Published online 20 December 2006 | Nature | doi:10.1038/news061218-6

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Your gut bacteria may help to determine your holiday weight gain.

Inside story: some gut bacteria absorb more calories from food than others.

The obese are often blamed for their own corpulence. But perhaps, just perhaps, some of the blame should be placed on another type of organism entirely: bacteria.

Researchers have shown that the intestines of obese people are swimming with a different make-up of microbes compared with those of slim people. And this microbial population could actually be helping them gain weight: bugs taken from an obese mouse and transplanted into another animal's intestine made the animals gain more fat than normal.

The researchers propose that the obese-prone microbes glean more calories from food, which are sucked up by the body and deposited as excess fat. "Minor differences in the calories you can harvest might play an important role in predisposition to obesity," says Jeffrey Gordon at the Washington University School of Medicine in St Louis, who led the studies.

The implications for people trying to lose weight are, for now, unclear. It isn't known how easy it is to change a person's microbial balance, for example, or whether that might have unwanted health consequences. On top of all that, notes obesity expert Stephen Bloom of Imperial College London, the body's other weight-regulating mechanisms might step in to compensate for any gut microbe changes.

Microbe medley

Every person's gut is home to a unique cocktail of trillions of bacteria and other minute bugs that help to break down food and fight off invading pathogens. In 2004, Gordon first proposed that this medley of microbes might help control body weight.

The studies he and his team publish in Nature this week are the strongest evidence in support of this idea 1,2. They strained the faeces of 12 willing obese volunteers, used genetic sequencing to identify the different species of bacteria in there, and compared them with five lean volunteers.

Most of the bacteria fell into two groups, called Firmicutes or Bacteroidetes. But the obese volunteers had more than 20% more Firmicutes and nearly 90% less Bacteroidetes than the lean ones.

The obese volunteers then spent one year on a low-fat or low-carbohydrate diet, and lost as much as 25% of their body weight. At the same time, the proportion of Firmicutes in their colon dropped and that of the Bacteroidetes rose, although these levels never reached those of the group who were slim to start with.

Cause and effect

This suggests that our bodies somehow communicate our weight to the microbes in our gut, and that obesity can upset the normal microbial balance. But studies in mice suggest that the reverse is also true: shifting the microbes can affect weight.

The researchers sucked microbes from the guts of either lean mice or the obese ones. They injected the microorganisms into the intestines of animals whose own innards were unnaturally bare of microbes because they had grown up in a sterile cage.

After two weeks, the mice injected with the 'obese' microbes gained roughly double the quantity of fat than those that received the 'thin' microbes, although this amounted to only a fraction of gram. "These changes might be minor, but over time would have a more dramatic effect," Gordon says.

Mouse experiments also suggest how the microbes make a difference. The researchers found that mice genetically engineered to be obese also have more intestinal Firmicutes, and their gut bacteria as a whole have more genes that break down otherwise indigestible fibrous components of food. That suggests that they can wring more calories from their food. An examination of the amount of food going in, and the amount of caloric content coming out in faeces, confirms that they're absorbing more calories.

A dose of fatness

The concept that our internal bacteria are partly determining our weight "is quite radical," says Randy Seeley of the University of Cincinnati, Ohio, who studies the regulation of body weight. Plenty about the idea is still unproven, he notes.

And it is unclear whether gut microbes are really a significant contributor to the ballooning obesity epidemic, or whether other factors are far more important.

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Gordon says that it might be possible to identify compounds manufactured by the bugs that influence fat deposition, and perhaps use these as obesity therapies. He is planning to study in more detail the differences between the microbes in overweight and thin individuals.

In the meantime, there seems little cause to worry that you might accidentally 'catch' a dose of fattening microbes from an obese friend. "It could feed a certain hysteria," says Seeley, "but there is no easy way to pass the obese bugs on to people."

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