Mice that exercise vigorously put on weight in the next 24 hours, whereas those that exert themselves moderately or not at all don’t, despite both groups eating the same amount of food, a study has found. This adds to the growing evidence that animals, including us, compensate for extra energy used during exercise by cutting energy expenditure in other ways.
“The real-world experiences of many individuals feeling too exhausted to move after strenuous exercise lend credence to our study’s findings being replicable in humans,” says at the University of Tsukuba, Japan.
It is often assumed that exercising more leads to weight loss, because we use extra energy while doing so. But trials involving people doing more exercise than they have been doing typically find that they .
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It used to be presumed that this was due to people who exercise also eating more. But in 2015, , now at Duke University in North Carolina, produced a study that measured the energy expenditure of Hadza hunter-gatherers in Tanzania using a highly accurate chemical method.
To his surprise, he found that despite their active lifestyle, the Hadza burn a similar number of calories each day as office workers. Pontzer proposed that when people are physically active, other forms of energy usage are reduced to compensate. In other words, exercising more might not increase the overall amount of calories burned each day, meaning people who exercise more won’t lose weight even if they don’t increase their food intake.
One way this could happen is that when we are tired, we move less when not exercising, an idea backed by subsequent animal studies. When mice are provided with exercise wheels, for instance, they are less active when not running on the wheel than mice caged without wheels.
Now, Matsui has shown there is more to it than just moving less when not exercising. His team implanted a small device in 30 mice that measured their core body temperature, as well as activity levels.
After all the mice had spent a week exercising on a treadmill for 30 minutes a day, they were divided into three groups. One did no exercise. Another ran for 30 minutes at a speed equivalent to people jogging at a pace that barely increases their breathing, says Matsui. The third group ran for 30 minutes at a faster pace. “This is akin to running at an intensity that causes pronounced breathing and sweating,” he says.
Over the next 24 hours, the overall physical activity of the group that exercised vigorously was around 30 per cent lower compared with previous days when they didn’t exercise. What’s more, the body temperature of this group was around 1 per cent lower during the same post-exercise period, which means these mice were burning less energy. They also gained weight despite eating no more food than usual. These changes weren’t seen in the other groups.
“Our experiments showed that high-intensity exercise diminished non-exercise physical activity and body temperature while increasing body weight post-exercise,” says Matsui.
The reduction in body temperature shows the animals aren’t just moving less, but also reducing other physiological tasks in response to exercise, says Pontzer. “I do think this adds to what we’ve known before regarding that adaptation to exercise, because they are showing that both behavioural changes and physiological changes contribute to the overall compensation,” he says. “This is precisely the sort of thing I would predict given the apparent compensation to exercise seen in human and animal studies, in which total energy expenditure is very stable even as daily activity levels increase.”
Matsui thinks the findings are relevant to people who are trying to lose weight. While most studies in people have focused on overall energy expenditure during exercise, the intensity of the activity might be more important.
“Overexerting oneself at high intensities to the extent of reducing subsequent non-exercise physical activity is counterproductive,” he says. “Thus, the advice for those seeking weight loss is to recognise the importance of non-exercise physical activity and to moderate exercise intensity to maintain overall daily activity.”
But Pontzer isn’t convinced. “I am not sure that we can extend these results to prescribe moderate activity rather than vigorous activity,” he says. “It’s not obvious that that particular result would extend to humans.”
This could soon become clear as Matsui is planning a similar study in people after securing the necessary funding and says his team will make the findings public as soon as possible.
While exercising more might not result in weight loss, there is no doubt it has many benefits, including making it less likely for people to regain weight previously lost.
bioRxiv
