Many things cause people to become obese. New research suggests that one of the biggest causes may be the billions of bacteria living in their guts.
There are as many, if not more, bacteria in and on a human body as there are human cells. Over the past few decades, we have been slowly untangling how these bacteria influence our health, fend against infections, and even change our brains.
A study in 2008 showed that, after accounting for all other factors, obese people had a slightly less diverse set of gut bacteria than lean people. The study had its limitations—a small sample size, for one—but the results were interesting enough that Michelle Beaumont, of King’s College London, and her colleagues wanted to do a larger study.
First they looked at data from more than 3,600 twins to find out whether visceral fat—the body fat stored around the waist—is a good indicator of obesity. (Using twins helps researchers determine how much genetic versus environmental factors affect a condition.) They found that it was.
They also obtained stool samples from 1,300 of the twins, which they analyzed to understand what types of bacteria lived in their guts. That analysis, published in Genome Biology, gave them two results. First, a certain proportion of gut bacteria seem to be inherited. Second, people with less bacterial diversity in their gut had a higher proportion of visceral fat—confirming, in other words, the 2008 finding that obesity is correlated with having less diverse gut bacteria.
This doesn’t prove that gut bacteria cause obesity, says Beaumont. But a 2014 study in mice hints that it might be the case.
In that study, researchers bred mice in bacteria-free environments. Then one mouse got gut bacteria implanted from those of an obese woman, while the other mouse got bacteria from her lean twin sister. Even though the mice were fed the exact same diet, the first mouse became obese while the other remained lean. It’s not clear why, but researchers suspect that the guts of obese people contain more bacteria that are efficient at forcing the body to convert carbohydrates into fat.
Putting all these results together, it’s clear that gut bacteria probably play a key role in adding fat to people’s bodies—and also that, to some extent, the gut bacteria you have are inherited from your parents. The solution to obesity for some people is simply to eat less and exercise more, but those who’ve inherited less diverse gut bacteria might need a more radical intervention.
The most promising treatment would be fecal transplants, to populate their guts with bacteria from lean people. Trials of such treatments have run into some problems recently. But with obesity at epidemic proportions in a growing number of countries, it may be a technique worth pursuing.