What if you could lose weight simply by lowering your body’s thermostat for a couple weeks? Sounds like the perfect solution to shedding those extra holiday pounds. Researchers have now shown this can happen—at least temporarily—in mice, and the reason may be microbes in their guts.
The new research is, “another striking example of how integral the microbiota is to our physiology,” says Alexander Khoruts, a gastroenterologist at the University of Minnesota, Minneapolis, who was not involved with the work. Such studies, he says, may ultimately lead to bacterial supplements that can help control obesity, which affects one in every three Americans.
Gut microbes—the collection of bacteria and other organisms that inhabit our intestines—have been shown to play a role in a wide variety of conditions from asthma to obesity. But researchers still don’t know precisely how they contribute to obesity. To gain a better understanding, a team led by physiologist Mirko Trajkovski from the University of Geneva in Switzerland has been following up on any leads he can to learn about the link between these microbes and metabolism. Trajkovski knew from past research that getting rid of some gut bacteria in mice makes them less able to maintain their internal body temperature in the cold. He wondered whether there might be a connection between gut microbes, external temperature, and weight control.
To find out, the researchers first surveyed what microbes were present in lab mice, which live in relative comfort at room temperature, 21°C. They then lowered the temperature to 6°C for 11 or 30 days, similar to conditions experienced by wild mice. During the experiments, the researchers took the temperatures of the mice, sampled their stool, and measured other metabolic parameters such as insulin uptake and fat type.
Normally, mice gain weight as they age and maintain a constant body temperature. In the experiment, the cold triggered an initial slight drop in temperature, causing the mice to burn fat and lose weight. Blood sugar levels dropped as well, and the cold mice became more sensitive to insulin. What’s more, their microbial makeup shifted significantly. Notably, a microbe called Akkermansia muciniphila, which is associated with obesity and diabetes, virtually disappeared, the researchers report online today in Cell. As this happened, the mice and their microbes seemed less adept at wringing energy from the food they digested.
After a little more than a week, however, weight loss leveled off and caloric intake from the gut improved. When the researchers looked inside the mice, they found something unusual: Their intestines had increased in length, helping to make digestion more efficient.
To see whether the shift in bacteria drove these physiological changes, the researchers transplanted bacteria from the cold-adapted mice into two other groups of mice without resident gut bacteria. Mice in both groups responded to the bacterial transplants as if they, too, had been exposed to prolonged cold. Their weight dropped, their blood sugar dropped, and they even grew longer intestines. “I was not expecting the magnitude of the response,” Trajlovski says. What’s more, when he and his colleagues gave the mice A. muciniphila, their intestines shrank, and the mice never stopped losing weight.
Barbara Cannon, a physiologist at Stockholm University, is surprised at the degree of the effects, particularly how quickly the metabolisms of mice living with the bacteria of cold-adapted mice changed. “I find it very remarkable that transplanted microbiota can continuously double the metabolic rate of mice and look forward to an explanation of the mechanism,” she says.
But whether these results apply to humans is a big “if,” Trajkovski cautions. Coming from a cold place himself, he thinks cold exposure is healthy—in moderation. “But you should not overdo it.”
Regardless, the work suggests that species like A. muciniphila may one day help control weight problems. “We have entered an age when microbiota therapeutics are becoming a reality,” Khoruts says. “Clearly we have to think of energy metabolism disorders as one of the main targets for this new class of therapies.”