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The benefits of exercise are real. And stories about people who have lost a tremendous amount of weight by hitting the treadmill abound. But the bulk of the evidence tells a less impressive story.
Consider this review of exercise intervention studies, published in 2001: It found that after 20 weeks, weight loss was less than expected, and that “the amount of exercise energy expenditure had no correlation with weight loss in these longer studies.”
To explore the effects of more exercise on weight, researchers have followed everybody from people training for marathons to sedentary young twins, and post-menopausal overweight and obese women who ramp up their physical activity through running, cycling, or personal training sessions.
Most people in these studies typically only lost a few pounds at best, even under highly controlled scenarios where their diets were kept constant.
Other meta-analyses, which looked at a bunch of exercise studies, have come to similarly lackluster conclusions about exercise for losing weight. This Cochrane Review of all the best-available evidence on exercise for weight loss found that physical activity alone led to only modest reductions. Ditto for another review published in 1999.
University of Alabama obesity researcher David Allison sums up the research this way: Adding physical activity has a very modest effect on weight loss — “a lesser effect than you’d mathematically predict,” he said.
We’ve long thought of weight loss in simple “calories in, calories out” terms. In a much-cited 1958 study, researcher Max Wishnofsky outlined a rule that many organizations — from the Mayo Clinic to Livestrong — still use to predict weight loss: A pound of human fat represents about 3,500 calories; therefore cutting 500 calories per day, through diet or physical activity, results in about a pound of weight loss per week. Similarly, adding 500 calories a day results in a weight gain of about the same.
Today, researchers view this rule as overly simplistic.
They now think of human energy balance as “a dynamic and adaptable system,” as one study describes. When you alter one component — cutting the number of calories you eat in a day to lose weight, doing more exercise than usual — this sets off a cascade of changes in the body that affect how many calories you use up, and in turn, your body weight.