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Receding gums reveal more bone loss around the roots of the teeth of obese mice (bottom), indicating more severe gum disease than that found in lean mice (top).

Salomon Amar/Boston University

Obesity Is Bacteria's Little Helper

Fat mice should consider flossing. Although not the exact conclusion of a new report, the study does indicate that--at least in rodents--obesity weakens the immune system's ability to fight off bacteria that cause gum disease. The finding helps explain why obese people are more likely to develop the oral ailment and suggests that they may be more vulnerable to other bacterial infections as well.

Obesity raises the risk for developing several serious conditions, including heart disease, diabetes, and certain cancers, and a growing body of evidence indicates that it also impairs the immune system. Earlier this year, a study in mice found that obese animals had more trouble fighting viruses, and previous research in people suggests that obesity suppresses key components of the immune system that help wipe out germs. Now a team led by oral biologist Salomon Amar of Boston University has shown for the first time that obesity also appears to reduce the immune system's ability to thwart bacteria.

The first thing the researchers needed to do was to plump up some mice. Five mice fed a calorie-laden, high-fat diet for 16 weeks bloated to 42 grams, or 1.5 times the weight of animals dining on standard mouse chow. Then, the researchers wrapped a silk thread soaked in a solution containing the bacteria Porphyromonas gingivalis, which cause gingivitis, around a tooth of each mouse to cause gum-disease infection. Compared to lean animals, the obese mice experienced 40% more bone loss around the roots of their teeth within 10 days after infection and had higher levels of bacteria in their plaque.

Further experiments showed that the obese mice had lethargic immune systems. When the researchers injected the gingivalis bacterium into the animals' tails, lean mice bumped up immune system components that respond to infections, including tumor necrosis factor-α and interleukin-6. This response was blunted in obese mice. The researchers report their findings online this week in the Proceedings of the National Academy of Sciences.

How obesity impairs the immune system is not yet clear, but infectious disease researcher Herbert Tanowitz of Albert Einstein College of Medicine of Yeshiva University in New York City suggests that the chronic low-level inflammation that occurs as a result of obesity may somehow be at fault.

Periodontist microbiologist Robert Genco of the University at Buffalo in New York state says the findings raise the possibility that another immune system component, neutrophils, is also suppressed in obesity. Neutrophils, which were not monitored in this study, kill bacteria, so if these cells are compromised, more severe disease could develop, he says.

Tanowitz adds that obese people probably have the same impaired immune response, a serious concern because gum disease can lead to tooth loss and raise the risk of heart disease and stroke. "There is good evidence that people who are obese have more problems, at least in the hospital setting, with bacterial infections," he says.

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