Ever heard the phrase ‘obesity epidemic' and thought it seemed a bit peculiar? Obesity's just about eating too much and exercising too little isn't it? It's hardly the bubonic plague or the Spanish flu.
Wrong, says Nikhil Dhurandhar from the Pennington Biomedical Research Center. For nearly 20 years he's been amassing evidence that an influenza-like virus called Ad-36 may be penetrating our systems and making us pile on the pounds.
But his argument may not be nearly as robust as the nation's expanding waistline.
It's true that if you inject Ad-36 into your average lab rat it makes his belly flabby. It looks like this happens by recruiting more cells to get busy turning energy into fat. Dhurandhar's also shown that very obese people attending obesity treatment are more likely to be infected than skinny ones out in the community.
But rats aren't people. And people attending obesity treatment are hardly typical of the population at large - they often have more severe cases linked with a host of health problems and lifestyle differences.
For a really thorough and critical review of all the evidence, you can take a look here. But it's probably fair to say the human obesity virus theory has yet to really...catch on.
Of course that's not to say that fatness isn't infectious in some other way.
Using weight and height data gathered from over 12,000 interconnected individuals, a team from Harvard looked at the spread of obesity through a social network. (I am still waiting for someone to replicate this experiment on Facebook or Twitter. We just need everyone to weigh themselves every week and own up to it in their updates...)
Chubby people evidently stuck together - there were clusters of obesity all over the network, extending through to people up to 3 degrees of separation from the index obese person. But heavier individuals also made their mates gain weight - a person was 57% more likely to become obese if he had a friend who also tipped the balance over the same time period.
Why? Well friends hang out together at the same places and do the same kind of things. Maybe it's no fun to get a salad when your companion just ordered a deep-fried grilled cheese sandwich. If your buddy's having a beer wouldn't it be rude to abstain? And who wants to walk to work when your boyfriend's driving and could give you a ride?
That said, catching your friends' habits needn't be a bad thing if they're better than your own. In my last job - also in obesity research - anyone who came in on the heavy side left feeling lighter. It was just impossible to enjoy a lunchtime burger and fries in the midst of all those celery sticks, fruit salads and wholegrain baps.
What should we take from all of this? First off, we probably don't need to start worrying about being doused in fatness germs the next time a large person sneezes in our vicinity. Secondly, although there's evidence that unhealthy habits can spread between people, we should take comfort in the knowledge that there's absolutely no reason why healthy ones shouldn't rub off too.
Maybe contagion needn't be such a bad thing after all.
