How Hearing Loss and the Gut Microbiome Are Linked to Dementia

Many factors influence the risk of developing dementia. A study led by researchers at the Johns Hopkins Bloomberg School of Public Health found that older adults with more severe hearing loss are more likely to have dementia, but that those who use hearing aids are less likely to have dementia than those who do not.

The findings from a nationally representative sample of more than 2,400 older adults are consistent with previous studies showing that hearing loss over time may be a risk factor for dementia and that treating hearing loss may reduce the risk of dementia. In addition, research suggests that the gut microbiome has an influence on brain health.

Link Between Hearing Loss and Dementia

The growing understanding that hearing loss could be linked to the risk of dementia, which affects millions, and other negative outcomes has drawn attention to the implementation of possible strategies for treating hearing loss. “This study refines our observations about the association between hearing loss and dementia and provides arguments for public health action to improve access to hearing care,” said lead author Alison Huang, PhD, MPH, senior research scientist in the Division of Epidemiology in the Bloomberg School and at the Cochlear Center for Hearing and Public Health, also in the Bloomberg School. Hearing loss is a critical public health issue, affecting two-thirds of adults over the age of 70.

For the study, Huang and his colleagues analyzed a nationally representative dataset from the National Health and Aging Trends Study (NHATS). Funded by the National Institute on Aging since 2011, the NHATS uses a nationwide sample of Medicare beneficiaries aged 65 and older, with a focus on those over 90 and on blacks. The analysis included 2,413 individuals, about half of whom were over 80 years old, and showed a clear link between the severity of hearing loss and dementia. The prevalence of dementia was 61 percent higher among participants with moderate/severe hearing loss than among those with normal hearing. Hearing aid use was associated with a 32 percent lower prevalence of dementia among the 853 participants with moderate/severe hearing loss.

The authors note that many previous studies were limited in that they relied on clinic-based data collection and excluded vulnerable populations who did not have the means or ability to visit a clinic. For their study, the researchers collected data from participants through testing and interviews at home. The link between hearing loss and dementia is not yet clear, and studies suggest several possible mechanisms. Huang’s research complements the work of the Cochlear Center for Hearing and Public Health, which is studying the link between hearing loss and dementia.

Gut Microbiome Plays a Role in How Lifestyle Affects Dementia Risk

The gut microbiome may play a role in how diet and exercise affect brain health and risk of dementia, according to a Baycrest study. This knowledge could help scientists and clinicians optimize strategies for preventing dementia. Lifestyle interventions that reduce the risk of dementia often include diet and exercise, which are known to influence the gut microbiome – the community of bacteria and other microorganisms that live in our gut. We know that imbalances in the microbiome are associated with cognitive impairment,” says Noah Koblinsky, lead author of the study, exercise physiologist and project coordinator at Baycrest’s Rotman Research Institute (RRI). The researchers wanted to find out whether lifestyle interventions could be customized to specifically target the gut microbiome and whether this would help optimize their effects on cognition.

To do this, Koblinsky and his team reviewed all existing research on diet and exercise interventions that looked at both the microbiome and brain health. They found that the gut microbiome appears to play a role in how diet and exercise affect brain health, although more research is needed to fully understand the links.

Dietary studies showed a large influence of diet on the microbiome, with foods associated with a Mediterranean dietary pattern (e.g., fiber and healthy fats) appearing to have the greatest benefits for a healthy gut microbiome and brain. A study of 1,200 older adults examined the influence of diet on both cognition and the microbiome. Half of the participants were asked to follow a Mediterranean diet for 12 months, while the other half did not. Participants in the Mediterranean diet group showed significant improvements in cognitive ability. In addition, those who adhered more closely to the concept had healthier microbiomes, which were associated with better brain health.

In another study, researchers used antibiotics to “kill off” the gut microbiome in a group of rats. They then gave these rats microbiome transplants (from feces) from rats that had been fed either an unhealthy or a healthy diet. The rats that received the transplant from the unhealthy diet group showed poorer memory performance and inflammation in the gut and brain. These results support the idea that the microbiome plays a role in how diet affects brain health. Researchers found fewer studies that looked at physical activity. However, those that did suggest that exercise, particularly cardio exercise, can lead to changes in the gut microbiome and, at the same time, brain health.