If you are spending more time running, walking or pumping iron in the gym and still not losing weight, fitness experts say it could be due to too big a reward for still too little exercise.

Although fitness has indisputable health benefits, it takes a lot of walking or running to burn off the calories in a doughnut.

"There's a war between exercise and nutrition in our heads," said American Council on Exercise spokesperson Jonathan Ross. "People tend to overestimate the amount of physical activity they get. They work out a little bit and treat themselves a lot."

A report by the University of Washington's Institute for Health Metrics and Evaluation showed that although Americans say they are more active, it has not made much of a dent in the obesity epidemic that affects more than one-third of US adults.

Ross, a personal trainer based outside Washington, DC, said exercise can play a role in weight reduction, but without broader lifestyle and nutritional changes, that role is limited.

"We put exercise in a box and once that exercise box is filled in we don't do much the rest of the day," he explained, adding that a post-workout calorie-dense treat doesn't help.

"Some (weight-loss) programmes stress nutrition, some stress exercise," he said. "But the two together are greater than the parts."

The National Weight Control Registry, which gathers information from people who have successfully lost at least 30 pounds and kept it off for a least one year, reports that 90 percent of its members exercise, on average, about one hour per day.

US health officials recommend that healthy adults get at least 150 minutes of moderate-intensity physical activity a week, or around 20 minutes a day.

Dr. Joseph E. Donnelly, an exercise physiologist with the American College of Sports Medicine said the U.S. government guidelines are for cardio vascular fitness, not weight loss.

"It was never intended for weight management," said Donnelly, a researcher who focuses on obesity at the University of Kansas. "People have misused it."

He added that studies suggest 250 to 300 minutes of exercise per week may be the minimum to lose weight.

"At 150 (minutes) the best you can hope for is weight maintenance."

Donnelly said if there is a success story for the role of exercise in weight control, it is in maintenance.

"Even among the naysayers who feel you can't lose much weight through exercise, most people agree it seems important to maintain weight," he said.

Dr. Michele Olson, professor of exercise physiology at Auburn University Montgomery, in Alabama, said it is difficult to shed pounds through exercise alone.

"One pound of fat has 3,500 calories," she explained. "If you ran a 26-mile marathon, where you burn about 100 calories per mile, you would burn 2600 calories, falling 900 calories short of burning one pound of fat."

She added that people must be physically active regardless of their size or whether they are losing weight.

"Moderately intense exercise done in as few as 10-minute increments two to three times a day markedly reduces our risk of all causes of mortality, heart disease most effectively but all other causes, including cancers, deaths due to hypertension and strokes, etc.," she said.

Ross said for many of his clients the goal is to maintain vitality and capability.

"For anyone out there who is frustrated about not losing weight, try to focus instead on what you love about your life," he said. "The No. 1 goal is to feel better in your body. That's what exercising is good for." 

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