The short and unsatisfactory answer is that you are human. A surprisingly large number of people who begin an exercise program subsequently pack on extra pounds. In a 2013 review of scientific studies related to exercise and weight loss, the researchers concluded that most sedentary people who start exercising lose far less weight than would be expected, given how many calories they burn while working out.
As Eric Ravussin, a professor at the Pennington Biomedical Research Center in Baton Rouge, La., told me some time back, “In general, exercise by itself is pretty useless for weight loss.”
The problem, most scientists agree, is that exercise makes us want to eat. Many studies have shown that if people start a new exercise program, their bodies begin to pump out much higher levels of various hormones that increase appetite. This reaction seems to be most pronounced if someone starts a new, moderate, aerobic exercise routine. (There are hints in some studies that intense exercise, such as interval training, may dampen appetite. But the relevant studies have been few and small.)
So if we take up jogging, swimming, walking or bike riding, for example, we feel unusually hungry afterward, and often wind up consuming as many or more calories than we just expended. A moderate 30-minute walk burns only about 100 calories. A smoothie afterward may contain two or three times that amount.
Simultaneously, many of us move less on days when we exercise, studies show. Scientists refer to this reaction as “compensatory inactivity.” Without consciously planning to, we spend more hours sitting during those days when we have worked out than when we have not, and ultimately burn fewer total calories.
In essence, starting to exercise often means we eat more and move less than we did before, consume more calories and gain weight.
But there is a solution, according to a comprehensive new review of decades’ worth of studies about exercise and body weight. Work out, the findings suggest, but also scrupulously watch what you eat. Pooling data from multiple past studies, it finds that if people exercise and restrict their calorie intake, they drop pounds — and in fact often lose more weight than by dieting alone.