By Ian Sample, Science correspondent
Bugs that lurk in the guts of slim people could be turned into radical new therapies to treat obesity, according to a study.
The claim follows a series of experiments which found that the different populations of bacteria that live in lean and overweight people caused mice to lose or gain weight. The findings build on a growing amount of research that gives the millions of microbes that live in the gut a major role in weight control.
Scientists at Washington University school of medicine in St Louis said the research paved the way for new therapies that tackle obesity by altering the types and numbers of bugs that make their home in the gut.
Researchers led by Jeffrey Gordon recruited four pairs of women who were twins. One woman in each pair was obese, but the other had a healthy body weight.
From each woman, the researchers collected faeces which contained a wealth of expelled gut microbes. Through a number of tests, the scientists investigated what happened when they transplanted these into mice bred to have no gut microbes of their own.
The scientists found that mice stayed slim when they received faecal transplants from slim women, but put on much more fat when the samples came from the obese twin. Tests revealed that one type of bug, called bacteroides, was more plentiful in slim women and protected the animals from putting on too much fat.
In a follow-up experiment, mice with microbes from the slim women shared a cage with mice that had microbes from obese women. Because of the animals' proclivity for coprophagia - their habit of eating each others' faeces - this caused a mixing of the animals' gut microbes. After the mice had spent 10 days as cage mates, the obese ones had become leaner. But this only happened if the animals were fed a healthy diet that was high in fibre and low in saturated fats. When the diet was switched to high-fat, low-fibre meals, the obese mice remained overweight.
The scientists think that a healthier diet allowed "good" microbes to thrive in the animals' guts, and even reverse obesity in the overweight mice. But a more typical western diet, high in fat and low in fibre, blocked the effect. That would explain why there is no "epidemic of leanness" in the US and elsewhere in the west, the scientists say.
Gordon said the findings, which are published in the journal Science, would steer the development of foods and new therapies that treat obesity by altering the makeup of microbes in the intestines.
"In the future, the nutritional value and the effects of food will involve significant consideration of our microbiota, and developing healthy, nutritious foods will be done from the inside out, not just the outside in," he said.
In an accompanying article, Alan Walker and Julian Parkhill at the Sanger Institute in Cambridge called the work
"a step toward the ultimate goal of developing relatively simple mixtures of bacteria for testing as anti-obesity therapeutics".
© 2013 Guardian Newspapers Limited