 Systematic reviews and meta-analyses have found that tooth loss, or periodontitis, is associated with both cognitive impairment and dementia. Reverse causation might be an intuitive explanation, dementia leading to a decline in oral hygiene, but prospective studies following people over time have found that tooth loss appears to predict future cognitive decline and the more missing teeth, the higher the associated risk. Genetic determinants of elevated periodontitis risk do not appear to be associated with development of Alzheimer's disease, suggesting the association may be due to shared or confounding risk factors. A poor diet or low socioeconomic status represent common pathways for both tooth loss and dementia. But even studies controlling for confounding factors found a connection between tooth loss and dementia. However, one factor that wasn't controlled for was intelligence. Those with higher IQ are both more likely to floss and have fewer tooth and gum problems, as well as have a delayed onset of clinically detectable Alzheimer's due to a greater cognitive reserve. Are there any potential causal mechanisms beyond the chronic inflammation? If you induce periodontitis in mice with P. gingivalis, the keystone pathogen in human periodontitis, the bacteria ends up in their brains and triggers an increase in the Alzheimer's associated plaque protein amyloid beta. Since then, evidence of the bacteria has been found in human Alzheimer's brains. Clinical trials are now on the way to see if blocking its neurotoxic effects will have any therapeutic benefit. A clue to a stranger potential mechanism was unearthed in a subgroup analysis of a tooth loss and dementia meta-analysis. Only tooth loss among those without dentures was associated with the later development of dementia. The same tooth loss with dentures was not. How does that make any sense? A review entitled Mastication for the Mind compiled a number of studies suggesting that chewing, mastication, and medical speak is somehow important for proper brain function. For example, the extraction of teeth from aged mice appears to upset their memory and learning, but maybe they're just upset, in pain and flame, from having their teeth removed. A more convincing series of experiments compared mice given food pellets versus the same pellets crushed into powder. Those eating the quenco soft diet, the powdered diet, also suffered a loss in memory and learning capacity. What does chewing have to do with cognition? Stick an ultrasound probe on someone's head, and you can detect an increase in cerebral blood flow of the largest artery branch in the brain when they clench their jaw or chew gum. In an fMRI machine, brain scans show enhanced cognitive arousal and chewing even tasteless sugarless gum, though translational studies on the acute cognitive effects of gum chewing are contradictory. Maybe it's not the chewing, though. Check this out. Noticing that people missing molars on one side of the mouths have asymmetrically sized pupils, a smaller pupil on the missing molar side, researchers tested whether implanting dental crowns to replace the missing teeth could reduce the discrepancy. And it did. The study, subtile new teeth for a brighter brain, suggested that a gap in the sensation of teeth pushing up against one another adversely affects brain function. To put it to that test, we'd want to randomize people with missing teeth to get dentures and see if their cognition improves over those who don't. No such studies exist. But there was an intriguing pilot study performed. 10 toothless individuals, 9 out of 10 of them cognitively impaired, 6 of them severely so, were given conventional dentures for a month before being fitted with overdentures, which are snapped into titanium implants surgically screwed into the jawbone. The conventional dentures, held in place by adhesives and natural suction, did nothing to significantly alter cognitive function. But the one securely attached to implants in the bone, presumably transmitting the same kind of chewing pressure sensations to the nerves in the jaw that natural roots of teeth might, had a dramatic effect. 9 out of the 10 subjects went into the study cognitive impaired, but 8 out of 10 left the study cognitively intact. This suggest well-fitting, secure dental prosthesis isn't just about improving self-confidence, social contact, quality of life, but proper brain functioning as well. Of course, even better though would be to preserve the teeth you have.