MAYBE A VIRUS IS TO BLAME

The research: At least 10 viruses are believed to cause obesity in animals, and two have been tenuously linked to people. Antibodies against one (SMAM-1, which causes obesity in chickens) were found in about 20% of a group of 50 obese people tested in 1992. (Scientists don't know how many non-obese people would have antibodies to this virus as well.) In 2005, in a study of 500 people tested, antibodies to Ad-36 -- a virus that causes symptoms similar to the common cold -- were found in about 30% of obese people, but only in 11% of non-obese people. And in a 2005 test of 89 twin pairs, if one had antibodies and the other didn't, the one who did was generally heavier.

After exposure to Ad-36, chickens, mice, monkeys and rats hardly act under the weather at all, but their body fat increases, sometimes even doubling. Strangely, though, their total cholesterol, "bad" cholesterol and triglyceride levels go down. People who have antibodies for Ad-36 also have better metabolic profiles than people who don't.

It's unknown whether more people are exposed to Ad-36 now than 30 years ago since no one was tested then.

Our experts weigh in: "Obesity due to infection is possible in some people. How big the group is I don't know. . . . Not all obesity is due to viral infection," says Nikhil Dhurandhar of Louisiana State University, who has studied SMAM-1 and Ad-36 and coined the term "infectobesity" for "obesity of infectious origin."

"I think the data are new and emerging, and we just don't know yet," says Dr. Julie Lumeng of the University of Michigan, while Dr. George Bray of Louisiana State University says, "The jury is out on whether these are important in humans."

"I don't buy into these at all," says Susan Roberts of Tufts University.