You’ve been running, walking or using the treadmill religiously and still you can’t manage to lose the belly (otherwise known as a heart attack waiting to happen). The answer could be simple: strength training. According to a new study, it attacks belly fat better than aerobic exercise.

It’s a fact of life: Most men’s bellies get bigger with age. For fighting belly bulge, "weight training had almost double the effect of lessening the increase in waist circumference," says Rania Mekary, PhD, a researcher at the Harvard School of Public Health who led the study, which was published in the journal Obesity.

''We aren't trying to discredit aerobic activity," Mekary says. In fact, she found aerobic activity was better than weight training at reducing overall body weight over the 12-year study. But body weight alone isn't as good a predictor of heart disease risk as belly fat. Ideally, men should do both weight training and aerobic exercise to minimize belly fat and maximize overall health, she says.

Previous studies over the years have produced conflicting reports, Mekary says, about which is better for losing the belly.

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Aerobics vs. weights

Mekary's team tracked the physical activity, waist circumference and body weight of more than 10,000 men, all age 40 and older, enrolled in the Health Professionals Follow-up Study between 1996 and 2008. The researchers compared levels and types of activity over the course of the study to see which activity had the most effect on men's waist sizes.

Overall, the men gained on average about 1.2 inches in their waist. Some gained six inches or more.

Here’s how the different types of exercise compared when it came to belly fat:

  • Weight training. Men who increased the time they spent on weight training by 20 minutes a day gained less in the waist than those who did aerobic activity. Some used weight machines; others used free weights. It didn't matter if they increased from zero to 20 minutes or added 20 minutes to what they already were doing, Mekary says. On average, they had about a quarter of an inch less waist gain than those who did aerobic activity; those who did 40 minutes had about half an inch less gain.
  • Aerobic activity. Guys who spent 20 more minutes a day doing aerobic activity gained less in the waist, too, but they didn't do as well at battling belly bulge as the weight training group. They minimized waist gain half as much as those who weight trained.
  • Everyday activities. Men who climbed stairs or did yard work controlled their expanding waistlines less well than either of the other groups.
  • Inactivity. Not surprisingly, those who increased their sedentary behavior, such as TV watching, did not minimize their gain at all. Their waists got larger.

So, if you have only 30 minutes today and want your waist to behave? Weight train, Mekary says.

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Why weight-training works

Weight training helps preserve muscle mass, Mekary says, which declines with age. Having more muscle mass can increase your metabolism, the rate at which you burn calories. "When you do weight training," Mekary says, "your total energy expenditure increases not only during the weight training but also after the exercise. Over the long term, it reduces overall body fat. Weight training is helping your body to use fat as energy."

What else might help?

Looking to get back to your college or wedding day waistline? That won’t be easy. On average, in the study, the best weight training or aerobic exercise could do was lessen the age-related increase in waist size, Mekary says, not return it to long-ago measurements. (Some men did much better than others, however.)

Adding these strategies could help.

Eating more fiber. Increasing soluble fiber (the kind found in oat bran, nuts, seeds and some fruits and vegetables) by just 10 grams a day helps decrease belly fat accumulation by nearly 4 percent, researchers reported in the journal Obesity in 2012. Two apples provide about 10 grams of soluble fiber.

Interval training. Interval training involves alternating between high and lower intensity aerobic activity (for instance, throwing some sprints into your jog). It can help burn belly fat, researchers reported this year in the Journal of Sports Medicine and Physical Fitness. Adults who did two days a week of regular gym training and two days of interval training reduced belly fat and waist circumference more than those who did four days of regular gym training during the eight-week study.

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Kathleen Doheny is a Los Angeles journalist specializing in health, behavior and fitness topics.