told you so…" Recently I told you I was not convinced that the widespread advice to start taking vitamin D supplements to lower you risk of cancer would work. Why? Because the evidence is pretty slim and suspect.
And now, it seems, I was right. According to a recent study in the prestigious Journal of the National Cancer Institute, people with higher blood levels of vitamin D do not have an overall reduced risk for cancer when compared to people with lower levels of vitamin D.
The background here is that a number of studies have linked intake of vitamin D supplements with lower rates of several cancers. One very much hyped study, from Iowa, claimed to find an astounding 60% lower cancer rate in a group of post-menopausal women taking vitamin D supplements. The women were taking vitamin D to lower their risk of osteoporosis (vitamin D is essential to help calcium maintain bone strength). And it is this study that seems to have convinced the Canadian Cancer Society to come out with the trumpeted advice that all Canadians should consider taking vitamin D supplements to cut their risk of cancer, unless there was a good reason not to.
But there are problems with all the pro-vitamin D evidence, as I see it, which is why the Canadian Cancer Society advice worried me. All of the studies that found a lower cancer risk in vitamin D supplement takers were “weak” studies. Why? Because there are always very basic and intrinsic differences between people who maintain a schedule of taking supplements regularly and those who don’t. And it’s those differences that I believe account for most, if not all the discrepancies we find in these studies.
You see people who take supplements regularly (as well as those people who take all the pills they’re supposed to take when they’re enrolled in studies) are often significantly more disciplined than those who don’t take supplements. Supplement takers tend to be more educated, are nearly always far more concerned about their health, meaning they’re significantly more likely to have better diets, exercise more, drink less, and on and on. Even though researchers try to account for such differences in their conclusions, they really can’t. So it’s never a surprise to me that supplement-takers (and pill takers) are found to have better health outcomes than people who don’t take supplements. But that’s not because of the supplements; it’s because of the type of people who take supplements.
So that’s why all the vitamin D ballyhoo worried me: I just can’t see that vitamin D alone accounts for the differences in cancer rates.
Which brings me to a recent study that started by determining the blood levels of vitamin D in roughly 17,000 Americans, then followed those people for up to 12 years. In the end, the researchers found no difference in cancer death rates between those with high vitamin D levels and those with lower levels. The one exception was death from colorectal cancer, which was much lower in the high-vitamin D people, but the overall numbers were pretty small, so for me, this is probably a spurious finding.
Does this mean that vitamin D doesn’t lower the risk of some cancers? No. It might still prove to do so, but we’re going to need a very good study to prove that link, and no one is rushing to do such a study because vitamin D can’t be patented.
The bottom line, folks: we do know that a healthy lifestyle can lower your risk of cancer, but that involves watching your weight, doing some exercise, eating a healthy diet, limiting your alcohol intake, and not smoking. It does not, however, include taking any supplements.
Sorry, but I just report ‘m the way I see ‘em.
Art
Contact Art with your comments or questions at ahister@knowitallhealth.com
Reviewed October 31, 2007
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