Gut Check: Based on New Microbe Study, Should We Blame Geography for Obesity?

By Brendan Buhler

Does this bacteria make me look fat?

I ask because in the latest search for causes of the obesity epidemic, the one victim of sausage-like finger pointing is the community of microbes residing in the human gut. And according to the surprising recent work by UC Berkeley grad student Taichi Suzuki, it seems that where you live goes a great way to determining what microbes you have.

The case, in short, is this: There are two dominant categories of bacteria in the human gut microbiome: firmicutes and bacteroidetes. Two households, both alike in dignity, in fair intestines, where we lay our scene… ahem. Anyhow, the gram-positive firmicutes tend to dominate and they tend to dominate most of all in the intestines of the obese subjects, be they zebrafish, mice or humans. Firmicutes are thought to increase the amount of energy you absorb from food. In smaller and slimmer subjects, you’ll find a greater number of gram-negative bacteroidetes.

Suzuki’s work compares the microbial census of over a thousand people around the world (the data is culled from previous studies by others) and finds that the higher your latitude, the greater the proportion of firmicutes in your gut microbiome. This fits with what’s called Bergman’s rule, an observation made by a German biologist more than 150 years ago that many animals, including humans, increase in size the further north they live. (It’s thought to be an adaptation to the cold.)

Aha! The north remembers (what it ate, because it goes straight to its thighs). Yes?

Well, maybe. On the other hand, maybe not.

For starters, it’s not entirely clear that the demon firmicutes cause obesity. They could just be correlated with it. In particular, with eating. When animals are fasting or on low calorie diets, the proportion of bacteroidetes to firmicutes increases. We could be blaming smoke for causing fire.

On the other firmicutes-blaming hand, if you have mice that are born sterile—free from any gut microbes—and kept in a sterile environment, they never grow as large as mice with a full compliment of microbes and on the same diet. The firmicute-having mice are better at extracting calories from food. Maybe it’s more like blaming oily rags for fire—they didn’t start it, but they don’t help.

But can we call firmicutes the driving cause of obesity? Here, Suzuki’s work indicates that we might not want to, because when we compare his work to a casual perusing of a list of national obesity rates we find at the top of the pack…American Samoa. Followed in the rest of the top ten by Nauru, the Cook Islands, Tokelau, Tonga, Samoa, Palau, Kiribati, the Marshall Islands and Kuwait. Rather equatorial climes—excepting possibly Kuwait, which has roughly the latitude of Tallahassee.

Europe doesn’t even show up in the rankings until the 30s and 40s. (The United States is number 18.) And the Netherlands, a country that in the map presented with Suzuki’s data has one of the highest proportions of firmicutes? Number 103.

And shocking nobody, obesity rates are lowest in countries that also happen to be among the world’s poorest. Not that that’s a perfect correlation, either, because the world’s richest people aren’t necessarily the most obese. When was the last time you heard anyone complaining about all the fatties in Liechtenstein?

Possibly this is a complex problem.

For an in-depth travel guide into the mysteries of the human gut, check out “The Teeming Metropolis of You” from the Fall 2011 issue of California magazine.

Filed under: Science + Health
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But, soft, what gas from yonder orifice breaks…

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