Almost one-quarter of adults in the U.S. are obese, and despite what seems to be a nationwide obsession with bodyweight, that figure is increasing, according to the National Institutes of Health.The reasons why the disease is so widespread have included poor diet, lack of exercise, metabolism problems, and heredity.
But now there may be another explanation. A study by researchers at two universities has shown that some obesity may be caused by a virus.
A study published in the August issue of the peer-reviewed International Journal of Obesity by scientists at Wayne State University and the University of Wisconsin may offer a clue to the mystery of why obesity is still on the riseóa human adenovirus known as Ad-36. Researchers inoculated chickens and mice with the virus and found that it caused them to gain excessive amounts of fat.
image: Dr. Milan Nermut, National Institute of Biological Standards and Control
"In a number of animal experiments that we did we have shown time and again that this virus definitely causes obesity and increased fat deposition in animals," says lead author Nikhil Dhurandhar, a nutrition researcher at Wayne State University.
Dhurandhar and his colleagues conducted three experiments with chickens and one with mice to find out the effects of the virus over varying amounts of time. While the chickens infected with the virus ate only slightly more than controls that were not infected, they developed 2.5 times more body fat. "The body weight of infected animals is [only] about five to seven percent more, but it is the body fat which is dramatically different," notes Dhurandhar. The researchers also infected chickens with a type of virus found in birds, called CELO, but these animals didnít gain body fat, which suggests that adenoviruses vary in their ability to increase fat.
Although Ad-36 didnít cause any typical symptoms of infection, it did have one strange one: Chickens infected with it had lower cholesterol and triglyceride (fat in the blood) levels. "We want to find out why we see this lowered cholesterol," says Dhurandhar. "Is it getting reduced because its production is going down, or are the cholesterol levels going down because it is getting deposited, for example, in your heart?" Right now, there is no answer to this question.
|Some chickens gained excessive amounts of body fat.|
While this is the first time that a human virus has been shown to cause obesity in animals, animal viruses have been known to do it before. At least four animal viruses have produced obesity in mice, chickens, and rats.
Experiments canít be done by infecting people with adenovirus, but an earlier study by Dhurandhar did look at obese people to find out if they carry antibodies to the virus. The presence of antibodies is indirect evidence that someone was once exposed to the virus.
When 154 obese and 45 lean human volunteers were tested, about 30 percent of the obese people had antibodies to Ad-36, and five percent of the lean people did.
Little is known about Ad-36, one of about 50 human adenoviruses. These viruses cause colds, upper respiratory infections, gastrointestinal problems and eye inflammation. Many adenoviruses are transmitted through the air, but scientist arenít sure if thatís the case with Ad-36. "We do not know if this virus can be transmitted in humans and if it does, how," says Dhurandhar. "We need more research and more understanding of the mechanism before we can comment on its role and how it is transmitted."
Obesity is a serious chronic disease. People who suffer from it are more likely to have high cholesterol, gallbladder disease, arthritis, sleep apnea, and lower-back problems. It can also lead to other debilitating illnesses, including heart disease, stroke, high blood pressure, diabetes, and some types of cancer. Even worse, according to the American Heart Association, obesity is a risk factor for five of the 10 leading causes of death in the U.S.
While everyone needs a certain amount of body fat, obesity is defined as having an unhealthy amount of it, or a Body Mass Index (BMI) of 30 or more. BMI is calculated by dividing a persons weight in kilograms by the square of height in meters.
Even though weíve become more health and diet-conscious in recent years, obesity continues to rise. Between 1991 and 1998, the prevalence of obesity increased by more than 50 percent in the U.S., according to a study published in the Journal of the American Medical Association.
Linking microbes to diseases... and cures
The link between a virus and obesity may seem surprising, but in recent years other microbes have been found to cause chronic illnesses in people. For example, both Chlamydia pneumoniae and cytomegalovirus (CMV) may cause heart disease. And most ulcers are caused by the bacteria Helicobacter pylori, not excess acid or stress. This means they can be easily treated with antibiotics.
But that doesnít mean that viruses or bacteria are always the sole causes of certain diseases. "All these pathogens might be acting in combination with other environmental factors which might precipitate that particular chronic disease," Dhurandhar points out.
So in the case of obesity, it could be that a combination of a virus, together with genes for obesity, a high-fat diet, and a sedentary lifestyle is the cause. If thatís the case, itís possible that treatmentsóor even a vaccineócould be developed for obesity caused by infections.
"I just hope this research opens doors, avenues for other researchers to consider infection as one of the possible causes for obesity," says Dhurandhar.
Elsewhere on the web:
American Obesity Association
Helping Your Overweight Child
Obesity information from the NIH
The North American Association for the Study of Obesity
The National Association to Advance Fat Acceptance (convention August 1 - 6 in San Diego)
Disease risk relative to weight
Weight loss and control from the NIH
Calculate your BMI
Understanding Adult Obesity