The guts of obese people are teeming with a distinctive mix of bacteria that seems to make them prone to gaining weight, a startling discovery that could lead to new ways to fight the obesity epidemic, researchers reported Wednesday.
Obese people have more gut microbes that are especially efficient at extracting calories from food, the researchers said, and the proportion of these superdigesting organisms ebbs as people lose weight. Moreover, when the scientists transplanted gut bugs from obese mice into lean mice, the thin animals started getting fat, providing more support for the theory that the bacteria that populate the gut play an important role in regulating weight.
"There appears to be a link between obesity and the type of bugs in your gut," said Dr. Jeffrey Gordon of Washington University School of Medicine in St. Louis, who led the series of experiments being published in Thursday's issue of the journal Nature. "The difference in the structure of microbial ecology of our gut may set us up for susceptibility to obesity."
Dr. 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 microbe-containing "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 world's biggest health problems, they said.
"In the future, we could potentially manipulate the structure and function of these microbial societies as a new approach toward preventing and treating obesity," Dr. 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 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's probably relatively minor.
"This is extremely interesting," said Hans-Rudolf Berthoud of the Pennington Biomedical Research Center in Baton Rouge, La. "But lifestyle and the environment are still the major factors in the obesity epidemic."
To explore the role of the organisms in weight regulation, Dr. Gordon's team first compared the gut flora of 12 obese people with that in lean subjects. The obese tended to have significantly greater proportion of one of the two main types of bacteria found in the gut, known as Firmicutes, than the other, known as 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 that restricted either their fat or carbohydrates. As they lost weight, the proportion of Firmicutes fell and the proportion of Bacteroidetes rose, the researchers found.
When the researchers conducted detailed molecular analyses of the two types of bacteria in the laboratory, 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 balance of Firmicutes to Bacteroidetes. And when they transferred gut flora from obese mice to sterile mice devoid of gut flora, the recipient animals tended to gain weight, confirming that pattern was associated with weight gain.
"This attribute of being able to harvest and store more energy appeared to be transmissible," Dr. Gordon said. "For the first time, we see that there is a correlation between the microbial gut ecology and the obese state."
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