David H. Freedman

Newsflash: Exercise is good for your health. So is cutting down on high-fat meats, and on sugar and refined carbs such as white flour and white rice. Are you surprised? I’m not, personally, but it sounds as if a lot of people out there are finding these notions hard to believe, because we keep seeing health headlines that announce the latest study to discover that following these straightforward and generally well-accepted suggestions lowers the risks of various diseases, and of obesity.

Why do studies keep pounding these points home, when most people who actually care what studies have to say on these questions (as most people in America probably do not) have long since come to believe they’ve heard enough to make up their minds? Well, for one thing, that’s how science is supposed to work–you keep validating theories with new tests, because the new tests may be better than the old ones, and might turn up different answers or at least new twists on them. And the accumulated results from many studies give scientists a much better handle on what’s really going on. (The far bigger problem in science is that the vast majority of published research isn’t validated at all, but that’s a different story.)

Not only that, but the fact of the matter is that every so often an at least fairly credible study turns up evidence that suggests these standard healthy habits may not do as much for us as we’d like to think, and may even hurt. Thus there was the recent scientific review that suggested there is little solid evidence that any of the recommended lifestyle and nutritional habits lower the risk of Alzheimer’s, even though we’ve been hearing that they do for years. There are studies that, in violation of what virtually everyone who has lost weight and kept it off knows, have found that exercise doesn’t make any difference in controlling weight, a claim transmitted without doubt or qualification by former U.S. FDA commissioner David Kessler in his best-selling book on obesity. There are studies that find that eating a lot of saturated fat is just fine (these are pushed by the Atkins diet fanatics, of course), and others that conclude people would be thinner and healthier if they loaded up every morning on a high-carb breakfast. More than one study has found that being overweight may help you live longer.

So what’s up with all that? Probably several things. First of all, bear in mind that scientists themselves have recognized (as I’ve documented at length elsewhere) that even most prestigiously published medical research is refuted within a few years. Research is hard enough when done impeccably, but it is in fact plagued with surprising levels of sloppiness, bias, fudging and other distorting shenanigans. So no results should be surprising–someone, somewhere will manage to screw a study up and get exciting but false results that some perfectly good journal will be thrilled to publish. In addition, different researchers who are supposed to be studying the same thing often end up measuring very different things in very different ways, and in very different sorts of people. Thus one study might conclude that a certain diet lowers the risk of, say, diabetes while another concludes it does not–simply because one of the studies measured blood-sugar levels in healthy patients while the other measured deaths from diabetes-related complications in very ill patients. (These measurement mismatches may be part of the problem with the studies that find obesity can be protective–it seems to prove so only for older patients and those with extensive heart disease, perhaps because old or sick people who are thin face greater risks from nutritional shortages than from the slower-acting ravages of obesity, or so it has been speculated.)

But there’s an even more basic reason why we shouldn’t be surprised that studies conflict on the healthfulness of nutrition and habits. There are thousands of factors that come into play when it comes to the ways various nutrients, habits and other environmental factors affect us, and they interact in a complex web that science may not yet have barely begun to unravel. Scientists try to tease the roles of individual factors out in their studies, but rarely can they honestly claim to have succeeded. (And let’s not even get started here on the fact that they often base their claims on animal studies.)

All of these challenges conspire to distort study results and further confuse an already confusing picture. At least with exercise, satured fat and refined carbs there’s a reasonably strong consensus of evidence, if not an unblemished one. That’s why I’m going to keep exercising, and limiting saturated fat and refined carbs. I’m not fully confident any of that will do much for me, but at least I know I’m playing the odds.