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A new study from Harvard reveals that the best way to burn belly fat is not through running, swimming, or cycling. While cardiovascular exercise is important to ensure whole-body health, it turns out the true ticket to a trim waistline is daily strength training.

After analyzing the fitness habits of 10,500 healthy men aged 40 and up, the researchers discovered that the guys who lifted weights for at least 20 minutes per day, every day accumulated half as much belly fat over 12 years than those who did only cardio. Although the men in the study who did both kinds of exercise staved off the most amount of fat gain as they aged, when the effects of one type of workout were compared directly to the other, it became clear that weight training trumps aerobic exercise in its ability to keep off excess fat.

The researchers assessed fat gain by measuring the men's waist circumferences, which they say provides a far more accurate picture of health than body weight does. "As you age, you lose muscle mass whether you like it or not," says study coauthor Rania Mekary. "With the loss of lean muscle, you therefore gain more fat mass, which we know weighs less than lean muscle." For guys, this age-related shift in body composition can be especially troublesome, because they tend to accumulate that flab in and around the gut. Having a lot of that visceral fat, regardless of how much you weigh, increases risk of heart disease, diabetes, and even cancer.

TRAINING PLAN: The Real Weight-Loss Workout

The big reason why weight training is so effective at beefing up and diminishing fat might seem obvious: By pumping iron, you're continually building and maintaining muscle, which doesn't necessarily happen when you rely solely on cardio for exercise. But there's more going on, too.  

"When you weight train and build more strength and muscle mass, over the long-term, your muscles will adapt in a way that they are able to take in more oxygen," Mekary says. Taking in oxygen is necessary for the body to be able to burn fat, she explains. Through this process of increasing oxygen uptake, the muscles develop more mitochondria, the energy sparkplugs in muscle cells. "When you have more mitochondria, your body is able to burn more fat as a source of energy."

The fat-shredding doesn't end once you put down the barbells, either. "Other studies have found that even between sets, during these very short breaks, your muscles are pumped up and you continue to burn calories," Mekary says. "Even 48 hours after you've weight trained, you're still burning more calories than a person who did not lift weights."

Although this study zeroed in on the effects of 20 minutes per day of weight training, the results suggest that spending additional time lifting can be even more beneficial. "We found a dose-response relationship between weight training and waist circumference," says Mekary. "Basically, the more you do, the better. But at 20 minutes per day, you will see results."

However, transforming your muscles into fat-burning machines takes time, Mekary says. If you're just starting to strength train, you can't expect a svelte physique right away. "It might take a few months or longer before you really see a difference," she notes. This may explain why several past studies, which have been conducted over short time frames, have concluded the opposite of what her team found — that weight training is not any more effective at fighting fat gain than cardiovascular workouts. "Of course you wouldn't see much change in body composition after weight training for just a few weeks or even a month," she says. "The results will only show over the long-term. Just look at men who do a lot of weight training for many years. They usually have V-shaped bodies, with very narrow waists. This is a very good indication of how weight training works."

The perfect prescription for keeping your gut in check and your entire body in the best shape possible: Do both weight training and cardio. "We are not trying to discredit the many proven health benefits of aerobic exercise," Mekary says. "It is very important for lowering risk of diabetes, cardiovascular disease, and cancer." But if you've only got 15 or 20 minutes to spend at the gym? "Don't feel guilty if you just do weight training — the results will show over the long run," she says.