What do we really know about how to keep ourselves healthy? There are the obvious steps: exercising, not smoking and avoiding obesity, high blood pressure and undesirable cholesterol levels. Having good genes is certainly important, but obviously that’s not under our control. Not being poor helps, too. Beyond that, there’s a great deal we don’t know about how to stay healthy. This generates continual guesswork and lots of studies.
Unfortunately, the results of these studies are often misleading or contradictory. In large part this is because of the fundamental limitations of some of the basic tools used by medical researchers, especially epidemiological studies. The Wellness Letter has discussed these limitations—we even ran a long article called “Why Do Those #&*?@! Experts Keep Changing Their Minds?” I think some key points are worth restating, since they color so much of what we report on.
Most of the studies that make news these days are epidemiological studies. An indispensable arm of research, epidemiology focuses on the distribution of disease among certain populations and tries to find links to diet, lifestyle or other factors. The downside of studying people in the real world is that so many variables come into play.
Though epidemiological studies can identify associations, they seldom can prove cause and effect. In our article on vitamin D, for instance, we say that many studies have found that people with high blood levels of D have a reduced risk of various disorders, such as heart disease and some cancers. That is not the same as saying “vitamin D prevents heart disease,” though it’s fair to say that it “may help prevent heart disease” (with the emphasis on may). People with high vitamin D levels may be health-conscious in many ways, which could account for the reduced risk. For instance, they may exercise outdoors more and thus get more sun, which will increase D levels. Well-designed research controls for such “confounders” statistically, but these hidden variables can’t all be identified and can distort results.
Epidemiology is most useful for identifying powerful risk factors—smoking, for lung cancer, for example. It is less effective at assessing risk when associations are weaker or more complex—say, between diet and cancer.
Such studies do generate hypotheses that can be tested in clinical trials. Some large, long-term trials are now underway concerning vitamin D. Most hypotheses turn out to be wrong, however, so acting upon them before they are confirmed by a body of research often turns out to be wishful thinking.
Science makes progress by the process of trial and error. I suggest that you retain a spirit of inquiry and a healthy skepticism, but not lapse into cynicism when you hear about flip-flopping research results or about common wisdom that is overturned. It’s always an interesting challenge to follow health news, and we’ll keep trying to present the big picture and provide clear-headed advice.