A team of Japanese neuroscientists may have bitten off more than they could chew. Or at least more than their research participants could chew. There may be some truth to the stereotype of the slow-witted toothless guy. According to their study, having fewer teeth may mean having a worse memory. Time also plays a role; the longer a person has been toothless, the more likely they are to experience cognitive decline. Now might be a good time to start following your dentist's advice about regular flossing.
Nozomi Okamoto and his collaborators were interested in parsing out which factors might predict memory decline as we age. The gums might not seem like the first place to look for brain power, but past research has shown that periodontal disease is associated with the development of dementia. In a rat model, removing the rodent's teeth resulted in decreased chewing, but peculiarly it also led to fewer brain cells in the hippocampus and a reduction in the brain chemical acetylcholine, two markers of Alzheimer's. Some evidence suggests that inflammatory agents originating from gum disease may lead to inflammation in the brain. Might dental health be an indicator of mental ability?
Okamoto recruited over 4,000 participants over age 65 from the Nara region of Japan, just south of Kyoto. Okamoto assessed participants for MMI, or Mild Memory Impairment, a preclinical stage of dementia. He also had two dentists screen them to count the number of remaining teeth and, for toothless subjects, the length of time without teeth. To account for other possible explanations of memory function, Okamoto also tested for level of depression, typical alcohol intake, smoking habits, lifestyle, education and medical history.
Even when controlling for other factors such as education and lifestyle, having fewer teeth was a significant risk factor for performing worse on memory tasks and demonstrating MMI. Participants who had been toothless for over 15 years were three times as likely to have MMI compared to their toothy peers. Now we know why they're called wisdom teeth.
The findings come with some caution. Okamoto only met with each participant across a few months, but most studies of aging require researchers to follow their subjects across many years to determine whether a relationship is causal or merely a correlation. Tooth loss may only be an indication of mental function, but we don't know if it precedes cognitive decline or follows from it. It could be the chicken or the egg. For example, Okamoto also didn't look at genetics, such as the Alzheimer's-linked ApoE4 gene, which may explain both tooth loss and cognitive decline.
The association between tooth loss and memory may be due to poor dental health. Periodontal disease often means high levels of inflammation in the gums and supportive tissue for the teeth, but the effects aren't limited to the mouth. Unhealthy gums are also linked to heart disease and stroke, possibly because the same bacteria which inflame the gums can enter the bloodstream and cause plaque buildup elsewhere in the body, including the brain.
Another possible explanation may be that tooth loss leads to the deterioration of the cerebral cortex because of the loss of sensory input. In rats, the more teeth were pulled, the more cells were lost in the brain region associated with memory formation, the hippocampus.
Perhaps the results of this study will take some of the bite out your next visit to the dentist. Taking care of your smile may be an effective mental floss. Who would object to that?
Reference: Okamoto et al. (2010) Relationship of tooth loss to mild memory impairment and cognitive impairment: findings from the fujiwara-kyo study. Behavioral and Brain Functions. 6:77