The cold, dark winter season certainly has its compensations — snowman building, warm fires and holiday food. Now a new study, published in the journal Cell, may offer us another: weight loss. A team of scientists from Switzerland have discovered that, though exposure to the cold prevents a certain gut bacterium from burning more fat, there may now be a way around it.
Scientists at the University of Geneva (UNIGE) found that mice exposed to cold experience a dramatic shift in their microbiota composition, making them leaner and more sensitive to insulin. Transplantation of this cold-modified microbiota to other mice gives them "complete tolerance" to cold — increasing brown fat levels and thus improves sensitivity to insulin, even if they themselves haven't experienced the cold.
The body temperature of mammals normally stays fairly constant. In fact, we warm-blooded creatures are very good at keeping our core temperature within a pretty narrow range — usually within a degree or so of 98.6°F (37°C). As warm weather makes our blood vessels expand and so lose heat through sweating, so cold weather makes them contract, preserving heat. As environmental temperature drops, so does that of the body — by few degrees, before slowly rising almost back to normal.
This heat exchange mechanism is regulated by brown adipose tissue, or brown fat, which the body produces in the cold and then burns to generate heat. According to a previous report in Trends in Endocrinology & Metabolism, brown fat turns glucose and fatty acids into fuel for the body "making it an appealing target for the treatment of obesity, diabetes, and other metabolic disorders." However, says the Swiss team, prolonged exposure to the cold can also slow down weight loss as the body uses more calories from food. There's a good reason we prefer those warmer, heavier dishes during the winter.
The slowdown, the researchers say, is due to the disappearance of a key bacterium — Akkermansia muciniphila — which affects the way nutrients are absorbed by the body. When the scientists artificially reintroduced the bacteria, weight loss resumed. This suggests that it is the way nutrients are absorbed that changes. "Otherwise, we would expect the mice exposed to cold to keep on losing weight, because they would continue to burn calories to generate heat, indicates Mirko Trajkovski, who led the research.
Over a period of a month, the UNIGE researchers exposed a group of mice to cold — slowly dropping their environmental temperature from 68°F (20°C) to 42°F (6°C) — and studied how their microbiota changed. "The changes we observed in the microbiota composition of the mice living in a cold environment were even more dramatic than the microbiota differences previously observed between obese and healthy individuals," say Claire Chevalier and Ozren Stojanovic, co-authors of the study.
And then, something even more surprising happened: when they transplanted the "cold microbiota" to mice that had not been exposed, they too became immediately resistant to cold. The animals' body temperature did not drop, say the researchers, "as if the transplanted microbiota also modified this adaptive mechanism." The results seem to suggest that it's the altered microbiota alone that provide resistance to cold. The new mice also showed a better metabolic profile in general, with greater sensitivity to insulin and additional "beige fat" — adipose tissue, which is similar to brown fat and which protects against excess weight.
"We were amazed to observe that the changes in microbiota during cold exposure actually promote increased gut and microvilli lengths," says Trajkovski. Microvilli are tiny projections that protrude from the intestinal wall enlarging the surface area of the gut and so increasing the potential absorption of nutrients. The reduction of Akkermansia muciniphila means the gut responds by changing its shape to absorb more nutrients. This explains why weight loss through exposure to cold is only temporary — the absence of this particular bacteria causes the body to compensate.
Crucially, studies of the microbiota of people suffering from obesity also seem to lack this same bacterium. Further research is needed but, if proven to be a valid strategy, it would "open a door," the researchers say, to new and potentially effective methods of treating obesity. And maybe more — "The gut is also our largest endocrine tissue which secretes many hormones acting in different parts of our body," says Trajkovski. "Thus, altering the gut morphology might be one of the ways by which microbiota impacts all other organs, including our brain."