Obesity linked to bacteria in gut

“There appears to be a link between obesity and the type of bugs in your gut,” said Jeffrey I Gordon of Washington University School of Medicine in St Louis, who led the series of experiments published in Nature this week.

Gordon and his colleagues stressed that more work is needed to explore the findings. And they cautioned against trying to manipulate “gut flora” with antibiotics or microbial “probiotic” pills sold in health-food stores. But if the findings are confirmed and better understood, they could lead to profound new insights into one of the biggest health problems.

“In future, we could potentially manipulate the structure and function of these microbial societies as a new approach toward preventing and treating obesity,” Gordon said.

The findings produced enthusiasm and caution from other researchers. Some praised the work for possibly offering a long-sought alternative explanation for the obesity epidemic. Perhaps some change, such as a food additive or antibiotic use, has caused a fundamental shift in gut flora, making it easier for many people to gain weight.

“This is very exciting,” said Barbara Corkey, an obesity researcher at Boston University. “We don’t know why the obesity epidemic is happening. People say it’s because of gluttony and sloth. I think there must be something else. It’s exciting to see some work being done on alternative explanations.”

Others suspect that if gut microbes do play a role, it is a minor one. “This is extremely interesting,” said Hans-Rudolf Berthoud of the Pennington Biomedical Research Center in Baton Rouge. “But lifestyle and the environment are still the major factors in the obesity epidemic.”

To explore the role of the organisms in weight regulation, Gordon’s team first compared the gut flora of 12 obese people to lean subjects’. The obese tended to have a significantly greater proportion of one of the two main types of bacteria found in the gut — Firmicutes — than the other, Bacteroidetes.

Next, the researchers spent a year meticulously measuring the gut flora of the obese volunteers as they tried to lose weight by eating low-calorie diets. As they lost weight, the proportion of Firmicutes in their digestive tracts fell and the proportion of Bacteroidetes rose.

When the researchers conducted detailed molecular analyses of the two types of bacteria, they discovered the Firmicutes were much better at extracting calories from food.

Moreover, when the researchers examined the gut flora of obese laboratory mice, they found a similar pattern in their ratios of Firmicutes to Bacteroidetes. After gut flora from obese mice was transferred to the lean ones, the recipient animals were found to gain weight.

Among other things, the findings could help explain why it becomes harder and harder to lose weight as people get fat, Gordon said.

“That’s part of the pathology of obesity. When you shift the amount of fat tissue, the amount of energy you can harvest becomes somewhat greater, and that’s going to fuel the obese state,” he said.

The researchers acknowledged that the difference in the number of calories extracted by the microbes is relatively small. But over time even a small differential could be significant.

Many questions remain, however. It is unclear what determines the make-up of a person’s gut flora. It might be the microbes they pick up from their mothers; it might be their exposure to antibiotics. It is also unclear how fat tissue and gut flora might affect one another, and whether the change in gut bacteria causes or is a result of the weight loss.

Despite those and other questions, scientists said the findings are sure to inspire more investigation.

–Rob Stein