If you’re trying to lose weight — and who isn’t, right? — you may be confused about how to achieve the optimal balance of aerobic exercise and resistance training. Aerobic activity is supposed to burn lots of calories, but it can take a big chunk out of your day. Lifting weights or otherwise challenging your muscles is supposed to build lean mass, which, in turn, has been thought to accelerate calorie burning. Plus, it isn’t quite as time-consuming as running, swimming, biking or other popular aerobic workouts. For those with plenty of time, a combination of the two would seem to be ideal.

(Jonathan Newton/The Washington Post)

Researchers at Duke University divided 234 overweight or obese adult participants into three groups as follows: one group engaged in resistance training consisting of three days per week of weight lifting, three sets per day, 8-12 repetitions per set; another engaged in aerobic training, covering about 12 miles per week; a third group engaged in both kinds of activity. Full sets of final data were collected for just 119 of the initial participants.

Their physical activity was monitored to ensure participants were putting in similar amounts of time and effort.  Dietary intake was estimated to be between an average of 2,000 and 2,100 calories per day.

After a four-week “run-up” period designed to weed out those who would likely drop out during the study period itself, all groups did their thing for eight weeks.

All in all, aerobic exercise proved, at least in this study, to be a more efficient way to lose body fat than the other two approaches. Although the aerobic exercisers spent an average of 133 minutes per week engaged in their activities while the resistance-training group spent about 180 minutes per week training, and folks in the combined-activity group about twice as many minutes as those in either of the other groups, the aerobics and aerobics-plus-resistance group lost more weight than those who did resistance training alone; in fact, the resistance-training group actually gained weight, though that gain was attributed to increases in lean body mass. And while the combined-activity group did shed pounds and fat mass, those reductions weren’t any greater than those in the aerobics-only group. Still, those in the combination group did whittle their waist circumference more than the others; the authors attribute that result to the sheer amount of time they spent exercising.

“If increasing muscle mass and strength is the goal, a program including resistance training is required,” the study concludes. “However, balancing time commitments against health benefits accrued, it appears that aerobic training alone is the optimal mode of exercise for reducing fat mass and total body mass.”

The study, funded by the National Institutes of Health, was first published online, ahead of the print edition, in September 2012.