 Right now, almost two-thirds of Americans are overweight, and by 2030, more than half our population may be clinically obese. Childhood obesity is tripled, and most of them will grow up to be overweight as well. The United States may be in the midst of raising the first generation since our nation's founding that will have a shorter predicted lifespan than that of the previous generation. The food industry blames inactivity— you just need to move more. But what is the role of exercise in the treatment of obesity? There is considerable debate in the medical literature today about whether physical activity has any role whatsoever in the epidemic of obesity that's swept the globe since the 80s. The increase in calories per person is more than sufficient to explain the U.S. epidemic of obesity. In fact, if anything, the level of physical activity over the last few decades has actually gone up in both Europe and North America. This has important policy implications. Yes, we still need to exercise more, but the priorities for reversing the obesity epidemic should focus on the overconsumption of calories. To work off the increased caloric intake, which for kids is like an extra can of soda and small fries, compared to what they're eating back in the 70s, and for adults like an extra big mac a day, to work that off, you'd have to walk like two hours a day, seven days a week. So exercise can prevent weight gain, but the amount required to prevent weight gain may be closer to twice the current recommendations. Public health advocates have been experimenting with including this kind of information. The fast food menu labeled with calories and the number of miles to walk to burn those calories appeared to be most effective in influencing the selection of lower calorie meals. Now exercise alone may have a small effect, and that small effect can make a big difference on a population scale. A 1% decrease in BMI nationwide might prevent millions of cases of diabetes and heart disease, thousands of cases of cancer. But why don't we lose more weight from exercise? Maybe because we're just not doing it enough. I mean, the small magnitude of weight loss observed from the majority of exercise interventions, you know, where they make people exercise, may be primarily due to how low the doses are of the prescribed exercise. People tend to overestimate how many calories are burned by physical activity. For example, there's this myth that a bout of sexual activity burns a few hundred calories. So hey, you could get a side of fries with that. But if you actually hook people up and measure energy expenditure during the act, and your study subjects don't get too tangled up with all the wires and hoses, though it may be nearly the metabolic equivalent of calisthenics, given that the average bout of sexual activity only lasts about six minutes, a young man might expend approximately 21 calories during sexual intercourse. Of course, he would have spent roughly one-third that just lying around watching TV, just basal metabolism. So the incremental benefit is plausibly on the order of 14 calories. So maybe we could have like one fry with that.