Western diets rich in fat and sugar are causing obesity not only because they are so dense in calories but because they are changing the bacterial contents of the human gut, new research suggests.

Scientists in the United States have discovered that switching mice with typically human gut bacteria onto a high-fat, high-sugar diet leads to lasting changes in the microbes that promote weight and fat gain.

The findings, from a team led by Jeff Gordon, of Washington University in St Louis, add to growing evidence that diet-induced changes in human gut flora are a significant factor in the obesity epidemic. In 2006, 24 per cent of adults in England and Wales were classified as obese, compared with 15 per cent in 1993.

Energy-dense diets appear to be feeding this trend with a double whammy to the human waistline. As well as providing more calories than people can burn through exercise, they appear to encourage the growth of gut bacteria that process food more efficiently, adding further to this energy excess.

The adult human body is home to about 100 trillion bacteria, the greatest concentration of which live in our intestines. These change the way we metabolise food and drugs, and can have significant effects on health.

One of gut bacteria’s main functions is to break down food that the body cannot digest, increasing its calorific value. This, however, also means that the ecology of the gut can affect obesity.

Dr Gordon has previously found evidence that the gut flora of obese people differ significantly from those of the lean, containing higher concentrations of a group known as the Firmicutes which are efficient processors of otherwise indigestible foods.

In the new research, details of which are published in the journal Science Translational Medicine, his team has now investigated this further using mice to model the human gut.

The scientists took mice raised in a germ-free environment, which had no native gut bacteria of their own, and infected their intestines with human gut bacteria taken from faeces. They then examined how these bacterial communities evolved when the mice were fed on different diets.

When these “humanised” mice were placed on a high-fat, high-sugar diet designed to mimic those common in Western societies, the contents of their gut bacteria changed overnight.

“Switching from a low-fat, plant polysaccharide-rich diet to a high-fat, high-sugar ‘Western’ diet shifted the structure of the microbiota within a single day,” the scientists wrote. The mice showed an increase in types of bacteria linked to obesity, and also increased in body fat.

The researchers then transplanted microbes from the guts of these mice into other germ-free mice. These animals also put on weight, even when fed on a low-fat diet.

As well as supporting the idea that gut bacteria can influence obesity, the research is significant because it establishes a workable animal model for further studies. It “will be useful for conducting proof-of-principle ‘clinical trials’ that test the effects of environmental and genetic factors on the gut microbiota and host physiology”, the researchers said.

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