"Hearing loss may be an under-recognized complication of diabetes. As diabetes becomes more common, the disease may become a more significant contributor to hearing loss," said senior author Catherine Cowie, Ph.D., of the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), who suggested that people with diabetes should consider having their hearing tested. "Our study found a strong and consistent link between hearing impairment and diabetes using a number of different outcomes."

The researchers discovered the higher rate of hearing loss in those with diabetes after analyzing the results of hearing tests given to a nationally representative sample of adults in the United States. The test measured participants’ ability to hear low, middle, and high frequency sounds in both ears. The link between diabetes and hearing loss was evident across all frequencies, with a stronger association in the high frequency range. Mild or greater hearing impairment of low- or mid-frequency sounds in the worse ear was about 21 percent in 399 adults with diabetes compared to about 9 percent in 4,741 adults without diabetes. For high frequency sounds, mild or greater hearing impairment in the worse ear was 54 percent in those with diabetes compared to 32 percent in those who did not have the disease.

Adults with pre-diabetes, whose blood glucose is higher than normal but not high enough for a diabetes diagnosis, had a 30 percent higher rate of hearing loss compared to those with normal blood sugar tested after an overnight fast.

The study, published early online June 17, 2008, in the Annals of Internal Medicine, was conducted by researchers from the NIDDK, the National Institute on Deafness and Other Communication Disorders (NIDCD), components of the NIH, and Social & Scientific Systems, Inc., which provides support on public health topics to NIH and other government agencies.

The researchers analyzed data from hearing tests administered from 1999 to 2004 to participants in the National Health and Nutrition Examination Survey (NHANES) conducted by the National Center for Health Statistics, part of the Centers for Disease Control and Prevention (CDC). Half of the 11,405 survey participants aged 20 to 69 were randomly assigned to have their hearing tested, and nearly 90 percent of them completed the hearing exam and the diabetes questionnaire. The hearing test, called pure tone audiometry, measures hearing sensitivity across a range of sound frequencies.

"Using the data from the hearing tests, we measured hearing impairment in eight different ways. Also, participants responded to questions about hearing loss in the questionnaire, which asked whether they had a little trouble hearing, a lot of trouble hearing, or were deaf without a hearing aid," said Cowie. In addition, 2,259 of the participants who received hearing tests were randomly assigned to have their blood glucose tested after an overnight fast.

Earlier U.S. studies that examined diabetes and hearing loss found a weaker association or no association, but these studies were based on smaller samples of older adults, and they were not nationally representative, according to co-author Howard Hoffman, an epidemiologist at NIDCD. "This is the first study of a nationally representative sample of working age adults, 20 to 69 years old, and we found an association between diabetes and hearing impairment evident as early as ages 30 to 40."