Researchers found that sedentary mice lost structures that are important in the auditory system—hair cells and strial capillaries—at a much higher rate than mice that exercised. This resulted in a roughly 20-percent hearing loss in sedentary mice compared with a 5-percent hearing loss in active mice.
Age-related hearing loss affects about 70 percent of adults age 70 and older and occurs when people lose hair cells, strial capillaries, and spiral ganglion in the cochlear system of their ear.
Hair cells sense sound, strial capillaries feed the auditory system with oxygen, and spiral ganglion are a group of nerve cells that send sound from the cochlea to the brain.
Shaped like a snail shell, the auditory system is always running, says lead author Shinichi Someya, associate professor in the aging and geriatric research department at the University of Florida College of Medicine.
“The cochlear or inner ear is a high energy–demanding organ. The auditory system is always on and always processing sound. To process sound, it needs a huge amount of energy molecules.”
The system needs to be well-fed with oxygen, delivered to the inner ear by strial capillaries, to generate those energy molecules.
To test how exercise affects the loss of strial capillaries, hair cells, and neurons, researchers separated mice into two groups: mice that had access to a running wheel and mice that did not have that access. The mice were also housed individually so that the researchers could keep track of how far the mice ran on their running wheels.
The exercise regimen for the mice peaked when the animals were 6 months old, or about 25 in human years. As the mice aged—to 24 months, or 60 human years—their exercise levels decreased.
At their peak, the mice were running about 7.6 miles per day, and at the lowest point, the mice were still running about 2.5 miles per day. The group of exercising mice was then compared with a control group of non-exercising mice.
The findings, published in the Journal of Neuroscience, suggest age-related inflammation damages the capillaries and cells, and that exercising provides protection against that kind of inflammation.
In another part of the study, inflammation in the bodies of the sedentary mice was compared to inflammation in the exercising group. The mouse runners were able to keep most markers of inflammation to about half that of the sedentary group, which may help preserve the capillaries and hair cells involved in hearing.
While epidemiological studies have shown a link between hearing sensitivity and exercise, this is the first research to show that regular exercise can prevent age-related hearing loss in mice, and that the research translates well to humans, Someya says.
“Exercise likely releases some growth factors yet to be discovered that maintain capillary density as compared to the control animals who were not exercising,” says co-author Christiaan Leeuwenburgh, professor and vice chair of research for the Institute on Aging. “Also, exercise may release other beneficial factors, but can also attenuate and blunt negative factors, such as inflammation.”