Half of the men were set to exercise for an hour a day, wearing a heart-rate monitor and calorie counter, while the second group only had to sweat for 30 minutes.
Research results showed that 30 minutes of exercise hard enough to produce a sweat was enough to turn the tide on an unhealthy body mass index.
On average, the men who exercised 30 minutes a day lost 3.6 kilo in three months, while those who exercised for a whole hour only lost 2.7kg.
The reduction in body mass was about 4 kg for both groups, researchers said in a statement.
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," researcher Mads Rosenkilde said in a statement.
The study was published in the American Journal of Physiology.