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It may sound far-fetched, but the theory that a virus can make you fat is gaining credibility. In 1986 Nikhil Dhurandhar was treating obese patients in Bombay, India, while working on a PhD in biochemistry, when he had a conversation with a fellow scientist about an avian virus that was killing poultry. The scientist mentioned an odd effect the virus had on the infected chickens: Their abdominal cavities were full of fat, and the dead birds were far heavier than their healthy counterparts. A sick chicken should be a skinny chicken, Dhurandhar thought. He wondered what would happen if he exposed normal chickens to the virus. Sure enough, the ones that got infected developed significantly more body fat than the healthy birds and, paradoxically, lower cholesterol and triglycerides.

The findings were so compelling that he decided to test his patients for antibodies to the virus—and he discovered nearly 20 percent of them had been infected. Not only that, these were among the heaviest people in his practice, and they had lower cholesterol and triglycerides than most of his other patients.

Today Dhurandhar is a scientist at the Pennington Biomedical Research Center in Louisiana, studying a field he has named infectobesity. He and others have found nine viruses that cause obesity in animals, four of which also infect humans. He may have discovered part of the mechanism as well: After animals are infected with one particular human virus, their prefat cells mature and proliferate, increasing the number of fat cells in the body.

Dhurandhar says we are a long way from being able to tell some overweight people that their problem is a virus, or better yet, offering an obesity vaccine. But he points out that there is exploding research in the area of germs causing other chronic illnesses such as heart disease, autoimmune diseases, even depression. And he cites the experience of the two Australian researchers who suggested that a bacterium was responsible for stomach ulcers and were scoffed at for years—until they won the Nobel Prize for Medicine in 2005. Famously, one of those researchers swallowed a Petri dish of the bacteria to prove his case. Is the slender Dhurandhar willing to infect himself with one of his viruses to prove his thesis? He laughs and says if he did it and gained weight, "people would just say I ate too much."

Chicken viruses, NEAT factor—maybe these culprits explain why we're fat. Or maybe we get too little sleep. Or have too much stress. Or modern indoor temperature control protects us from the hot, sweaty, appetite-dulling days and shivering cold that used to keep people trim. Every few months, there seems to be a new theory. But whatever pans out among these ideas, science knows right now what works to lose weight and keep it off: Move more and eat less. That means making active choices whenever possible—getting up and changing the TV channel, taking the stairs, and being more conscious when it comes to diet (keep a food diary; look up calories at nal.usda.gov/fnic/foodcomp/search, and check out NWCR.ws). As for those people who holler that they eat like birds but complain they look like butterballs, whatever the cause of this dilemma, if you're mindful about what you put in your mouth, it feels as if you're eating a lot more.

Emily Yoffe is a columnist for Slate.com and the author of What the Dog Did (Bloomsbury).

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