Getting the slimming bug

The bacterial make-up of the intestines may help determine whether people gain weight or lose it,according to two new studies,one in humans and one in mice.

The research also suggests that a popular weight-loss operation,gastric bypass,which shrinks the stomach and rearranges the intestines,seems to work in part by shifting the balance of bacteria in the digestive tract. People who have the surgery generally lose 65 per cent to 75 per cent of their excess weight,but scientists have not fully understood why. Now,the researchers are saying that bacterial changes may account for 20 per cent of the weight loss.

The findings mean that eventually,treatments that adjust the microbe levels,or microbiota, in the gut may be developed to help people lose weight without surgery,said Dr Lee M. Kaplan,director of the obesity,metabolism and nutrition institute at the Massachusetts General Hospital,and an author of a study published Wednesday in Science Translational Medicine.

Not everyone who hopes to lose weight wants or needs surgery to do it,he said. There is a need for other therapies, Dr Kaplan said.

In no way is manipulating the microbiota going to mimic all the myriad effects of gastric bypass. But if this could produce 20 per cent of the effects of surgery,it will still be valuable.

Dr Kaplan said his groups experiments were the first to try to find out if microbial changes could account for some of the weight loss after gastric bypass. Earlier studies had shown that the microbiota of an obese person changed significantly after the surgery,becoming more like that of someone who was thin. But was the change from the surgery itself,or from the weight loss that followed the operation? And did the microbial change have any effects of its own?

The researchers used mice,which they had fattened up with a rich diet. One group had gastric bypass operations,and two other groups had sham operations in which the animals intestines were severed and sewn back together.

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The point was to find out whether just being cut open,without having the bypass,would have an effect on weight or gut bacteria. One sham group was kept on the rich food,while the other was put on a weight-loss diet.

In the bypass mice,the microbial populations quickly changed,and the mice lost weight. In the sham group,the microbiota did not change much even in those on the weight-loss diet.

Next,the researchers transferred intestinal contents from each of the groups into other mice,which lacked their own intestinal bacteria. The animals that received material from the bypass mice rapidly lost weight; stool from mice that had the sham operations had no effect.

Exactly how the altered intestinal bacteria might cause weight loss is not yet known,the researchers said. But somehow the microbes seem to rev up metabolism so that the animals burn off more energy.

Weight-gain microbes

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A second study by a different group found that overweight people may be more likely to harbour a certain type of intestinal microbe. The microbes may contribute to weight gain by helping other organisms to digest certain nutrients,making more calories available. That study was published Tuesday in the Journal of Clinical Endocrinology amp; Metabolism.

The study involved 792 people who had their breath

analyzed to help diagnose digestive orders. They agreed to let researchers measure the levels of hydrogen and methane; elevated levels indicate the presence of a microbe called Methanobrevibacter smithii. The people with the highest readings on the breath test were more likely to be heavier and have more body fat,and the researchers suspect that the microbes may be at least partly responsible for their obesity.