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updated 2/1/2011 8:33:29 AM ET 2011-02-01T13:33:29

Kids who have their tonsils removed seem to gain weight after the surgery and may be more likely to become overweight compared to children who never went under the knife, a new study suggests.

In the research published in the February issue of Otolaryngology — Head and Neck Surgery, scientists reviewed data from nine different studies spanning a 40-year period. They looked at the weight of 795 children ranging in age from 0 to 18 years old before tonsillectomy and for up to eight years after it.

"We found a greater-than-expected weight gain in normal and overweight children after tonsillectomy," said Dr. Anita Jeyakumar, who led the research team.

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In one analysis of 127 children six months to one year after surgery, the average body mass index of the kids increased by about 7 percent. In another analysis of 249 children, 50 to 75 percent of kids had weight gain after surgery. While most weight gain happened in the first year after surgery, scientists don't know definitively whether it levels off after that.

The scientists wonder whether this post-surgery weight gain in young people is contributing to the nation's obesity epidemic. That's because tonsillectomy is the most common major operation done in childhood and more than 500,000 surgeries take place each year in the United States on kids under 15.

"This weight gain has been going on for a long time, almost four decades. Now, we need to figure out why it's happening," said Jeyakumar, a pediatric otolaryngologist (ear, nose, and throat specialist) at Cardinal Glennon Children's Medical Center in St. Louis, Mo.

Tonsils are small clumps of infection-fighting tissue found on either side of the throat. When swollen, they can cause frequent throat or ear infections. Their enlarged size can also block a child's airways at night.

There are several theories about why some kids may be putting on pounds.

One is that when children have enlarged tonsils, they're spending more energy (calories) to breathe. Once they're removed, breathing is easier and uses less calories, Jeyakumar explained.

Another is that when tonsils are big and swallowing is difficult, children may limit the foods they eat or have less of an appetite. After the surgery, kids typically feel better and food probably tastes better, too.

While there appears to be a potential for weight to increase after the operation, it's unclear how much and it doesn't happen to every child. Other factors, like growth spurts and lifestyle habits, also play a role.

"There's a huge difference between suggesting an association between getting your tonsils out and gaining weight and demonstrating a cause-and-effect relationship," said Dr. Julie Wei, a pediatric otolaryngologist and associate professor at the University of Kansas School of Medicine in Kansas City.

In young and school-age children there's evidence of both a weight gain and a "growth spurt" after tonsillectomy that may be triggered by higher levels of growth factors.

For now, the best advice Wei has for parents is to "Keep a close eye on children after surgery and if you think they are gaining excessive weight, discuss this with your pediatrician."

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