Currently, nearly two-thirds of Americans are overweight. By 2030 it is estimated more than half our population may be clinically obese. Childhood obesity has tripled, and most children 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 life span than that of the previous generation.
The food industry blames inactivity. We just need to move more, they say. 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 has swept the globe since the 1980s.” 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. We still need to exercise more, but the priorities for reversing the obesity epidemic should focus on the overconsumption of calories. American children are currently eating about an extra 350 calories (equal to about a can of soda and small fries), and adults are eating about an extra 500 calories (equal to about one Big Mac). We’d have to walk two hours a day, seven days a week to burn off those calories. So exercise can prevent weight gain, but the amount required to prevent weight gain may be closer to twice the current recommendations. It’s more effective to stick to foods rich in nutrients but poor in calories: see my video Calculate Your Healthy Eating Score. It’s cheaper too, see Best Nutrition Bang For Your Buck.
Public health advocates have been experimenting with including this kind of information. One study found that fast food menus labeled with calories and the number of miles to walk to burn those calories appeared the most effective in influencing the selection of lower calorie meals.
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 and thousands of cases of cancer. But why don’t we lose more weight from exercise? It may be because we’re just not doing it enough. “The small magnitude of weight loss observed from the majority of exercise interventions may be primarily due to low doses of 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 may think, “Hey, I could get a side of fries with that.” But if we actually hook people up and measure energy expenditure during the act (and the study subjects don’t get too tangled up with all the wires and hoses) it may be only close to 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. Due to baseline metabolic needs, he would have spent roughly one third of that just lying around watching TV, so the incremental benefit is plausibly on the order of 14 calories. So maybe he could have one fry with that.
Michael Greger, M.D.
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