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See Inside May / June 2010

Preventing Hearing Loss

Scientists discover why we get deafer with age-and how antioxidants might slow the process

Old age brings with it a host of physical woes, and among the most common is hearing loss. Forty percent of Americans older than 65 suffer from hearing loss, and by 2030 some 65 million Americans will be hard of hearing.

Now joint work by researchers at the universities of Wisconsin, Florida, Washington and Tokyo has uncovered the mechanism behind age-related hearing loss, and with the help of simple chemicals, they have managed to keep old mice hearing as well as young pups.

The team investigated a molecular mechanism that has been implicated in many age-related maladies but had not yet been tied to hearing loss. Our bodies are constantly exposed to short-lived organic molecules known as free radicals, which harm cells in a process called oxidation. When cells are stressed by oxi­dative damage, they release a protein called Bak, which triggers a cascade of events culminating in cell suicide.

To test whether this mechanism was responsible for age-related hearing loss, the researchers compared normal mice with genetically engineered mice that do not have the gene necessary to make Bak. These Bak-deficient mice failed to develop hearing problems as they aged, but the ordinary mice, subjected to the same oxidative stress, became hard of hearing. Although most cells in the body are replaced with new cells after they die, the inner ear’s sensory nerve cells and ganglion neurons do not regenerate, so hearing loss is permanent.

After determining the cause of hearing loss, the researchers combed through published literature to see what kind of intervention might stave off free radical damage. Two antioxidants—molecules that prevent free radicals from harming cells—stood out: alpha lipoic acid (found in organ meats) and coenzyme Q10 (abundant in meat, fish and poultry). “When we fed [normal] mice these antioxidants in their food, they were protected from free radical damage in the cochlea,” says the study’s first author, Shinichi Someya of the University of Wisconsin–Madison. The team focused exclusively on the inner ear in its studies, but Someya says other body systems might also benefit from the antioxidants.

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