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Microbiota, obesity and northern populations

The researchers looked at data from more than 1,000 people from around the world. The blue represents the proportion of obesity-related bacteria in the gut, while red is the proportion of bacteria associated with slimness (Credit: Image courtesy of Univer
Human gut obesity microbes differ by hemisphere

People living in cold, northern latitudes have bacteria in their guts that may predispose them to obesity, according to a new study by researchers at the University of California, Berkeley, and the University of Arizona, Tucson.

The researchers' analysis of the gut microbes of more than a thousand people from around the world showed that those living in northern latitudes had more gut bacteria that have been linked to obesity than did people living farther south. The meta-analysis 'Geographical variation of human gut microbial composition.' is of six earlier studies and is published in the online journal Biology Letters by Taichi Suzuki (Berkeley) and Professor Michael Worobey (Arizona).

"People think obesity is a bad thing, but maybe in the past getting more fat and more energy from the diet might have been important to survival in cold places," said Suzuki, noting that one theory is that obesity-linked bacteria are better at extracting energy from food. "Our gut microbes today might be influenced by our ancestors. This suggests that what we call 'healthy microbiota' may differ in different geographic regions."

Studies of gut microbes have become a hot research area among scientists because the proportion of different types of bacteria and Archaea in the gut seems to be correlated with diseases ranging from diabetes and obesity to cancer. In particular, the group of bacteria called Firmicutes seems to dominate in the intestines of obese people, and obese mice, while a group called Bacteroidetes dominates in slimmer people and mice.

As there is evidence that humans have adapted to colder climates by increasing their body mass (e.g. Bergmann's rule), they tested whether Firmicutes increase and Bacteroidetes decrease with latitude, using 1,020 healthy individuals drawn from 23 populations and six published studies.

They found a positive correlation between Firmicutes and latitude and a negative correlation between Bacteroidetes and latitude. The overall pattern appears robust to sex, age and bacterial detection methods. Inaddition, comparisons between African Americans and native Africans and between European Americans and native Europeans suggest no evidence of host genotype explaining the observed patterns.

"This observation is pretty cool, but it is not clear why we are seeing the relationship we do with latitude," Worobey said. "There is something amazing and weird going on with microbiomes. Maybe changes to your gut community of bacteria are important for allowing populations to adapt to different environmental conditions in lots of animals, including humans.”

"Bergmann's rule - that body size increases with latitude for many animals - is a good one and presumed to be an adaptation for dealing with cold environments," said Suzuki's advisor Michael Nachman, professor of integrative biology and director of UC Berkeley's Museum of Vertebrate Zoology. "Whether gut microbes also help explain Bergmann's rule will require experimental tests, but Taichi's discovery adds an intriguing and completely overlooked piece of the puzzle to this otherwise well-studied evolutionary pattern."

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