The study, published in the journal Neurology, found that gum disease and tooth loss were linked to brain shrinkage in the hippocampus, which plays a role in memory and Alzheimer's disease.
The study, however, does not prove that gum disease or tooth loss causes Alzheimer's disease; it only shows an association.
"Tooth loss and gum disease, which is inflammation of the tissue around the teeth that can cause shrinkage of the gums and loosening of the teeth, are very common, so evaluating a potential link with dementia is incredibly important," said Satoshi Yamaguchi, from Tohoku University in Sendai, Japan.
"Our study found that these conditions may play a role in the health of the brain area that controls thinking and memory, giving people another reason to take better care of their teeth."
The study involved 172 people with an average age of 67 who did not have memory problems at the beginning of the study.
Participants had dental exams and took memory tests at the beginning of the study. They also had brain scans to measure volume of the hippocampus at the beginning of the study and again four years later.
Researchers found that the number of teeth and amount of gum disease was linked to changes in the left hippocampus of the brain.
For people with mild gum disease having fewer teeth was associated with a faster rate of brain shrinkage in the left hippocampus.
However, for people with severe gum disease having more teeth was associated with a faster rate of brain shrinkage in the same area of the brain.
