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Obese teenagers seeking to trim body fat and slim their waistlines will yield greater health benefits by combining both heart-pumping and muscle-strengthening activities, a new Canadian study suggests.

Researchers at the University of Calgary and the University of Ottawa led the Healthy Eating Aerobic and Resistance Training in Youth (HEARTY) trial. In the paper, published in the Journal of the American Medical Association Pediatrics, researchers cited the study's importance was due to the "little evidence" that exists on which type of exercise is "optimal" for obese teens.

"Traditionally, aerobic exercise has always been prescribed to adults, not to children because it's been most studied in the literature and with research," said study co-author Angela Alberga, a post-doctoral fellow at the University of Calgary. Dr. Alberga said their study is the first that has the largest sample to evaluate differences between solely aerobic and resistance training – as well as a combination of the two – and its effects on teens.

The study recruited 304 previously inactive obese teens between the ages of 14 and 18, 70 per cent of whom were female. Exercise training took place at six community-based facilities in Ottawa and Gatineau, Que. For the first month, all participants took part in both aerobic and resistance training. Aerobic training involved use of bikes, elliptical machines and treadmills, while resistance training referred to use of pulley-type weight machines or exercises using body weight as resistance, such as pushups and situps.

After the first month, participants were divided into four groups: one group that only did aerobic training, one that did only resistance training, another that did combined aerobic and resistance training and a fourth group that didn't exercise at all. Each group was monitored for an additional five months.

Among the groups who exercised, the teens who did so at least three to four times a week had greater health benefits – regardless of the type of training, Dr. Alberga said. However, those that did both aerobic and resistance training had a noticeable advantage.

"When we looked at teenagers who exercised three to four times per week, we found that the combined training group had larger decreases in per cent body fat compared to the group that only did aerobic exercise alone," said Dr. Alberga, noting the combined group had a 2.4-per-cent decline versus 1.2 per cent for the aerobic group. Among those who exercised three to four times a week, the combined training group also saw a significant slimdown in waist circumference, a reduction of 6.9 centimetres compared with the resistance group (4.8 centimetres) and the aerobic group (3.7 centimetres), Alberga said.

The combination of aerobic and resistance training has benefits because they target different energy systems, she noted.

"The resistance training is well-known to increase muscular strength and muscular endurance and actually affects some of the muscle fibre types," Dr. Alberga said. "Aerobic training is known for improvements in cardio-respiratory fitness – so that's the ability of your heart and your lungs to really perform well during exercise.

Ultimately, activity in any form is beneficial for kids and teens "of all shapes and sizes," Alberga said.

"Regardless of what exercise they do, they will get health benefits."