Contagious Obesity

Dhurandhar first discovered that viruses could cause obesity while working with patients in India. He noticed that people who had been exposed to a chicken adenovirus called SMAM-1 were consistently fatter than those who had had no exposure. Later on, having moved to the U.S., Dhurandhar collaborated with other researchers to study whether human adenoviruses, which are common throughout the human population, could be having a similar effect. This led to the discovery that the human adenovirus Ad-36 caused significant fat increases in animals, and what's more, was associated with obesity in humans.

"Because we knew that Ad-36 causes obesity or increased fatness in animals and is associated with obesity in humans, we wanted to determine if any of the other more than 50 human adenoviruses could be having the same kind of effect," Whigham explains. "So we chose some additional viruses to test."

Adenovirus

"The chickens infected with the Ad-37 virus had three times more visceral fat and about twice as much total body fat as the control chickens," she says.

The researchers conducted a second series of experiments that involved infecting pre-fat cells with the different viruses in the lab. "We actually infect the cells in cell culture, and then, in the case of Ad-36, we see that these cells take on fat and become mature fat cells more quickly and to a greater extent than the control cells," Whigham explains.

In this case, both Ad-37 and Ad-31 viruses caused the fat cells to multiply and store more fat. As Ad-31 was not found to increase fat in the chickens, Whigham says, "This indicates that we cannot simply rely on the cell culture model to test all of these additional — more than 50 adenoviruses — for their potential to increase fatness."

As it's unethical to test these viruses in humans, for now all of the evidence for their effect on people comes from associative data. Previous work done by Dr. Dhurandhar and his colleagues showed that in a randomly tested population, 30 percent of obese people had been exposed to Ad-36 as compared to only five percent of lean people.

"We even have some twin data that indicates that if you have twins who are discordant for [Ad-36] — meaning one has been exposed to the virus and one hasn't — the twin who has been exposed to the virus and has antibodies to that virus is more obese than the twin," Whigham adds.

However, she says once the mechanism of the viruses is better understood, it will be possible to develop tests that more definitively determine whether the virus is causing obesity in humans. She says that all that can be said about the mechanism of the viruses at this point is that they work on a cellular level, turning on and off genes related to fat cell maturity.

"When I first heard that obesity might be caused by a virus, I was quite surprised. As one now looks back at the increase in obesity, clearly it's rising very fast and it has to be something that's in the environment," Greenway says. "Now, I don't think that viruses are causing the whole problem, but they may well be part of it."

Whigham says infection with one of these viruses is likely not a guaranteed sentence to obesity. "At this point there's no reason to cause people to panic that if they get exposed to the virus they will definitely become fat. And I also remind people that there's lots of good reasons to follow a healthy diet and a regular exercise program," she adds.

Important next steps in this line of research will include determining how many of the other adenoviruses could be causing increased fat. Once they are all identified, it may be possible to start developing a vaccine. Till then, our best bet is to keep our hands clean.

Anyone who feels their exercise and dieting is failing for more than just the ordinary reasons can actually get tested — Obetech, LLC has developed a test for antibodies to Ad-36. The company was founded by Richard Atkinson, another of the main researchers in this field, and one of the authors on this study.