A good pair of dentures may fend off dementia. That's the message from Japanese researchers, who say that chewing helps prevent memory loss as we grow old.

New memories are briefly stored in the hippocampus, a brain area critical for learning. But as we age, hippocampal cells start to deteriorate and our short-term memory gets worse. Elderly people with missing teeth often chew less, too, and some studies have suggested that bad memory and tooth loss might be linked.

The researchers looked at mice that have been genetically altered so they rapidly develop signs of human aging, such as hair loss, cataracts and failing memory. They extracted the molar teeth of some mice, so they could still eat, but couldn't chew.

The memories of the mice were then tested in a water maze. Young mice quickly learned to find a hidden platform, regardless of whether they had molars missing or not, and old mice with a full set of teeth performed only slightly worse. But the old molarless mice couldn't remember how to find the platform, consistently heading out into the pool in the wrong direction. Looking at their hippocampi suggested why: essential cells called glia had deteriorated far more than usual.

The findings suggest that chewing is essential to preserve our ability to form memories in old age. To pursue this, the researchers used magnetic resonance imaging (MRI) to study brain activity while people were chewing. The task causes an increase in hippocampal signals. Chewing may improve our memories by reducing stress. The hippocampus helps control levels of stress hormones in the blood. So if older people chew less, their stress levels might rise enough to cause a decline in short-term memory.

Behavioral Brain Research (vol 108, p 145)