Can working out help us drop pounds after all?
The findings also indicate that, to benefit, we may need to exercise quite a bit.
In theory, exercise should contribute substantially to weight loss. It burns calories. If we do not replace them, our bodies should achieve negative energy balance, use stored fat for fuel and shed pounds.
But life and our metabolisms are not predictable or fair, as multiple exercise studies involving people and animals show. In these experiments, participants lose less weight than would be expected, given the energy they expend during exercise.
The studies generally have concluded that the exercisers had compensated for the energy they expended during exercise, either by eating more or moving less throughout the day.
These compensations were often unwitting but effective.
Some researchers had begun to wonder, though, if the amount of exercise might matter.
Many of the past human experiments had involved about 30 minutes a day or so of moderate exercise, which is the amount generally recommended by current guidelines to improve health.
But what if people exercised more, some researchers asked.
Would they still compensate for all the calories that they burned?
To find out, scientists from the University of North Dakota and other institutions decided to invite 31 overweight, sedentary men and women to a lab for measurements of their resting metabolic rate and body composition.
The volunteers also recounted in detail what they had eaten the previous day and agreed to wear a sophisticated activity tracker for a week.
The scientists then randomly divided them into groups.
One group began a program of walking briskly or otherwise exercising five times a week until its members had each burned 300 calories, which took most of them about 30 minutes. The sessions were individualized.
During the week, these volunteers burned 1,500 extra calories with their exercise program.
The other group began working out for twice as long, burning 600 calories a session, or about 3,000 calories a week.
The exercise program lasted for 12 weeks. The researchers asked their volunteers not to change their diets or lifestyles during this time and to wear the activity monitors for a few days.
After four months, everyone returned to the lab and repeated the original tests.
The results must have been disconcerting for some of them.
Those men and women who had burned about 1,500 calories a week with exercise turned out to have lost little if any body fat, the tests showed.
Some were heavier.
But most of those who had walked twice as much were thinner now. Twelve of them had shed at least 5 percent of their body fat during the study.
The researchers then used mathematical calculations, based on each person’s fat loss — if any — to determine whether and by how much they had compensated for their exercise.
The men and women in the group that had burned 1,500 calories a week with exercise proved to have compensated for nearly 950 of those calories, the numbers indicated.
Interestingly, those in the other group had also compensated for some of the calories they had burned, and to almost the exact same extent as those who had exercised less, adding back about 1,000 calories a week, the calculations showed.
But since they had expended 3,000 calories a week, they had wound up with a weekly deficit of about 2,000 calories from exercise and lost fat, the researchers concluded.
The findings were published in the American Journal of Physiology-Regulatory, Integrative and Comparative Physiology.
How the volunteers had compensated was not absolutely clear, said Kyle Flack, an assistant professor at the University of Kentucky, who conducted the experiment as part of his graduate research.
People’s resting metabolic rates had not changed during the study, he said, whichever group they had been in.
Their activity monitors also showed few differences in how much or little they moved during the day.
So the caloric compensation must have involved overeating, he said.
But the volunteers did not think so.
“Their food recall did not show differences” in how much they reported eating at the start and end of the study, Flack said. “I think they just did not realize that they were eating more.”
There probably also are complicated interconnections between exercise, appetite and people’s relationships to food that were not picked up during this study and can affect eating and weight, he said. He hopes to study those issues in the future.
But already, the results from this experiment are encouraging, if cautionary.
“It looks like you can lose weight with exercise,” Flack said.
But success may require more exertion and will than we might hope, he added.
“Thirty minutes of exercise was not enough” in this study to overcome the natural drive to replace the calories we burn during a workout. “Sixty minutes of exercise was better,” he said.
But even then, people replaced about a third of the calories they expended during exercise.
“You still have to count calories and weigh portions” if you hope to use exercise to control your weight, he said.