Could obesity be infectious?

Scientists have recently found that obesity can spread between friends if they copy each other's eating habits. Now, a new study has suggested it could actually be infectious.

In the study, published in the journal Nature, mice were engineered to have a immune deficiency leading to fatty liver disease and got fatter when fed a Western-style diet.

But strikingly, when these immune-deficient mice were put in the same cage as healthy mice, the healthy mice started to come down with symptoms of liver disease, and also got fatter, researchers found.

The culprit? Microbes in the stomachs of the mice, said Richard Flavell, a professor of immunobiology at Yale School of Medicine who led the study.

Because the mice had their immune systems disturbed, the bacteria in their guts got "out of wack," said Flavell.

"We normally live in symbiosis with the bacteria in our guts, but in the study, the number of 'bad' disease-associated bacteria increased 1,000-fold in mice with immune problems," Flavell was quoted as saying by LiveScience.

And it's these bad bacteria that were transmitted from mouse to mouse, causing the healthy mice to also experience changes in their gut microbes -- and making them fat, he said.

"We could make a mouse fatter just by putting it in the same cage as the other mouse," he added.

The crucial question is: Could this happen in people?
It's possible, but we will need much more research to find out, Flavell said. The contagiousness of obesity seen in this study is probably more likely in mice than in people because mice eat each other's poop, a very efficient way to transmit gut bacteria, the researchers added.

At least, the study suggested that "this should be very seriously looked at in people", Flavell said.

Fatty liver disease is very common among obese people, affecting 75 per cent to 100 per cent of the obese population. And in about 20 per cent of these individuals, the disease progresses and becomes severe, the researchers said.

Previously, if two family members living in the same household both developed liver disease or became obese, people would have blamed genetics. But the new study suggests the environment may play a role as well, they pointed out.

If the findings apply to people, they would suggest that we need to take approaches to obesity and fatty liver disease that address gut bugs -- perhaps antibiotics or probiotics -- in addition to traditional treatments, Flavell said.

"This is a very thought-provoking study that underlines the role of the bugs that we all carry inside us in determining our susceptibility to liver disease and its complications," said Dr Jasmohan Bajaj, an associate professor of gastroenterology at Virginia Commonwealth University,  who was not involved in the study.

More work is needed in humans, who are much more complex than mice, to understand the role of gut bacteria in liver disease, but "these experiments form a key step forward," he said.