Gut microbes may hold the key to a pain-free way of tackling obesity, research suggests.
Scientists found that by modifying gastric bug populations in mice, they were able to induce rapid and significant weight loss.
The change occurred after bacteria from obese mice that had undergone gastric bypass surgery were transplanted into ordinary animals.
Surgery had the effect of altering the make-up of the gut flora, introducing a different balance which promoted slimming.
When this new mix of microbes was transferred to non-obese mice, the weight loss benefits were transferred too.
The US research shows that gastric bypasses do more than prevent food being digested. Much of their impact is due to altered ecology in the gut.
"It may not be that we will have a magic pill that will work for everyone who's slightly overweight," said study leader Dr Peter Turnbaugh, from Harvard University in Boston. "But if we can, at a minimum, provide some alternative to gastric bypass surgery that produces similar effects, it would be a major advance."
Gastric bypasses work by rearranging the gut so that it accommodates less food.
The research showed that after surgery different kinds of microbe began to take over. In particular, the gut became dominated by verrucomicrobia and gammaproteobacteria. In contrast levels of the Firmicutes family of bugs fell.
It took less than a week for the rebalancing to occur, and the effect continued for months afterwards.
The new population of bugs appeared to drive weight loss, and continued to do so when transferred to a non-obese group of mice that had not undergone a gastric bypass.
"Simply by colonising mice with the altered microbial community, the mice were able to maintain a lower body fat and lose weight - about 20% as much as they would if they underwent surgery," said Dr Turnbaugh.
He suspected an even more dramatic result would have been seen if the mice receiving the bugs had been fattened up beforehand.
How particular populations of microbes induce weight loss remains unclear.
The answer may be linked to waste products the bugs excrete, according to the research published in the journal Science Translational Medicine.
Along with the altered microbes, the scientists found changes in the concentration of certain short-chain fatty acids. Previous studies have suggested the molecules may trigger signals that cause the body to speed up metabolism, or store fewer calories as fat.
"A major gap in our knowledge is the underlying mechanism linking microbes to weight loss," said Dr Turnbaugh. "There were certain microbes that we found at higher abundance after surgery, so we think those are good targets for beginning to understand what is taking place."
Co-author Dr Lee Kaplan, from Massachusetts General Hospital in Boston, said: "We need to learn a good deal more about the mechanisms by which a microbial population changed by gastric bypass exert its effects, and then we need to learn if we can produce these effects - either the microbial changes or the associated metabolic changes - without surgery.
"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."