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,” Bennett, who wasn’t involved in the new study, told Reuters Health.

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, according to the analysis. 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 does not just burn off calories, more importantly it helps to build muscle mass which is beneficial for long-term weight loss and/or weight maintenance,” lead author Mandy Ho, from the University of Sydney, Australia, told Reuters Health in an email.

“This is particularly important for the growing kids because over restricting dietary intake may cause adverse effects on normal growth and development.”

Ho and her colleagues found some measures of cholesterol and blood sugar, including insulin and HDL (“good”) cholesterol, improved with the addition of regular exercise. But changes in other levels, such as 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, the study team wrote in JAMA Pediatrics.

Helping young people lose weight, and especially keep it off, has proven a difficult challenge. One recent study, for example, found little evidence that at-home weight loss programs can affect kids’ BMI (see Reuters Health story of June 12, 2013 here: reut.rs/1a59b7d).

Bennett said programs that can change both diet and exercise habits probably are most effective. But for parents who are struggling with a heavy child, “diet is absolutely critical,” he said.

“That’s really where I would tell parents to focus their time.”

SOURCE: JAMA Pediatrics, online June 17, 2013

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