The Scientist and the Stairmaster: Why most of us believe that exercise makes us thinner—and why we're wrong. - By Gary Taubes
Let us begin with a short quiz: a few questions to ponder during the 30 (or 60 or 90) minutes a day you spend burning off excess calories at the gym, or perhaps while feeling guilty because you’re not so engaged. If lean people are more physically active than fat people—one fact in the often-murky science of weight control that’s been established beyond reasonable doubt— does that mean that working out will make a fat person lean? Does it mean that sitting around will make a lean person fat? How about a mathematical variation on these questions: Let’s say we go to the gym and burn off 3,500 calories every week—that’s 700 calories a session, five times a week. Since a pound of fat is equivalent to 3,500 calories, does that mean we’ll be a pound slimmer for every week we exercise? And will we continue to slim down at this pace for as long as we continue to exercise?
For most of us, fear of flab is the reason we exercise, the motivation that drives us to the gym. It’s also why public-health authorities have taken to encouraging ever more exercise as part of a healthy lifestyle. If we’re fat or fatter than ideal, we work out. Burn calories. Expend energy. Still fat? Burn more. The dietary guidelines of the U.S. Department of Agriculture, for instance, now recommend that we engage in up to 60 minutes daily of “moderate to vigorous intensity” physical activity just to maintain weight—that is, keep us from fattening further. Considering the ubiquity of the message, the hold it has on our lives, and the elegant simplicity of the notion—burn calories, lose weight—wouldn’t it be nice to believe it were true? The catch is that science suggests it’s not, and so the answer to all of the above quiz questions is “no.”
Just last month, the American Heart Association and the American College of Sports Medicine published joint guidelines for physical activity and health. They suggested that 30 minutes of moderate physical activity five days a week is necessary to “promote and maintain health.” What they didn’t say, though, was that more physical activity will lead us to lose weight. Indeed, the best they could say about the relationship between fat and exercise was this: “It is reasonable to assume that persons with relatively high daily energy expenditures would be less likely to gain weight over time, compared with those who have low energy expenditures. So far, data to support this hypothesis are not particularly compelling.” In other words, despite half a century of efforts to prove otherwise, scientists still can’t say that exercise will help keep off the pounds.
The 30 minutes recommended by the AHA-ACSM report is a departure from the recent guidelines of other authoritative organizations—the Institute of Medicine of the National Academies and the International Association for the Study of Obesity—both of which, like the USDA, have recommended that we exercise for up to 60 minutes a day to avoid what the USDA calls “unhealthy weight gain.” But the reason for this 60-minute recommendation is precisely that so little evidence exists to support the notion that exercising less has any effect.
The report that these experts cite most often as grounds for their assessments was published in 2000 by two Finnish researchers who surveyed all the relevant research on exercise and weight of the previous twenty years. Yet the Finnish report, the most scientifically rigorous review of the evidence to date, can hardly be said to have cleared up the matter. When the Finnish investigators looked at the results of the dozen best-constructed experimental trials that addressed weight maintenance—that is, successful dieters who were trying to keep off the pounds they had shed—they found that everyone regains weight. And depending on the type of trial, exercise would either decrease the rate of that gain (by 3.2 ounces per month) or increase its rate (by 1.8 ounces). As the Finns themselves concluded, with characteristic understatement, the relationship between exercise and weight is “more complex” than they might otherwise have imagined.
This is not to say that there aren’t excellent reasons to be physically active, as these reports invariably point out. We might just enjoy exercise. We may increase our overall fitness; we may live longer, perhaps by reducing our risk of heart disease or diabetes; we’ll probably feel better about ourselves. (Of course, this may be purely a cultural phenomenon. It’s hard to imagine that the French, for instance, would improve their self-esteem by spending more time at the gym.) But there’s no reason to think that we will lose any significant amount of weight, and little reason to think we will prevent ourselves from gaining it.
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Ultimately, the relationship between physical activity and fatness comes down to the question of cause and effect. Is Lance Armstrong excessively lean because he burns off a few thousand calories a day cycling, or is he driven to expend that energy because his body is constitutionally set against storing calories as fat? If his fat tissue is resistant to accumulating calories, his body has little choice but to burn them as quickly as possible: what Rony and his contemporaries called the “activity impulse”—a physiological drive, not a conscious one. His body is telling him to get on his bike and ride, not his mind. Those of us who run to fat would have the opposite problem. Our fat tissue wants to store calories, leaving our muscles with a relative dearth of energy to burn. It’s not willpower we lack, but fuel.
For the last 60 years, researchers studying obesity and weight regulation have insisted on treating the human body as a thermodynamic black box: Calories go in one side, they come out the other, and the difference (calories in minus calories out) ends up as either more or less fat. The fat tissue, in this thermodynamic model, has nothing to say in the matter. Thus the official recommendations to eat less and exercise more and assuredly you’ll get thinner. (Or at least not fatter.) And in the strict sense this is true—you can starve a human, or a rat, and he will indeed lose weight—but that misses the point. Humans, rats, and all living organisms are ruled by biology, not thermodynamics. When we deprive ourselves of food, we get hungry. When we push ourselves physically, we get tired.
Our bodies, like all living organisms, have evolved a fantastically complex web of feedback loops. These physiological mechanisms serve fundamentally to work against the inevitable pull of thermodynamics (which is entropy, a.k.a. death) and so make life possible. The necessary condition of life, as the great French physiologist Claude Bernard noted 140 years ago, is to keep the internal environment of an organism stable and conducive to life, regardless of what’s happening on the outside. This is what the Harvard physiologist Walter Cannon, in the thirties, called homeostasis—or the “wisdom of the body,” as he put it. “Somehow the unstable stuff of which we are composed,” Cannon wrote, “had learned the trick of maintaining stability.”
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If it’s biology, and not a lack of willpower, that explains why exercise fails so many of us as a weight-loss tool, then we can still find reason for optimism. Since insulin is the primary hormone affecting the activity of LPL on our cells, it’s not surprising that insulin is the primary regulator of how fat we get. “Fat is mobilized [from fat tissue] when insulin secretion diminishes,” the American Medical Association Council on Foods and Nutrition explained back in 1974, before this fact, too, was deemed irrelevant to the question of why we gain weight or the means to lose it. Because insulin determines fat accumulation, it’s quite possible that we get fat not because we eat too much or exercise too little but because we secrete too much insulin or because our insulin levels remain elevated far longer than might be ideal.
To be sure, this is the same logic that leads to other unconventional ideas. As it turns out, it’s carbohydrates—particularly easily digestible carbohydrates and sugars—that primarily stimulate insulin secretion. “Carbohydrates is driving insulin is driving fat,” as George Cahill Jr., a retired Harvard professor of medicine and expert on insulin, recently phrased it for me. So maybe if we eat fewer carbohydrates—in particular the easily digestible simple carbohydrates and sugars—we might lose considerable fat or at least not gain any more, whether we exercise or not. This would explain the slew of recent clinical trials demonstrating that dieters who restrict carbohydrates but not calories invariably lose more weight than dieters who restrict calories but not necessarily carbohydrates. Put simply, it’s quite possible that the foods—potatoes, pasta, rice, bread, pastries, sweets, soda, and beer—that our parents always thought were fattening (back when the medical specialists treating obesity believed that exercise made us hungry) really are fattening. And so if we avoid these foods specifically, we may find our weights more in line with our desires.
As for those people who insist that exercise has been the key to their weight-loss programs, the one thing we’d have to wonder is whether they changed their diets as well. Rare is the person who decides the time has come to lose weight and doesn’t also decide perhaps it’s time to eat fewer sweets, drink less beer, switch to diet soda, and maybe curtail the kind of carb-rich snacks—the potato chips and the candy bars—that might be singularly responsible for driving up their insulin and so their fat.
For the rest of us, it may be time to take a scientific or biological view of our excesses rather than a biblical one. The benefits of exercise include the joys of virtuousness. I worked out today, therefore I can eat fattening foods to my heart’s content. But maybe the causality is reversed here too. Maybe it’s because we eat foods that fatten us that the workout becomes a necessity, the best we can do in the battle against our own fat tissue.
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Related book: Good Calories, Bad Calories: Fats, Carbs, and the Controversial Science of Diet and Health
Related speech: The Quality of Calories: What Makes Us Fat and Why Nobody Seems to Care
Related article: “What if It's All Been a Big Fat Lie?” By Gary Taubes
Related previous post: Why Exercise Won't Make You Thin - By John Cloud
Related paper: “Why I Do All This Walking, or How Systems Become Fragile” By Nassim Taleb
