Harvard university just unveiled a

Upon checking the fitness habits of 10,500 healthy men aged 40 and up, the research showed that the guys who lifted weights for at least 20 minutes per day, every day got half as much belly fat over 12 years than those who did only cardio.

Although the male study participants who engaged in both kinds of exercise reduced significant fat gain as they aged, as weight training tops aerobic exercise in being able to keep off excess fat.

The researchers checked fat gain by measuring the men's waist circumferences, which shows a more accurate picture of health than body weight does. "As you age, you lose muscle mass whether you like it or not," according to 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 worrisome, as they seem to gain the flab in and around the gut. Having loads of that visceral fat, shoots up the risk of heart disease, diabetes, and even cancer.

The number one reason why weight training is great at beefing up and diminishing fat might be a no brainer: By lifting iron, you're constantly building and maintaining muscle, which doesn't really happen when you rely only on cardio exercises. However, there’s more to it.

"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," according to Mekary.

Receiving oxygen is vital for the body to be able to burn fat, she explains. As you increase oxygen uptake, the muscles develop more mitochondria, the energy increases in muscle cells. "When you have more mitochondria, your body is able to burn more fat as a source of energy."

However, it doesn’t end when you drop the barbells. "Other studies have found that even between sets, during these very short breaks, your muscles are pumped up and you continue to burn calories," according to Mekary. "Even 48 hours after you've weight trained, you're still burning more calories than a person who did not lift weights."

The study also revealed that spending additional time lifting can be even more beneficial, although it focused on only 20 minutes per day training. "We found a dose-response relationship between weight training and waist circumference," according to Mekary. "Basically, the more you do, the better. But at 20 minutes per day, you will see results."

However, it’s not easy transforming your muscles into fat-burning machines, Mekary says. If you're just starting to strength train, you can't expect miracles overnight. "It might take a few months or longer before you really see a difference," she notes.

This may be the reason why several past studies, done over short time frames, have shown the opposite of what her team concluded, which is that weight training is not more effective 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 most accurate way to keep your gut in check and your entire body in the best shape possible is to combine weight training and cardio. "We are not trying to discredit the many proven health benefits of aerobic exercise," according to Mekary.

"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.