Obesity can weaken the body's immune system and reduce its ability to fight off infections, according to scientists.

Previous studies have hinted at a link between obesity and increased risk of bacterial infections, but there has been little research into how serious the effects are.

It is a pressing issue because obesity has reached epidemic proportions. In the UK, more than a fifth of adults are obese and of the remaining population half of men and a third of women are overweight.

Worldwide, there are an estimated 300 million obese people and obesity is already linked to heart disease, diabetes and premature death.

In their experiment, researchers led by Salomon Amar at Boston University infected obese mice with bacteria which cause gum disease. They found that compared with lean mice, the obese mice showed a 40% greater increase in tooth decay and loss of bone 10 days after being infected.

Writing in the Proceedings of the National Academy of Sciences yesterday, the Boston researchers found that the obese mice also had more bacteria around their gums compared with lean mice, concluding that this was due to their inability to mount a proper immune attack on the bugs. This meant the bacteria could linger in the mouth and cause more damage.

Though it is unclear exactly what causes obesity to affect the immune response, the researchers suggested that gaining weight might upset a mechanism in the body that reacts to foreign organisms. They added that the effect would have implications for many bacterial infections.