Overweight? Change you gut bacteria!

Your gut reaction influences your weight

• 18:00 20 December 2006
• NewScientist.com news service
• Roxanne Khamsi

The amount of weight an animal gains from eating depends partly on the types of microbes found in its gut, a new mouse study reveals.

Mice with microbes that are better at extracting energy from food end up fatter, researchers say. And when obese people diet, they lose some of these fattening microbes from their guts, according to a related report.

The new findings could help nutritionists develop new ways of preventing and treating weight problems.

People, like mice, are born without any bacteria in their gut. But within the first few years of life a wide range of microbes colonise their intestine and help them to breakdown and digest food.

To explore the influence of these bacteria, Jeffrey Gordon at the University of Washington in St Louis, Missouri, US, and colleagues turned to a special “germ-free” breed of laboratory mice that lack the trillions of microbes that normally reside in the rodent gut.

Previous studies have shown that these mice stay slim if kept inside a sterile, microbe-free environment (see Slimming for slackers).

Grossly obese

The team inoculated these animals with a sample of gut microbes from either normal mice or mice that have a mutation in the “obese” gene. This gene controls the expression of an appetite-regulating hormone – animals that carry a mutated copy are grossly obese because they overeat excessively.

The germ-free mice that received bacteria from normal mice subsequently experienced a 25% increase in body fat. Those that received microbes from obese mice, on the other hand, experienced a 45% increase in body fat.

To explain why injections from obese mice caused a greater weight gain, scientists compared the microbial makeup of these mice with their normal counterparts. They found that, compared with normal mice, the obese rodents had more microbes from the bacterial group known as Firmicutes and fewer of those belonging to the group Bacteroidetes.

Compared with Bacteroidetes bacteria, Firmicutes bacteria might have a wider range of enzymes for breaking down complex carbohydrates, the researchers say. This could make them more efficient at extracting energy from food, which our bodies ultimately convert into fat.

"Striking result"

In a related study, Gordon’s team determined the microbial makeup of 12 obese people by conducting a genetic analysis of their faeces. This revealed that these individuals had a higher ratio of Firmicutes to Bacteroidetes bugs than lean people involved in the study. For example, Bacteroidetes microbes constituted 5% of the obese people’s gut flora, but 20% of the lean subjects’.

After a year of either a carbohydrate- or fat-restricted diet, the obese individuals lost weight and the ratio of Firmicutes to Bacteroidetes bugs shifted towards that of their lean counterparts. In the end, Bacteroidetes made up about 15% of their gut flora.

“It’s a striking result and it’s constant across all of these different people,” comments David Relman at Stanford University in California, US, who did an earlier, landmark genetic analysis of human gut bugs (Science, vol 308, p 1635). “It’s not just having microbes, it’s the particular makeup that might matter,” he adds.

Journal: Nature (DOI: 10.1038/nature4441022a) and (DOI: 10.1038/nature05414)