Almost 10 million people in the United States have hearing loss that stems from exposure to excessive noise. Animal data now suggest that some share of that hearing loss can be avoided if people consume a specific combination of dietary supplements prior to encountering a dangerous din.
Noise bombardment can trigger tissues in the ear to produce free radicals, molecular fragments also known as oxidants. These biologically damaging chemicals can kill the inner ear's hair cells, the ones that sense sound. Perhaps not surprisingly, the newfound dietary defense consists of heavy doses of antioxidants: vitamins A, C, and E. The new supplement combo, which had a protective effect in a recent test on guinea pigs, also includes magnesium, a mineral that had already shown some ability to minimize noise trauma.
Colleen G. Le Prell, a neuroscientist at the University of Michigan's Kresge Hearing Research Institute and her colleagues administered high oral doses of the supplement combo to six guinea pigs for 5 days. One hour after the treated animals got their first dose, the researchers subjected these and other guinea pigs to a 5-hour-long exposure to a 120-decibel, broad-frequency noise.
Sound of that intensity is about what a person hears from a nearby lightning strike or a close encounter with a propeller-driven plane preparing for takeoff.
Five days after the final doses of the supplement, Le Prell's team assessed the animals' permanent hearing loss by tallying dead hair cells. Although all the guinea pigs had sustained some hearing loss, the amount was dramatically greater in those animals that hadn't received the supplements.
If the new supplement combination protects as well against hearing loss in people as it did in the rodents, Le Prell says, it will provide people with an alternative to bulky ear protectors and uncomfortable earplugs in loud situations.
What an earful
Some 30 million U.S. residents confront hazardous levels of noise each year—sounds rated at 85 decibels (dB) or higher. Dins that loud can occur when a person uses a power lawn mower or even when someone sits inside an apartment next to an urban freeway (SN: 6/5/82, p. 377).
Many people regularly encounter significantly louder noises. Street traffic can run to 90 dB, the sounds of a jackhammer can reach 100 dB, and the ears of patrons attending a live rock concert can be assaulted by a blaring 110 dB. At least some degree of partial hearing loss can persist for up to a day after a loud concert.
Because the decibel scale is logarithmic, a 10 dB difference between sounds means that the larger one is 10 times as loud.
Although loud noises aren't always preventable, a person's exposure can be lowered to a nontraumatic volume by ear protectors. Unfortunately, Le Prell observes, such devices "are hard to wear correctly," so even people who don them typically "don't get as much protection as [the devices are] rated for."
And some noises are just too much for even properly worn ear protectors. Le Prell notes that the Swedish military sometimes assigns soldiers to urban-warfare exercises in which they fire machine guns within concrete bunkers. In this "very loud, reverberating environment," Le Prell says, the gunfire can create impulse noises—the most dangerous kind—of up to 150 dB. "The sound level measured in the ear canal, underneath the hearing protector, is typically 130 dB," she says.
These troops can sustain a temporary 10-dB loss in hearing sensitivity, studies have shown.
A noise-dampening recipe
Over the past couple of decades, studies have hinted that various antioxidants can diminish the hearing loss triggered by noise. However, none of the supplements worked very well in animals, even when researchers administered them in repeated, high doses for a month prior to a noise-exposure test, Le Prell notes.
Other studies suggested a possible protective role for magnesium. Although not an antioxidant, the mineral partially blocks cells' docking sites for glutamate, the chemical signal that overexcites nerve cells when noise becomes excessive.
Le Prell says she and her coworkers chose to team magnesium with the three antioxidant vitamins because the substances all affect cells differently. They might be therapeutic in different parts of hair cells traumatized by noise, the researchers reasoned. For instance, in addition to dampening the glutamate response to loud sounds, magnesium limits the blood vessel constriction in the inner ear that typically accompanies intense noise. This change might permit more bloodborne antioxidants to reach noise-affected tissues, the scientists reasoned.
When the researchers counted dead hair cells, the numbers suggested that the nine animals untreated with supplements experienced a 50-dB loss in hearing. In people, such a shift would be enough to convert loud speech into a whisper. The six animals that had received the full nutritional treatment suffered only a 10 dB drop in hearing sensitivity. Groups of animals receiving only the trio of vitamins or only the magnesium received a benefit midway in between, Le Prell's team reports in the May 1 Free Radical Biology and Medicine.
"You're talking about a handicapping hearing loss in the untreated animals," Le Prell told Science News Online, versus "a hearing loss that would require minimal or no intervention in the treated animals."
The supplement therapy was so impressive—and free of side effects—that a human trial is slated to begin within a few months. Le Prell notes that her group will participate in research involving some of those Swedish troops during urban-warfare training. The work will be done in collaboration with the Swedish military and the Karolinska Institute in Stockholm.
The doses for the troops will be about a tenth of the amount, per unit bodyweight, that was administered to the guinea pigs. This lower dose is near the "safe upper limit" for each ingredient, as established by the U.S. Institute of Medicine.
Ultimately, Le Prell says, "we want to look into long-term clinical trials, studying people who are exposed to noise on a daily basis as part of their jobs."
She argues that most such people—like everyone else—get little exterior protection from noise, so a dietary supplement that offers an internal defense would be invaluable.
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American Hearing Research Foundation (AHRF)
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American Speech-Language-Hearing Association (ASHA)
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Colleen G. Le Prell
Kresge Hearing Research Institute
University of Michigan
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National Institute on Deafness and Other Communication Disorders
National Institutes of Health
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Occupational Safety & Health Administration
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Washington, DC 20210
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National Institute on Deafness and Other Communication Disorders. Noise-Induced Hearing Loss. Available at [Go to].
Occupational Safety & Health Administration. Noise and Hearing Conservation. Available at [Go to].
Raloff, J. 1982. Noise can be hazardous to our health. Science News 121(June 5):377-381.
______. 1982. Occupational noise—the subtle pollutant. Science News 121(May 22):347.