Weight Training Helps Keep Men's Waistline from Expanding

People who lift weights every day may accumulate less belly fat over the years, compared with people who spend the same amount of time doing aerobic exercises, according to a new study. The results come from a study among men.

Combining weight training and aerobic activity, which is particularly beneficial for heart health, would be optimal, researchers said.

"When we age, we lose muscle mass and we tend to accumulate more body fat. If you only engage in aerobic activities, such as running, jogging, you will end up losing fat, as well as muscle mass," said study co-author Rania Mekary, a researcher at Harvard School of Public Health. "That's why you need to supplement your workout with resistance training, in order to preserve the muscle mass." [7 Common Exercise Errors and How to Fix Them]

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However, that is not to say that aerobic activities are less important for health, Mekary emphasized. Doing cardio could help prevent conditions such as diabetes, heart disease and even cancer, she said.

For the study, the researchers looked at 10,500 healthy American men ages 40 and over who participated in the Health Professionals Follow-Up Study between 1996 and 2008. Specifically, the researchers examined participants' weight, physical activity and waist circumference to study how changes in the men's activity levels over 12 years affected their waistlines. For older adults, waist circumference is a better indicator of healthy body composition than body mass index, the researchers said.

Men's waist sizes tend to increase as they age, and the men in the study showed a 1.2-inch (3 centimeters) increase in their waists, on average. But the men who engaged in weight training for 20 minutes a day had a smaller increase than f men who spent the same amount of time doing aerobic exercises, according to the study. The results held after controlling for other factors that affect weight, such as diet.

The researchers also found that the men who increased their sedentary behaviors, such as watching TV, over the course of the study had a larger increase in their waistlines than those who didn't increase sedentary time, according to the study published today (Dec. 22) in the journal Obesity.

"Engaging in resistance training or, ideally, combining it with aerobic exercise could help older adults lessen abdominal fat while increasing or preserving muscle mass," Mekary said. Although the study was among men, the researchers suspect that the results are true for women as well, she said.

Some previous studies have found that aerobic exercises work better than weight lifting for people who are trying lose belly fat. But those studies looked at the effects over shorter terms, often over several months, compared to the new research.

A phenomenon called excess post-exercise oxygen consumption could explain why people in the new study gained less fat over the years if they engaged in weight training, Mekary said. In order to burn fat, the body needs oxygen, and in weight training, the body's metabolic rate remains high up to two days after exercise, resulting in more fat-burning.

Email Bahar Gholipour. Follow Live Science @livescience, Facebook & Google+. Originally published on Live Science.