Tacking regular exercise on to a diet program for obese kids and teens typically doesn't help them lose any more weight, a new review of past data suggests.
"Changing diet, improving diet, reducing calories is enormously important for weight loss both in kids and adults," said Gary Bennett, who studies obesity prevention at Duke University in Durham, North Carolina. "Exercise is important too, but I think we sometimes overemphasize how important exercise is."
For the new study, researchers analyzed results from 14 earlier trials that assigned overweight and obese youth to a diet and exercise program or a diet-only intervention. Those programs lasted anywhere from six weeks to six months.
Most studies found kids tended to have a lower body mass index (BMI) - a ratio of weight in relation to height - and a smaller percentage of body fat after completing either type of intervention. Adding aerobic exercise such as jogging or dance to a restricted-calorie diet had little effect on weight loss.
However, kids who did resistance training lost more body fat than those who didn't exercise. Strength training for an hour or less each week was tied to an extra half a percent drop in body fat and a greater increase in muscle.
Exercise burns off calories, and importantly it helps to build muscle mass which is beneficial for weight loss and long term weight maintenance.
The researchers also found levels of insulin (the hormone the clears sugar from the bloodstream) and HDL ("good") cholesterol, improved with the addition of regular exercise. But changes in LDL ("bad") cholesterol were greater with a diet plan alone.
In many of the studies, kids gained back the weight - and any cholesterol or blood sugar benefits went away - once the programs were over.
Helping young people lose weight, and especially keep it off, has proven a difficult challenge. Experts say that programs that can change both diet and exercise habits probably are most effective. But diet is an absolutely critical focus.
Source: JAMA Pediatrics, online June 17, 2013.
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