Poor eating habits, sedentary lifestyles, and genes have all been implicated as contributors to obesity. Now researchers have evidence that suggests another possibility -- that a virus may trigger fat accumulation in some people.

In experiments with chickens and mice, researchers found that a virus known as human adenovirus-36 (Ad-36) caused the animals to gain excessive amounts of fat. In contrast, chickens infected with a bird adenovirus called CELO did not pack on excess fat. While these results do not point to Ad-36 as a cause of human obesity, they do suggest that one or more viruses may be behind some people's weight woes.

In general, adenoviruses cause colds, diarrhea and eye infections, but little is known about how Ad-36 is spread or the types of infections it might cause. The researchers settled on Ad-36 because it does not cross-react with other adenoviruses, meaning it likely has "unique qualities."

The idea that a virus may trigger obesity is not new to the animal world. Four viruses have been shown to cause fat accumulation in animals, one reasearcher noted. "This," he said, "is the first time a human virus has been associated with obesity."

Researchers came across their first circumstantial evidence of a viral cause for human obesity in thier native India, where they discovered that a chicken adenovirus isolated in Bombay caused chickens to accumulate fat. Surprisingly, the virus also lowered the animals' cholesterol levels. Researchers next looked for evidence of infection with the chicken virus in a group of 52 obese people. The researchers found that 10 showed signs of infection.

More importantly, these people were the most obese, yet had lower cholesterol than those who were not infected with the chicken adenovirus.

Because the US would not allow researchers to import the Bombay chicken virus for study, they decided to investigate whether a human virus might be involved in fat accumulation.

In four separate experiments, researchers found that the animals infected with Ad-36 packed on fat, but showed "paradoxically low" cholesterol levels. Why this happened is unclear. Damage to the areas of the brain that control calorie use is a possibility, but they found no evidence of that in these experiments.

Much more study is needed to determine whether obesity is indeed an infectious disease. It may be that in some cases, Ad-36 or another virus interacts with obesity-linked genes or poor diet. Or, researchers say it's possible that a virus alone leads to obesity in some people, something that has been seen in animals.

What's important, according to researchers, is that scientists recognize the possibility that infectious organisms could play a role -- just as they are thought to do in ulcers and heart disease.

International Journal of Obesity August 2000;24:989-996