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The ability to achieve even some of these effects without surgery would give us an entirely new way to treat the critical problem of obesity.”

Dr Lee KaplanHarvard Medical School

In the latest study, researchers compared three groups of obese mice on a high-calorie diet.

One group was given a gastric bypass

One was given a sham operation, and the high-calorie diet continued

One was given the same fake operation but then fed a low-calorie diet to promote weight loss

A week later the mice who had undergone the real obesity surgery had different bacteria in their guts, with an increase in types usually seen in lean individuals and a drop in types associated with obesity.

There was little change in micro-organisms present in the mice who had had sham operations, even though the group on the low-calorie diet lost just as much weight as the mice who had had the bypass surgery.

Metabolism impact

Researchers then transferred samples from the guts of the three groups of mice into other germ-free mice.

Those who received bacteria from the bypass mice, lost a significant amount of weight in two weeks but the others saw no change.

It is not yet clear how the microbes influence weight loss, but one theory is that they have an impact on metabolism.

"We need to learn a good deal more about the mechanism by which a microbial population changed by gastric bypass exerts its effects," said study author Dr Lee Kaplan, an associate professor of Medicine at Harvard Medical School.

"The ability to achieve even some of these effects without surgery would give us an entirely new way to treat the critical problem of obesity, one that could help patients unable or unwilling to have surgery."

Co-author Peter Turnbaugh added: "It may not be that we will have a magic pill that will work for everyone who's slightly overweight.

"But if we can, at a minimum, provide some alternative to gastric bypass surgery that produces similar effects, it would be a major advance."

Prof David Haslam, from the National Obesity Forum, said: "We know the effects of bariatric surgery are not just mechanical and we don't know the full reasons why it works so well, especially in the resolution of diabetes.