Thursday June 20 2013
As we get older, we tend to lose teeth and memories
A gap-toothed grin and absent-mindedness "could go hand-in-hand", the Daily Mail reports, saying that "research suggests that losing our teeth is actually a cause of memory loss".
The news is based on recent research examining the link between oral health and memory in older people. However, the type of research carried out can't tell us whether tooth loss causes memory loss, despite the Mail's claim.
This small study of older people found that the number of natural teeth they had was significantly associated with performance on several cognitive tests. This was true even after accounting for several potential confounding factors.
The association was small and it is unclear whether it would have any noticeable effect on people. In fact, the other variables included in the statistical model, including age and education, had a greater impact on memory test performance than the number of teeth did.
This study appears to support previous research in animals that suggests a link between "dental status" and memory. The researchers have called for larger studies to investigate whether the link between teeth and memory is "clinically meaningful", and to find out the reasons why.
Where did the story come from?
The study was carried out by researchers from Umeå University and Stockholm University in Sweden, and the University of Tromsø in Norway. It was funded by the Swedish Council for Social Research, Västerbotten County Council and other organisations throughout Sweden.
It was published in the peer-reviewed European Journal of Oral Sciences.
The Daily Mail's coverage of this story incorrectly conflates correlation with causation. The paper reports that the "research suggests that losing our teeth is actually a cause of memory loss". However, this is not in fact supported by the available evidence.
What kind of research was this?
This was a cross-sectional study that examined the relationship between the number of natural teeth and memory in healthy older people.
As a cross-sectional study, this research does not provide evidence that tooth loss causes memory loss – it can only show whether the two factors are related.
What did the research involve?
Researchers randomly selected 273 people between the ages of 55 and 80 who were participating in an ongoing cohort study on memory and health. The selected participants had their mouths examined, a health assessment and cognitive tests. People with possible dementia or other neurological conditions were excluded from the study.
The oral exam assessed the participants':
- number of teeth
- occlusion (how the upper and lower teeth come together when closing the mouth)
- periodontal conditions (diseases of the tissue surrounding the teeth, including the gums and bones)
- dental caries
- dental fillings
- root fillings
- prosthetic treatments
The health assessment included a self-reported medical history component, where participants were asked if they had ever suffered from any of 28 specific diseases. Of the 28 diseases, heart disease, high blood pressure, unconsciousness, head injury and eye disease were included in the statistical analysis.
In addition to medical history, the researchers collected information on length of education, occupation, living conditions and perceived stress. They also conducted a series of cognitive tests that assessed:
- "episodic memory" – testing the participants' ability to recall or recognise faces, words and sentences
- "semantic memory" – testing their ability to list words beginning with a particular letter or recognise synonyms during a vocabulary test
- "working memory" – testing the participants' ability to recall things they had just read
- "visuospatial ability" and processing speed
During the statistical analysis, the researchers first determined whether any of the following factors were significantly associated with performance on the cognitive tests:
- living conditions
- perceived stress
- number of teeth
Factors that showed a statistically significant association with cognitive ability were included in a three-tiered model. This model assessed the association between perceived stress, diseases and number of teeth while controlling for age, education, gender, occupation and living conditions.
When setting the threshold for determining whether any of the factors were significantly associated with cognitive ability, the researchers applied a statistical correction called the "Bonferroni correction", which reduces the likelihood of false positives.
What were the basic results?
On average, the participants had approximately 22 (of 32) natural teeth. Molars (the big chewing teeth at the back) were the most likely teeth to be missing.
The researchers found that both age and education were significantly associated with performance on cognitive tests. Older people were likely to have lower scores, and participants with higher education levels were more likely to have higher scores.
Older people had significantly fewer teeth than the younger participants. Participants with more education, higher occupation levels and better living conditions had significantly more teeth than their peers.
Separately, the researchers found that the number of natural teeth was associated with better performance on episodic recall and recognition tests, as well as on the vocabulary test. The number of natural teeth accounted for 3-4% of the variability in scores on these tests after considering the other factors in the model. There were no significant associations between the number of natural teeth and the other cognitive measures.
The demographic factors controlled for in this model – age, length of education, gender, occupation and living conditions – accounted for the majority of the variance in cognitive test scores (11-52%). The other factors included in the model – perceived stress, history of heart disease, high blood pressure, unconsciousness, head injury or eye disease – were not significantly associated with cognitive scores.
How did the researchers interpret the results?
The researchers conclude that, "the presence of natural teeth seems to have an impact on cognitive function" and that this may be due to reduced sensory input from the nerves connecting the teeth to the surrounding tissues in the mouth, which send signals to the brain during chewing.
The cross-sectional study suggests that there is an association between the number of natural teeth and certain types of memory in older individuals. However, this study cannot tell us why this association exists, or whether it is clinically significant.
The researchers report that previous studies in animals have found similar associations between tooth loss, impaired chewing ability and cognitive ability. They suggest two possible interpretations of this association:
- the lack of natural teeth may lead to degeneration of key sensory receptors in the mouth, which in turn may reduce the sensory input to the part of the brain responsible for episodic memory, or
- there may be dietary consequences of tooth loss, whereby impaired chewing ability may cause people to choose foods that are easy to chew, which may lead to poor nutrition and reduced cognitive ability
Despite these possible explanations, this study should not be taken as showing that tooth loss leads to cognitive decline. Media stories suggesting otherwise have interpreted the study incorrectly, confusing correlation with causation.
The researchers point out several limitations and considerations arising from their study:
- They say that, "Although significant, the amount of variance explained by the number of natural teeth is not impressive", but may nonetheless point to a small, important finding that corroborates previous research.
- Because of the small study size, they suggest that caution should be exercised before generalising the results to larger populations.
An additional limitation to note is the use of self-reporting of the potential confounders included in the statistical model. Ideally, more objective measures such as official medical records would be used to determine medical history, especially in a study examining cognitive ability in older adults.
The authors call for further investigation into the relationship between oral health and memory, preferably through large-scale epidemiological studies, to determine the potential clinical significance of the number of teeth on memory. Such studies would ideally involve the objective measurement of medical history and demographic factors.
Overall, this was a small but well-conducted study. While it suggests that there may be a link between tooth and memory loss, any association is likely to be small, of unclear clinical significance, and needs to be confirmed in larger prospective trials.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter.