If you're looking to shape up, researchers at the University of Copenhagen say 30 minutes of rigorous exercise can be as effective as an hour when it comes to shedding weight.

The team studied 60 heavy but healthy men between 20 and 40 years old who wanted to lose weight. Twenty-one were directed to get 30 minutes of aerobic exercise — running, cycling and rowing — daily. Twenty-one were told to get 60 minutes, and 18 were assigned to a control group that remained sedentary. The results showed that exercising for 30 minutes at a pace hard enough to break a sweat was sufficient to promote weight loss.

"I would have expected that more training would be better, perhaps not double as good," Mads Rosenkilde, a doctoral student who participated in the study, said in an e-mail. "What surprised me the most was the larger than expected loss of “body calories” (exclusively fat mass) in the [moderate] group."

The men who got active for 30 minutes daily burned 300 kcal per session and lost 3.6 kilograms in 13 weeks, while those who exercised for an hour a day burned 600 kcal and lost 2.7 kilos, according to the research, published in the American Journal of Physiology. The reduction in body mass was about 4 kilos for both groups, the paper notes.

"In fact we can see that exercising for a whole hour instead of a half does not provide any additional loss in either body weight or fat," Rosenkilde said in a statement announcing the results. "The men who exercised the most lost too little relative to the energy they burned by running, biking or rowing. Thirty minutes of concentrated exercise give equally good results on the scale."

Rosenkilde speculates that getting 30 minutes of exercise daily is so easily achieved that study participants remained active throughout the day. He also speculates that those who exercised for 60 minutes consumed more calories during the day and therefore lost less weight than expected.

The typical American spends just two hours a week exercising, according to researchers at Penn State and the University of Maryland, even though the U.S. Centers for Disease Control and Prevention recommends adults age 18 to 64 get four hours of exercise per week.

"These findings means that engaging in endurance training for loss of body fat doesn’t have to demanding task," Rosenkilde told Wired. "In fact you can do less for more."