Abnormally high body fat, also known as obesity, has become an epidemic in the United States. The National Institutes of Health estimates that today approximately two-thirds of the population is overweight and nearly one-third is obese. This is a marked increase from 1960 when less than half the population was overweight and less than a sixth were categorized as obese. Obesity has serious health and economic consequences, and as a result, a great deal of research has probed the causes and consequences of obesity. The psychology behind obesity has been well studied as has certain genetic predispositions for becoming obese, but at a recent American Chemical Society conference in Boston, another potential cause of obesity was discussed– a virus. Could a virus be responsible for this epidemic?

The Many Causes of Obesity

In addition to the many widely recognized causes of obesity (over-eating high-fat or high-sugar foods, choosing a sedentary lifestyle, harboring genetic predispositions, and taking certain medications) some researchers hypothesize that infection with a virus could be added to that list. Beginning in the 1980’s, research has shown that animals infected with particular viruses have an increased propensity for obesity. The reasons why these viruses cause weight gain differ from virus to virus, some alter appetite or hormone levels, whereas others increase fat cell (adipocyte) production. Only a couple of these viruses can potentially infect humans, and only one, SMAM-1, has been linked to human obesity.

Fat: a Symptom of the Common Cold?

More recently, another virus called Ad-36 (short for adenovirus-36) was shown to affect weight in several different animal models. Ad-36 is well known for its ability to cause coughing, sneezing and cold-like symptoms in humans. However, a group at the Virginia-based research company Obetech tested the ability of this virus to cause weight gain in mice. They also conducted a study to determine if a connection exists between human obesity and Ad-36 infection. They wanted to know if obese people have a higher incidence of infection with the virus. To answer this question, they screened volunteers for antibodies to Ad-36. Antibodies, blood-borne proteins the body makes in response to specific infections, are made for years after a single infection and are fairly specific to particular viruses. The researchers found that 30% of the obese people in their study had been exposed to Ad-36, in comparison with only 11% of the non-obese people in their study. Interestingly, exposure to the virus was also associated with lower cholesterol levels, similar to what had been previously observed in mice. No correlation was found between other, similar adenoviruses, and weight. These data suggest a link between obesity and Ad-36 infection, but do not prove that Ad-36 causes any weight gain in humans.

Stem Cells to Fat Cells

The most recent development in this story is from a meeting in Boston. In August 2007, Dr. Magdalena Pasarica of the Pennington Biomedical Research Center in Louisiana, presented the first data showing that Ad-36 can induce human stem cells to become fat cells. Human stem cells have the ability to become many, although not all, types of cells in the body. The majority of cells infected with Ad-36 in the laboratory dish quickly became fat cells, compared to only a few of the uninfected cells. Even when infected cells were exposed to signals that normally cause them to become bone cells, they still became fat cells. This indicates that Ad-36 can work at a very early stage of cell fate determination to induce more fat cells.

What’s Next?

While this latest work has been presented at a scientific meeting, it has yet to be critically reviewed by other scientists or published in a scientific journal. Furthermore, because this work was mostly performed by one research group, the scientific process requires that the results be replicated and published by several independent researchers for confirmation. Nevertheless, this work is thought-provoking because it represents a different way of thinking about weight gain and obesity in our society. While the concept of being pre-disposed genetically to weight gain is accepted, might certain viral infections alter our chances of becoming obese? If so, how might this change our views about obesity or how to prevent it? These are interesting questions to consider, but for now, it is premature to worry about catching an “obesity virus”. First of all, it is important to note that a virus’s ability to increase one’s risk for obesity is not the same as actually causing obesity. Furthermore, it is unclear how infection with Ad-36 might increase one’s risk for obesity. Does the virus bias the development of stem cells into fat cells in humans as was shown to occur in the laboratory dish? Are there other explanations as well? Given all the unanswered questions, it may be some time before it is clear what role (if any) viruses play in contributing to human obesity. But at least one thing is clear, avoiding obese people will not lower your chances of catching Ad-36, because even if the individual had been infected, the virus is only infectious for a few weeks. The bottom line is that even if someday a vaccine is developed to prevent infection with Ad-36, the best strategy for remaining lean and healthy remains maintaining a healthy diet together with regular exercise.

–Kathryn M. Levasseur, Harvard Medical School

For More Information:

American Chemical Society press release: Common virus may contribute to obesity in some people, new study shows:
< http://www.eurekalert.org/pub_releases/2007-08/acs-cvm081007.php >

Science News Online: Infectious Obesity: Adenovirus fattens stem cells:
< http://www.sciencenews.org/articles/20070825/fob2.asp >

National Institutes of Health: Obesity Statistics:
< http://win.niddk.nih.gov/statistics/index.htm >

The California Solar Initiative:
< http://www.gosolarcalifornia.ca.gov/csi/index.html >

Primary Literature:

Atkinson et al. Human adenovirus-36 is associated with increased body weight and paradoxical reduction of serum lipids. Int J Obes (Lond). 2005 Mar;29(3):281-6.

Dhurandhar et al. Human adenovirus Ad-36 promotes weight gain in male rhesus and marmoset monkeys. J Nutr. 2002 Oct;132(10):3155-60.

Vasilakopoulou and le Roux. Could a virus contribute to weight gain? Int J Obes (Lond). 2007 Sep;31(9):1350-6.