Led byTaichi Suzuki from the University of California, Berkeley, the team behind the study noted that although it is known that there is considerable variation in gut microbial composition within host species, very little is known about how this variation is shaped and why such variation exists.
For example, in humans, obesity has been associated with the relative abundance of two dominant bacterial groups - an increase in the proportion of Firmicutes and a decrease in the proportion of Bacteroidetes.
The meta-analysis findings, pooled from more than 1,000 people living in 23 different populations and involved in six earlier studies, suggest that people living in northern latitudes have a greater proportion of the obesity-related bacteria Firmicutes in their microflora.
"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. Our gut microbes today might be influenced by our ancestors," said Suzuki, noting that one theory is that obesity-linked bacteria are better at extracting energy from food. "This suggests that what we call 'healthy microbiota' may differ in different geographic regions."
Suzuki reasoned that, since animals and humans in the north tend to be larger in size – an observation called Bergmann's rule – then perhaps their gut microbiota would contain a greater proportion of Firmicutes than Bacteriodetes.
"It was almost as a lark," explained senior author Professor Michael Worobey of the University of Arizona. "Taichi thought that if Firmicutes and Bacteroidetes are linked to obesity, why not look at large scale trends in humans."
"When he came back with results that really showed there was something to it, it was quite a surprise."
Writing in Biology Letters, the Suzuki and his colleagues used data published in six previous studies, totalling 1,020 people from 23 populations in Africa, Europe, North and South America and Asia.
After analysing the data, he found that the proportion of Firmicutes increased with latitude and the proportion of Bacteriodetes decreased with latitude, regardless of sex, age, or detection methods.
Indeed, African Americans showed the same patterns as Europeans and North Americans, not the pattern of Africans living in tropical areas, he noted.
"This observation is pretty cool, but it is not clear why we are seeing the relationship we do with latitude," said Worobey. "There is something amazing and weird going on with microbiomes."
Indeed, the senior author added that the results are also fascinating from an evolutionary biology perspective.
"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," he said.
Source: Biology Letters
Published online ahead of print, doi: 10.1098/rsbl.2013.1037
"Geographical variation of human gut microbial composition"
Authors: Taichi A. Suzuki, Michael Worobey