Will better oral health protect an aging brain? The answer isn't clear.

Over the past two decades, an increasing number of studies have been looking for relationships between oral health and cognitive problems in older adults.

A study earlier this year, for example, linked gum disease with faster cognitive decline in people with Alzheimer's disease.

The authors of a more recent review of the evidence say any such link would be important, because many older adults keep their natural teeth and over one third of people over age are cognitively impaired.

"We thought it would be interesting to look at the current state of the findings," lead author Bei Wu, of Duke University's School of Nursing in Durham, North Carolina, told Reuters Health.

As reported in the Journal of the American Geriatrics Society, Wu's team analyzed 16 studies that had tracked participants over time, plus another 40 studies that only looked at people at one point in time.

Some studies found that markers of oral health, like number of teeth and presence of gum disease, were tied to rate of cognitive decline or dementia risk. But the links weren't stable in every study.

Other studies found no link between oral and brain health.

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Also, the idea that both conditions are linked by underlying inflammation was only examined by one study that the researchers found "to be only marginally relevant."

"This field is promising, but we really need to have a more rigorous studies to look into the relationship," said Wu.

Wu said the idea that oral health could affect cognition is appealing, because it's something that people can modify on their own.

"Having this kind of systematic review is extremely helpful for us to know what are the strengths and weaknesses of this area of research and what direction we should go in," she said.