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WEIGHT gain may be infectious. Evidence is growing that many people who are overweight may have a virus to blame.

Some have long thought that a virus is involved in some cases of obesity, but the idea is still highly controversial. Now a study shows for the first time that this virus, called adenovirus-36, is more often found in people who are obese than in those of a healthier weight. We don鈥檛 know yet how people catch the virus, but it may spread from person to person.

鈥淲hen the adenovirus was injected into mice and monkeys it made them put on weight鈥�

Being overweight is usually attributed to overeating or getting too little exercise, with both genetics and environmental factors thought to contribute. In the 1980s, Nikhil Dhurandhar, now working at Texas Tech University, heard of a virus affecting chicken flocks in India that made the birds grow fat. He found that a relative of this virus, adenovirus-36, made mice and monkeys put on weight. And when fat cells are grown in a dish, infecting them with the virus makes them store more fat.

In humans, adenoviruses usually cause colds, diarrhoea or eye infections. For ethical reasons, we can鈥檛 just inject adenovirus-36 into people and see if they put on weight. Instead, Dhurandhar and other groups looked to see whether people who are overweight are more likely to have antibodies to this virus 鈥� a sign that their immune systems have encountered it. They found that they did: , for instance, reported that 30 per cent of obese people had these antibodies, compared with 5 per cent of those who were a healthy weight.

Now Wilmore Webley at the University of Massachusetts Amherst has searched for the virus itself in people, rather than antibodies. His team looked at 80 biopsies taken from women with breast cancer. Analysing healthy breast tissue from them, the team found that 81 per cent of the samples from overweight women contained the virus, while just 19 per cent of samples from healthy-weight women did. 鈥淭hat is a big difference,鈥� says Webley. The findings were presented at a conference of the American Society for Microbiology in Atlanta, Georgia.

鈥淎ll scientists in the field of virus-induced obesity, including me, have been waiting for this kind of data,鈥� says Jae-Hwan Nam at the Catholic University of Korea in Seoul, South Korea.

Ongoing infection

Antibodies against a virus persist in our bodies long after a virus has been eliminated. Dhurandhar says the latest findings, based on measuring viral DNA instead of antibodies, suggest that obesity may be caused by an ongoing infection with the virus, rather than a past one that the body has defeated. It is possible that most people catch the virus from those showing no signs of infection.

Yet the findings still may not be enough to convince everyone. 鈥淭he big problem to me is that no matter how many association studies are reported, they do not prove causality,鈥� says Nick Finer at University College London.

One possibility is that people who are overweight are more susceptible to catching this virus. 鈥淭his data could be a consequence of obesity increasing the risk of particular infections,鈥� says Keith Godfrey at University Hospital Southampton in the UK.

Weighing against that explanation, however, are the earlier studies that found injecting the virus into monkeys makes them put on weight.

The infection rate in the women with breast cancer was higher than previous estimates of how common the virus is among overweight people. That may be because the virus helps trigger cancer directly, says Richard Atkinson at the University of Wisconsin-Madison, who did the initial work on the virus alongside Dhurandhar.

In unpublished work, Atkinson has found that breast cells infected with the virus in the lab multiply faster than normal, suggesting it may promote cancer. Other tumours, such as cervical cancer and liver cancer, are known to be caused by viruses.

Atkinson has helped develop a vaccine to prevent infection with adenovirus-36. This was successful at stopping mice from putting on weight after being exposed to the virus, a . However, no company has so far wanted to invest in the vaccine. 鈥淧eople just don鈥檛 accept that a virus causes obesity,鈥� says Atkinson. 鈥淏ut there鈥檚 precedent for the idea that a virus could cause cancer.鈥�

This article appeared in print under the headline 鈥淚s a virus causing obesity?鈥�