The researchers describe the findings of their randomized controlled trial in a study reported online recently in the American Journal of Physiology.
The researchers suggest one reason for the surprising result is that the exercise felt "doable" for the participants in the 30 minutes a day group, who even felt afterwards that they could have done more. Whereas in the 60 minutes a day group, they probably compensated by eating more, therefore losing less weight than expected.
This second point would fit in with the results of previous research that the researchers point to in their background information. This suggests that the reason exercise often produces a disappointing amount of weight loss is because a diet-induced negative energy balance (where calories consumed aren't enough to cover daily energy needs) often triggers "compensatory mechanisms", such as lower metabolic rate and increased appetite.
Perhaps 60 minutes of exercise results in more overcompensation than 30 minutes.
On average, the men who exercised 30 minutes a day lost 3.6 kg in three months, and those who exercised 60 minutes a day lost 2.7 kg. The reduction in body fat was about 4 kg for both groups.
The result is significant because 40% of Danish men are thought to be moderately overweight. Overcoming barriers to exercise in a group that does none at all, should be easier if the aim is to attain 30 minutes a day than 60 minutes a day.
The study is part of an interdisciplinary trial called FINE, a a Danish acronym for Physical Activity for a Long Healthy Life, which has generated strong data in a group of 60 or so participants.
Researchers suggest that 30 minutes of exercise hard enough to cause a sweat produces as much loss in body weight and fat as a whole hour.
Ethnologists were also part of the FINE research team: they were interested in exploring the barriers to exercise and in helping the participants overcome entrenched cultural habits.
For the study, the researchers randomly assigned each of 62 healthy, sedentary, moderately overweight young men to one of three groups: a high exercise group (burning about 600 kcal per day with about 60 minutes of aerobic exercise), a moderate exercise group (300 kcal per day, 30 minutes exercise), and a control group that continued to be sedentary.
They monitored the men as they followed their program for 13 weeks.
The participants trained every day through the study period. The training sessions were planned to produce a light sweat, but the participants were also instructed to increase the intensity three times a week.
The results showed that body weight went down by 2.7 kg in the high exercise group, and 3.6 kg in the moderate exercise group. Fat mass went down by 4.0 kg and 3.8 kg respectively.
But what was very surprising was although the energy burned during exercise in the high exercise group was double that of the moderate exercise group, they had roughly the same accumulated energy balance.
Energy balance is the balance of calories consumed through eating and drinking compared to calories burned through physical activity.
In this study, the researchers calculated it from changes in body composition.
The results showed that the accumulated energy balance was 83% more negative than expected in the moderate exercise group (ie better than expected), and 20% less negative than expected in the high exercise group (ie worse than expected).
"No statistically significant changes were found in energy intake or non-exercise physical activity that could explain the different compensatory responses associated with 30 vs. 60 min of daily aerobic exercise," write the researchers, who conclude:
"... a similar body fat loss was obtained regardless of exercise dose. A moderate dose of exercise induced a markedly greater than expected negative energy balance, while a higher dose induced a small but quantifiable degree of compensation."
First author Mads Rosenkilde, a PhD student in the Department of Biomedical Sciences, told the press:
"Participants exercising 30 minutes per day burned more calories than they should relative to the training program we set for them. 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."
"The men who exercised the most lost too little relative to the energy they burned by running, biking or rowing. 30 minutes of concentrated exercise give equally good results on the scale," he added.
Rosenkilde said they were surprised by the results, and the team now wants to study the effect of other forms of exercise.
He would like to explore ways of building exercise into daily life, for instance as a form of transport:
"Training is fantastic for your physical and mental health. The problem is that it takes time. If we can get people to exercise along the way - to work, for example - we will have won half the battle," said Rosenkilde.