"Increasing physical activity—if people control caloric intake—will lead to weight loss," says William Haskell of Stanford University who helped craft the HHS, ACSM and AHA guidelines. But he cautions that exercise alone is unlikely to lead to the instant results most people want, leading them to become frustrated and give up. "[Suppose I do] 30 minutes of brisk walking five days per week," says Haskell. "If you say walking a mile expends 100 calories, and if I walk at 3 miles per hour, I burn an extra 150 calories per day," he says. "[Since one pound of fat is equivalent to about 3,600 calories], it could take three weeks to lose one pound. For most people, they are going to find this disappointing, [and] probably won't stick with it."
So for the average person, caloric intake—rather than calorie burning from exercise—appears to be the most important factor in weight loss. But even if calorie intake trumps exercise, this does not mean exercise does not play a key role in helping people stay trim.
"If you talk about energy balance [when calories consumed equal calories burned], definitely there is evidence that exercise contributes to energy balance," says David Stensel, an exercise physiologist at the School of Sport & Exercise Sciences at Loughborough University in Leicestershire, England. A study published this month by Stensel's team suggests that vigorous exercise suppresses the key hunger hormone, ghrelin, for up to 30 minutes after workouts and increases levels of the appetite-suppressing hormone peptide YY for as long as three hours after exercise.
Stensel also points to studies showing that exercising may encourage people to crave healthier fare, such as unrefined foods (like fiber-rich beans and veggies) rather than foods loaded with refined sugar (such as cookies and cakes).
Some past researchers claimed that exercise would lead to weight gain in the long run because it ups one's appetite. But Arthur Leon of the University of Minnesota says that theory has been shot down over the past decade. Some research suggests that it might lead to greater caloric intake, Stensel notes, but that does not necessarily translate into extra pounds. The increased calories, he says, are not enough to offset the calories burned—or energy consumed—during exercising.
The bottom line: couch potatoes may applaud the exercise naysayers but the bulk of research suggests that workouts make us physically and perhaps mentally healthier.