If you give a mouse a running wheel, it will run.
But it may not burn many additional calories, because it will also start to move differently when it is not on the wheel, according to an interesting new study of the behaviors and metabolisms of exercising mice.
The study, published in Diabetes, involved animals, but it could have cautionary implications for people who start exercising in the hopes of losing weight.
In recent years, study after study examining exercise and weight loss among people and animals has concluded that, by itself, exercise is not an effective way to drop pounds.
In most of these experiments, the participants lost far less weight than would have been expected, mathematically, given how many additional calories they were burning with their workouts.
Scientists involved in this research have suspected and sometimes shown that exercisers, whatever their species, tend to become hungrier and consume more calories after physical activity. They also may grow more sedentary outside of exercise sessions. Together or separately, these changes could compensate for the extra energy used during exercise, meaning that, over all, energy expenditure doesn’t change and a person’s or rodent’s weight remains stubbornly the same.
Proving that possibility has been daunting, though, in part because it is difficult to quantify every physical movement someone or something makes, and how their movements do or do not change after exercise. Mice, for instance, skitter, dart, freeze, groom, eat, roam, defecate and otherwise flit about in frequent fits and starts.
But recently, animal researchers hit upon the idea of using infrared light beams to track how animals move at any given moment in their cages. Sophisticated software then can use that information to map daily patterns of physical activity, showing, second-by-second, when, where and for how long an animal roams, sits, runs or otherwise spends its time.
Intrigued, scientists at Vanderbilt University and other institutions thought that this technology would be ideal for tracking mice before and after they started exercising, especially if the technology were used in specialized metabolic-chamber cages that can quantify how much energy an inhabitant is expending throughout the day.
So the scientists fitted out cages, added locked running wheels, and let young, healthy, normal-weight, male mice loose in them to roam and explore for four days, providing the researchers with baseline data about each mouse’s metabolism and natural peripateticness.
The wheels then were unlocked and for nine days, the mice could run at will, while also eating and moving around off the wheels as much as they chose.
The mice, which seem to enjoy running, hopped readily on the wheels and ran, off and on, for hours.
They showed a subsequent spike in their daily energy expenditure, according to the metabolic measures, which makes sense, since they had added exercise to their lives.
But they did not change their eating habits. Although they were burning more calories, they did not gorge on more chow.
They did, however, alter how they moved. Almost immediately after they started using the wheels, they stopped roaming around their cages as they had before the wheels were unlocked.
In particular, they stopped engaging in the kind of lengthy meanders that had been common before they began to run. Instead, they now usually jogged on their wheels for a few minutes, hopped off, rested or roamed in short spurts, and then climbed back on the wheels, ran, rested, briefly roamed, and repeated.
These changes in how they spent their time neatly managed to almost counteract the extra calorie costs from running, says Daniel Lark, a research fellow in molecular physiology at the Vanderbilt University School of Medicine, who led the new study.
In general, the running mice showed a slightly negative energy balance, meaning that they were burning a few more calories over the course of the day than they were taking in by chowing down.
But that caloric deficit would have been about 45 percent greater, the metabolic calculations showed, if they had not also begun moving around their cages less.
What prompted the running mice to roam less is still uncertain.
“But it does not seem to have been fatigue or lack of time,” Dr. Lark says.
Wheel running is not arduous for mice, he points out, and did not fill their waking hours.
Instead, he says, it is likely that the animals’ bodies and brains sensed the beginnings of an energy deficit when the mice began to run and sent out biological signals that somehow advised the animals to slow down, conserve energy, maintain homeostasis and not drop weight.
He and his colleagues would like, in future experiments, to explore how, physiologically, the rodents’ bodies sensed the changes in their energy balance and at what point they might begin to eat more. They also would like to study female, older and obese animals.
Mice will never be people, of course, so we cannot say whether the results of this and any follow-up experiments would directly apply to us, Dr. Lark says.
But the results do intimate that if we hope to exercise off excess pounds, we watch what we eat and try not to move less while we work out more.