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A new review assesses the evidence suggesting that oral and cognitive health may be linked. Image credit: Gabrielle Lurie/The San Francisco Chronicle via Getty Images.
  • Although dementia is associated with an increased risk of poor oral health, evidence supporting the role of oral health in the development of dementia has been mixed.
  • A recent meta-analysis synthesizing data from previous studies suggests that gum disease and tooth loss are associated with an increased risk of cognitive decline and dementia.
  • These results underscore the importance of preserving oral health to reduce the risk of cognitive decline and dementia.

Individuals with dementia are often unable to maintain proper oral hygiene and are at increased risk of poor oral health. However, recent studies suggest that this relationship could be bidirectional.

A recent meta-analysis published in the Journal of the American Geriatrics Societysuggests that gum disease and tooth loss could indeed be associated with later cognitive decline and dementia.

The evidence included in the meta-analysis showed considerable methodological variation and was of low quality. Yet the potential effects of cognitive decline on gum disease cannot be ruled out.

Dr. Ella Cohn-Schwartz, a professor at Ben-Gurion University, Israel, not involved in this research, commented on it for Medical News Today:

“Poor oral health and non-optimal mastication have been found to be potentially modifiable risk factors for cognitive impairment, but previous evidence is limited. This meta-analysis provides a much-needed comprehensive synthesis of a large number of longitudinal studies regarding periodontal health, cognitive decline, and dementia.”

“This paper is novel in several regards, such as taking reverse causality into account, covering the spectrum of periodontal deterioration, including tooth loss, and examining both cognitive decline and dementia as outcomes. Their findings can indicate the importance of policies and interventions that address tooth loss, even partial loss, in old age,” Dr. Cohn-Schwartz added.

Dementia is characterized by a gradual decline in cognitive function, including memory, thinking, and reasoning, which impairs the ability of the individual to perform daily activities. Dementia is often preceded by milder forms of cognitive decline, such as mild cognitive impairment.

Mild cognitive impairment involves a decline in cognitive function that is greater than that typically observed during aging. Individuals with milder forms of cognitive decline do not show deficits in daily functioning but are at increased risk of dementia.

Around 55 million people across the globe are currently living with dementia, and the prevalence of this condition may rise to an estimated 139 million by 2050. There is a lack of effective treatments for cognitive decline and dementia, making it vital to understand the modifiable risk factors for these conditions.

Previous studies have shown that factors such as cardiovascular disease, diabetes, and an unhealthy diet can increase the risk of dementia. However, the role of oral health in the development of dementia has received less attention.

Until recently, some experts thought that inadequate oral hygiene due to the impairment of daily functioning was responsible for the poor oral health observed in individuals with dementia. However, emerging evidence suggests that poor oral health, including gum disease, could contribute to cognitive decline and dementia.

A recent meta-analysis synthesized data from previous longitudinal studies examining the potential role of oral health in cognitive decline and dementia. Specifically, the meta-analysis examined the impact of periodontitis, also known as gum disease, on cognitive health.

Periodontitis refers to the bacterial infection of gums that causes inflammation. The inflammation of the gums can damage the tissue and bones that support the teeth and can lead to tooth loss in severe cases.

The common signs of periodontitis include bleeding gums, loss of alveolar bones that support the teeth, and tooth loss. Periodontitis is also characterized by an increase in the depth of periodontal pockets, which are the spaces between gums and teeth.

The meta-analysis included 24 longitudinal studies examining the association between cognitive decline and periodontitis, and another 23 studies assessing the link between periodontitis and dementia.

It evaluated periodontal health assessed based on the presence of periodontitis, alveolar bone loss, increased periodontal pocket depth, and tooth loss.

The meta-analysis revealed that periodontitis was associated with an increased risk of cognitive decline and dementia. Among the various criteria used to assess periodontitis, further analysis revealed that tooth loss was also independently linked to cognitive decline and dementia.

Partial tooth loss, involving the loss of a few but not all teeth, was associated with cognitive decline. In contrast, complete tooth loss, but not partial tooth loss, was linked to an increased risk of dementia.

Previous studies have shown that individuals with dementia or mild cognitive impairment could lead to poor oral health.

The decline in cognitive function and changes in the brain associated with mild cognitive impairment or dementia occur gradually over many years. Studies with a follow-up duration of fewer than 10 years could thus potentially reflect the impact of cognitive decline on oral health.

Hence, the researchers reexamined the association between cognitive health and periodontal health after excluding studies with a follow-up duration of fewer than 10 years. After excluding these studies, the meta-analysis found a weaker association between periodontitis and dementia.

The weaker association between periodontitis and dementia in this additional analysis indicates that the results from the initial analysis could have been partly influenced by the effects of cognitive impairment on oral health.

Although the results in the follow-up analysis indicated that poor oral health may contribute to the development of dementia, some researchers remain skeptical about the evidence linking poor oral health to cognitive decline.

This is because dementia and tooth loss share the same risk factors, such as low education levels, socioeconomic status, and diabetes.

Moreover, individuals with higher cognitive function in childhood tend to have better oral health and access to dental care in adulthood than those with lower cognitive ability in early life.

Children with higher cognitive abilities are also more likely to maintain better cognitive function in old age. Thus, poor oral health may not have a causal role in the development of dementia.

Dr. Murray Thomson, a professor of dentistry at the University of Otago, not involved in the current research, noted:

“The findings of this review are not a surprise, given that gum disease and cognitive decline share the same risk factors throughout life. I would expect any investigation at any age in adulthood to show an association between gum disease and cognitive function because of this. The key issue is that there is no good evidence that gum disease causes poor cognitive function, but there is very good evidence that people with poorer cognitive function have more gum disease.”

The authors cautioned that the evidence reviewed in the meta-analysis was of low quality. There was considerable variation among the analyzed studies in the measures used to assess periodontal health and the tests used to evaluate cognitive function.

Moreover, the studies assessing the impact of cognitive decline on periodontal health generally included individuals over the age of 65 years and with shorter follow-up duration. This could have potentially biased the results in favor of a positive association between periodontal health and cognitive decline.

Thus, there is a need for more research conducted using standardized methods to further assess the link between periodontal and cognitive health.

Yet, these results suggest that the prevention and early treatment of oral health problems could help reduce the risk of cognitive decline and dementia.

The mechanisms that could explain the impact of periodontal health on cognitive impairment are not well understood. The bacterial infection responsible for gum disease is known to cause an increase in markers of systemic inflammation.

Several studies suggest that systemic inflammation may contribute to the development of dementia. Thus, systemic inflammation induced by periodontitis could potentially lead to a decline in cognitive function.

The bacterial infection and inflammation involved in periodontitis could also weaken the blood-brain barrier, which prevents toxic substances from entering the brain.

Weakening of the blood-brain barrier could allow the bacteria and inflammatory molecules to enter the brain via the bloodstream, subsequently contributing to brain inflammation. Periodontitis could thus cause brain inflammation, which plays a critical role in the development of dementia.

Deficits in chewing ability due to tooth loss may also contribute to the decline in cognitive function. This is because the process of chewing food is associated with increased blood flow to brain regions involved in cognition and helps maintain cognitive function.

Furthermore, the reduced ability to chew food due to tooth loss may influence dietary habits, including higher sugar consumption and a lower intake of dietary fiber. Unhealthy dietary habits associated with the decline in chewing ability due to tooth loss could also increase the risk of dementia.