TUESDAY, Aug. 21 (HealthDay News) -- New research is bolstering the theory that obesity may stem, a least in part, from a common virus -- one that helps create new, heftier fat cells.

The roots of obesity are probably complex and various, the U.S. team stressed. However, their lab tests showed that exposure to adenovirus-36 (Ad-36), which causes respiratory and eye infections, also causes stem cells to develop into fat cells.

This is the first time anyone has identified a viral "fattening effect" in humans, said lead researcher Dr. Magdalena Pasarica, an obesity researcher with the Pennington Biomedical Research Center at Louisiana State University in Baton Rouge.

The same team was also the first to have shown that Ad-36 is much more prevalent among obese people than among leaner men and women. In that earlier work, the virus was spotted among 30 percent of obese individuals compared with just 11 percent of non-obese people.

In the new study, "we took this a step further, and showed for the first time that Ad-36 stimulates human adult stem cells to become pre-fat cells and store more fat," said Pasarica.

Her team was scheduled to announce the finding Monday at the national meeting of the American Chemical Society, held this week in Boston.

Pasarica emphasized that it's not yet been proven that Ad-36 actually causes obesity. Even if that does turn out to be the case, not all people who are infected with the virus will inevitably pack on the pounds.

Nevertheless, the possibility that a virus helps drive some, if not all, cases of obesity raises the notion of a vaccine or anti-viral drug that could be developed to target such infections and fight off fat, the team said.

The potential treatment benefit could be enormous, as underscored by recent U.S. National of Institute of Health (NIH) statistics indicating that 97 million American adults are currently either overweight or obese. Excess weight gain exacerbates risks for many health complications, including heart disease, diabetes, and arthritis.

Funded by the NIH, Pasarica's current effort focused on the impact of Ad-36 viral infection on fatty tissue extracted from adult male and female liposuction patients.

Stem cells can develop into a variety of cell types. In this study, the researchers obtained adult stem cells from the tissue samples and then infected half of the cells with the virus. The cells were allowed to grow in culture for about a week.

By the end of that period, most of the infected cells infected with Ad-36 had developed into so-called "pre-fat" cells, but the uninfected cells did not.

The more virus they inserted into the cells, the greater the fat growth observed, the researchers said. This effect was evident among tissue taken from both men and women.

In addition, the virally-exposed pre-fat cells were found to collect fat at a faster than normal rate, resulting in a double-whammy effect -- more, and bigger fat cells.

The team was also able to identify a particular gene in the Ad-36 virus -- known as E4Orfl -- that appears to be directly responsible for the promotion of fat growth. The identification of this gene provides a target for future anti-obesity therapies, the team said.

Exactly how the virus might cause obesity remains unclear. The researchers are also unsure of the length of time the virus might remain in infected patients and how long its effects continue even if it is eradicated.

Pasarica said much more research is needed to explore all these questions.

Dr. Barbara B. Kahn, chief of the division of endocrinology, diabetes, and metabolism at Beth Israel Deaconess Medical Center in Boston, said the findings were still "inconclusive" and not a license to overeat.

"The bottom-line on the viral thing is it's very intriguing, (and) it definitely deserves more investigation, but while it may be a contributing factor, it's certainly not going to explain the majority of obesity in our society," said Kahn, who is also a professor of medicine at Harvard Medical School.

"In general, to become obese there has to be some sort of mismatch between energy intake and energy expenditure," she noted. Again, that gets back to the same old advice: Eat healthfully and exercise regularly to keep weight under control.

More information

There's more on the causes of obesity at ObesityinAmerica.

SOURCES: Magdalena Pasarica, M.D., Ph.D., obesity researcher, Pennington Biomedical Research Center, Louisiana State University, Baton Rouge; Barbara B. Kahn, M.D., chief, division of endocrinology, diabetes, and metabolism, Beth Israel Deaconess Medical Center, and professor, medicine, Harvard Medical School, Boston; presentation, national meeting, American Chemical Society, Aug. 19-23, 2007, Boston

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