A new study from UC Riverside shows that calorie restriction does not deter mice from exercising, challenging the belief that dieting drains energy from training.
The study, published in the journal Physiology and behavior, shows that reducing calories by 20% did not significantly reduce the distance mice voluntarily chose to run each day.
The researchers set out to understand what happens to mice when the amount of available food is reduced. They hoped the findings would be relevant to wild animals who don’t always get as much food as they want on a given day, and also to humans, whose doctors often prescribe them diets.
It’s somewhat difficult to get accurate data on how much voluntary exercise humans engage in. While it’s easy to categorize what people recognize as voluntary exercise, such as a trip to the gym, there are many gray areas that are difficult to quantify, such as walking to a cafeteria to buy lunch instead of eating a meal from a nearby lunch box.
It’s much easier to track what lab mice choose to do, and these animals typically like to run on wheels for many hours a day. In this study, researchers observed that mice chose to run at similar levels regardless of how much they ate.
“Voluntary exercise was remarkably resistant to reducing the amount of food by 20 percent and even 40 percent,” said UCR biologist and corresponding author of the study Theodore Garland, Jr. “They just kept running.”
The researchers spent three weeks getting the mice to a baseline level of running activity, then one week with 20% reduced calories and another week with 40% fewer calories. This experiment was done with both normal mice and “high-performance” mice bred to enjoy running.
Although the high-performance runners slightly reduced their total distance with a 40% calorie restriction, the reduction was only 11%. Because they started out running three times as far a day as the normal mice, the reduction is considered slight. “They’re still running at extremely high levels,” Garland said. The normal mice did not reduce their daily distance, even with a 40% calorie reduction.
Researchers believe running produces a kind of euphoria, in part by increasing levels of dopamine and cannabinoids in the brain, so the mice were motivated to keep running even with less food. “Running on wheels is a personally rewarding behavior,” Garland said.
Furthermore, the researchers were surprised to find that the 20% calorie reduction did not significantly affect body mass, either in the normal-running mice or in the high-running mice. Although there was some reduction in body mass with a 40% reduction, it was not as high as predicted.
“People typically lose about 4 percent of their body mass when they’re on a diet. That’s the same range as these mice,” Garland said.
This study contributes to our understanding of why some people like to exercise and others do not. In the future, the researchers are planning additional studies to understand why both the amount of voluntary exercise and body mass are so resistant to calorie restriction.
“There has to be some kind of compensation if food intake goes down 40% and weight doesn’t go down much,” Garland said. “It might be cutting back on other types of activity or becoming more metabolically efficient, which we haven’t measured yet.”
Since habitat destruction causes food shortages for wild animals, this kind of information could be useful to those trying to preserve species. And for the many people interested in improving their health, the implications could be equally significant.
“We don’t want people who are dieting to say, ‘I don’t have enough energy, so I’m going to make up for it by not exercising.’ That would be counterproductive, and now we know that doesn’t have to be the case,” Garland said.