The size of your gut may be partly shaped by which microbes call it home, according to new research linking obesity to types of digestive bacteria.

Both obese mice —and people — had more of one type of bacteria and less of another kind, according to two studies Thursday in the journal Nature.

A "microbial component" appears to contribute to obesity, said study lead author Jeffrey Gordon, director of Washington University's Center for Genome Sciences.

Obese humans and mice had a lower percentage of a family of bacteria called Bacteroidetes and more of a type of bacteria called Firmicutes, Gordon and his colleagues found.

The researchers aren't sure if more Firmicutes make you fat, or if people who are obese grow more of that type of bacteria.

But growing evidence of this link gives scientists a potentially new and still distant way of fighting obesity: Change the bacteria in the intestines and stomach. It also may lead to a way of fighting malnutrition in the developing world.

Obesity infectious?

Nikhil Dhurandhar, a professor of infection and obesity at Louisiana State University's Pennington Biomedical Research Center, wasn't part of the research, but said it may change the way obesity is treated eventually.

"We are getting more and more evidence to show that obesity isn't what we thought it used to be," Dhurandhar said. "It isn't just (that) you're eating too much and you're lazy."

He said the field of "infectobesity" looks at obesity with multiple causes, including viruses and microbes.

In another decade or so, the different causes of obesity could have different treatments. The current regimen of diet and exercise "is like treating all fevers with one Aspirin," Dhurandhar said.

In one study, Gordon and colleagues looked at what happened in mice with changes in bacteria level.

When lean mice with no germs in their guts had larger ratios of Firmicutes transplanted, they got "twice as fat" and took in more calories from the same amount of food than mice with the more normal bacteria ratio, said Washington University microbiology instructor Ruth Ley, a co-author of the study.

It was as if one group got far more calories from the same bowl of Cheerios than the other, Gordon said.

Bacteria levels change in dieters

In a study of a dozen dieting people, the results also were dramatic.

Before dieting, about 3 per cent of the gut bacteria in the obese participants was Bacteroidetes. But after dieting, the now normal-sized people had much higher levels of Bacteroidetes — close to 15 per cent, Gordon said.

"I think that gut bacteria affects body weight," said pathology Prof. Richard Atkinson, a Virginia Commonwealth University pathology professor, who wasn't part of the research team and is president of Obetech Obesity Research Center in Richmond.

"I don't think there's any doubt about that and they showed that."

The growing field of research puts more importance in the trillions of microbes that live in our guts and elsewhere, crediting it with everything from generations of people getting taller, to increases in diabetes and asthma.

People are born germ free, but within days they have a gut blooming with microbes. The microbes come from first foods — either breast milk or formula — the exterior environment, and the way the babies are born, said David Relman, a Stanford University medicine and microbiology professor, who was not part of the study.

For decades, doctors have treated bacteria in a "war-like" manner, yet recent research shows "most encounters we have with microbes are very beneficial," Gordon said.

Relman said: "Much of who we are, and what we can do and can't do as human beings is directly related to microbial inhabitants."