If you want to burn fat and lose weight, aerobic exercise may be your best workout option, according to a study in the Journal of Applied Physiology.

It was more effective than a weight-lifting routine, and about as beneficial as workouts combining cardio and strength training, researchers found.

"If a person is going to give me three hours of exercise a week,the most effective way to lose fat is to spend that time doing aerobic training," says lead study author Leslie Willis, an exercise physiologist at Duke University Medical Center in Durham, North Carolina.

If you lift weights, this doesn't mean you should stop, but if your time is limited and your main goal from exercise is to lose weight, cardio may be better than weight training, according to Willis.

The study

Because people often short-change exercise, the scientists wanted to find out what type of routine would provide overweight people the most efficient workout for weight loss. For eight months, more than 200 middle-aged adults were assigned to one of three exercise programs.

The first group did aerobic training for 133 minutes a week. This averaged out to three 45-minute sessions, with most people choosing to work out on treadmill or elliptical machines. Those in the second group spent the same amount of time getting fit, but lifted weights instead of doing cardio. The others did both types of exercise, which took twice as long. The adults in the study were all overweight or obese, about 50 years of age, and did not have diabetes.

The results

The scientists found that routines that included only weight lifting built muscle, but did not help people lose fat or shed pounds.

Willis said this surprised some experts who say increasing muscle mass helps us burn more calories by increasing our metabolism. Unlike fat, it takes a certain amount of energy to simply maintain muscle. But the Duke researchers say the calories burned from having more muscle were not enough to make a difference when it comes to weight loss.

The researchers also found that cardio workouts proved to be as beneficial for losing fat and weight and required only half the time commitment as the combination workouts.

"They (the combination aerobic/weight training group) did lose more weight around the middle, but it was not statistically significant," says Willis. "We're a little bit unclear as to why that is, but we would say that time-wise, it was not that much more beneficial to do double that work and not get that much more benefit."

Workout advice

But Willis and other experts agree strength training shouldn’t be discounted when it comes to overall health. Lifting weights helps build and maintain strong bones and helps prevent the gradual loss of muscle that comes with aging.

Weight training may not be the best way to take the weight off initially, it does help adults keep it off, says Dr. Melina Jampolis, a physician nutrition specialist and CNN's diet and fitness expert. And though having added muscle may not burn as many calories as once thought, it does pay off over time.

“If you are short on time and are not dieting, cardio is better for weight loss in the short-term, but we don’t know in the long-term. And in my opinion, to maintain weight loss and achieve optimal health, strength training is still very important,” explains Jampolis.

But some have to get the weight off first before we can then turn their attention to keeping it off, Willis says. And if there’s only time for one type of workout, cardio may be the best choice.

“The point here is that the general public who comes to us to exercise almost unanimously has one main goal, and that is to lose weight and fat," Willis says. "And we’re just not seeing that resistance training would be the most efficient way to do that."