U.S. scientists have found a surprising relationship between obesity and the kind of food digesting bacteria we have in our intestines. As VOA's David McAlary reports, the findings suggest that the microbes living in our guts might influence how prone we are to being overweight and might offer new solutions to the growing obesity epidemic worldwide.

Our intestines house two main groups of beneficial bacteria that help us break down otherwise indigestible foods so we can use the energy from them.

Researchers at Washington University in St. Louis have found that the relative proportion of one of these kinds of bacteria, called Bacteriodetes is lower in fat people compared with those who are lean. Stool samples showed that when obese people lost weight, the Bacteriodetes microbes in their guts increased in proportion to the other type, called Firmicutes.

They saw the same thing in mice.

"We found that the obese humans did actually have exactly the same shift in the ratio of Firmicutes to Bacteriodetes that we had seen in the mice," said Ruth Ley.

That is Washington University scientist Ruth Ley speaking to Nature magazine, which has published her group's two studies on this topic.

"As they lost weight, the amount of Bacteriodetes increased and began to resemble what you would see in a normal lean person, and the amount of the increase was proportional to the amount of fat that they had actually lost over time," she said.

The researchers also wanted to know if they could make lean mice fat simply by altering the intestinal proportion of their microbes, what they call flora. Co-investigator Peter Turnbaugh told Nature they could do just that.

"What we saw was the mice that were exposed to the microbes from an obese mouse actually gained more fat over the course of the experiment than the mice that were given a lean microbial community," said Peter Turnbaugh.

Together, the findings suggest that obesity involves more than eating too much and not being very active. Another member of the research team, Jeffrey Gordon, puts it this way.

"What this study shows is that there is also a microbial component to determining how much adipose tissue you might have, and that the microbes in the gut are part of the question that affects predisposition to or the pathophysiology of obesity," he said.

In a Nature magazine commentary, Matej Bajzer and Randy Seeley at the University of Cincinnati's Genome Research Institute say this is a potentially revolutionary idea that could change our views of what causes obesity. They write that the differences in the way the body extracts calories from food may be determined by the microbes, contributing to differences in body weights.

But Ruth Ley says the biological pathways governing the intestinal bacteria are unknown.

"We know there is some sort of linkage between the fat that is carried on the body," she said. "We know that fat cells produce hormones, and there could be some signaling between the adipose tissue in the and the kinds of bugs [microbes] that are in the intestine, but at this point, we do not understand the mechanism."

The researchers say that once these mechanisms are understood, manipulating the microbes in the gut could offer another approach to treating obesity.