Obese people find it harder to fight infections, and a weakened immune response may be to blame, suggests a new study from Boston University researchers.

In experiments with mice infected with the bacteria Porphyromonas gingivalis, obese mice had less ability to battle gum infection than their normal-weight counterparts, according to the report in this week’s early online edition of the Proceedings of the National Academy of Sciences.

“For years, we have had difficulty understanding why obese people have difficulty clearing an infection,” said lead researcher Dr. Salomon Amar, associate dean for research at the university’s School of Dental Medicine.

“Now we understand that dysfunction in some of the mechanisms, as a result of the obesity, explain difficulty in clearing the infection and also the difficulty in wound healing,” Amar said.

In the study, Amar’s team tied silk threads infected with the bacteria around the molars of obese and normal-weight mice. They then compared the animals’ responses to infection, by measuring both the amount of bone loss and the growth of the bacteria around their teeth.

The researchers found that the obese mice had a compromised immune response to the bacteria, which made the animals more susceptible to the infection.

Amar’s group also looked at the animals’ white blood cells, which are the main line of defense against infection. The white cells of obese mice had lower levels of an important signaling molecule, and some of the genes that fight inflammation were altered, the researchers found.

Why obesity has this effect isn’t clear, but the researchers think it may involve a signaling pathway that controls a protein called NF-kB. Alterations in this protein may be caused by constant exposure to food, Amar explained. “At some point, the body doesn’t respond properly to infection,” he said.

The same mechanism is at work in humans, Amar added. In fact, studies in obese people have shown they are more likely to have gum disease than non-obese people. The disease is caused by bacteria, which causes inflammation and destruction of the bone underlying teeth.

Amar thinks that obese people need to be treated differently to help them fight infections. “We need to be more aggressive in the use of targeted antibiotics in infections among obese people,” he said. “Also, we need to boost the immune response.”

One expert agreed the finding sheds light on the connection between obesity and infection.

“Very interesting paper,” said Dr. Sara G. Grossi, a senior research scientist at the Brody School of Medicine of East Carolina University. “This is a study that needed to be done, with very interesting results and implications for both obesity and periodontal disease — two diseases that are easier to prevent than to treat.”

(Source: NHIC)