Two new studies show that there are different colonies of bacteria in the intestines of the obese than there are in the innards of the slim. The research, published in today's edition of the journal Nature, finds that the microbes in an overweight body are more efficient at extracting calories from food.

"Not everyone sitting down to a bowl of cereal will necessarily absorb the same number of calories from it," says Jeffrey Gordon, lead author of the papers and a professor of medicine at Washington University in St. Louis.

About two-thirds of adults, about 136 million Americans, are overweight or obese, the government says. These findings open up a new area of research, says Sam Klein, a study co-author and professor of gastroenterology at the university.

"It's not just your brain and your body fat and your body organs involved in your energy balance equation," he says. "It may also be the bugs that are in your body as well."

One study focused on mice, the other on humans. They found that in both man and rodent, a family of bacteria known as firmicutes were more plentiful in the obese than in the lean. Conversely, bacteria called bacteroidetes were less abundant than in normal-weight subjects.

The research showed that obese mice were more efficient than lean mice at harvesting calories from complex sugars found in fruits, vegetables and grains, and depositing those calories in fat — most likely because of the bacterial colonies.

And when they transplanted the microbes from obese and thin mice into mice raised in a sterile environment, those that got microbes from the obese mice gained twice as much fat.

When obese people lost weight, virtually all the bacteroidetes increased, while the firmicute group shrank, Gordon says.

The bacteria inside us are a huge and mysterious part of life. "There are trillions of them, they outnumber the human cells in our bodies," Klein says. Meaning, Gordon quips, "you never eat alone."

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