Are gut bacteria the reason people who live in cold regions get fat?

Numerous animals, humans included, have adapted to living in cold climates by increasing their body size. And now, researchers have found that people living in colder regions have more obesity-related gut microbes than their warm-region counterparts. Are bacteria the reason why it's harder to lose weight in the snow?

"Bergmann's rule" states that populations in higher latitudes tend to be larger than those living in warmer latitudes. This trend, which was first described by German biologist Carl Bergmann nearly two centuries ago, typically applies to mammals and birds.

The rule also applies to many human populations, such as the large-bodied Inuit, according to Taichi Suzuki, an ecology and evolutionary biology PhD student at the University of California Berkeley.

Bergmann's rule is all about the ratio between body mass and surface area. Larger animals tend to have greater body mass, and thus produce more body heat. Compared with smaller animals, they also have a smaller surface area relative to their body mass — this means, essentially, that they will radiate less heat per unit of mass, helping them to stay warm in colder climates. On the opposite side of the coin, animals that are relatively small-bodied and live in a warm climate radiate more body heat per unit of mass, allowing them to stay cool.

Naturally, one factor contributing to body size and mass is fat. In the last decade, researchers have investigated the role that gut microbes — particularly those of the bacteria phyla Bacteroidetes and Firmicutes play in obesity. Specifically, it appears that obese people have an increase in the proportion of Firmicutes and a decrease in the proportion of Bacteroidetes, compared with lean people. What's more, mouse research has shown that you can fiddle with body weight by changing gut microbial compositions. "If you take the gut microbes from a fat mouse and put them into the lean mouse, the lean mouse will become fat," Suzuki tells io9.

So Suzuki wondered: Are these gut communities somehow connected to the geographical trend in human body size?

Firmicutes vs. Bacteroidetes

To find out what role — if any — bacteria play in the matter, Suzuki and his colleague Michael Worobey decided to look at how the proportions of Bacteroidetes and Firmicutes in your stomach changes, depending on your latitude.

The pair gathered data from six studies, which contained gut microbial information on 23 populations (1020 people) from various latitudes, including places in South America, North America, South Africa and Europe.

"Our major finding is that in colder regions, at higher latitudes, you see more Firmicutes and less Bacteroidetes," Suzuki says. People living at higher latitudes, such as in Europe, had a significantly larger abundance of Firmicutes and smaller abundance of Bacteroidetes than people living at lower latitudes. That is, so-called "healthy micrbiota" differ by geographical regions.

Are gut bacteria the reason people who live in cold regions get fat?

The abundances of Firmicutes (blue) increase at higher latitudes, while the abundances of Bacteroidetes (red) decrease at higher latitudes. Credit: Suzuki et al.

When Suzuki and Worobey teased apart the data, they found that no single factor, including age, sex, or the studies' methods used to detect the gut microbes, could explain the relative microbial abundance pattern they found. For instance, men and women displayed similar relative abundances of the two bacterial phyla. Likewise, the bacterial communities of children and adults were similar. However, the abundances of elderly people ran counter to the trend — the researchers note that the elderly (and infants) have a distinct and less stable gut microbial communities than other people.

At this point, it's not clear what the trend means, exactly. "It doesn't necessarily mean that microbes are causing the humans in colder regions to get fat," Suzuki says. It could be the case, for example, that people in colder regions eat a more fatty diet, and this affects their gut microbial composition.

As part of his dissertation work, Suzuki conducted a similar experiment in mice. "I knocked on people's doors and collected microbe samples from mice across North America," he explains. He found that house mice in colder, higher-latitude regions had greater body mass and more obesity-associated microbiota than mice living at lower latitudes.

Suzuki is now planning to do transplant studies with mice. He will be taking the gut microbes from mice in colder and warmer regions and putting them into lab mice, and then watching what happens to the mice's weight. This approach will allow him to see if the microbes are causing the change in body size, or if food is behind everything. Chances are, both factors play a role, he says.

"My interest is how microbes affect our health and evolution," Suzuki says. "People think obesity is a bad thing, but maybe it's not a bad thing in terms of human evolution and natural populations. It could be a good thing if you live in a cold place and need more fat to survive."

Check out the full study in the journal Biology Letters.

Image via Wikimedia Commons.