Numerous animal studies have suggested protective properties of certain vitamins in various pathologic cochlear processes,8
but there has been scant human data. This is the first large epidemiologic human study of the association between these vitamins with hearing loss. Overall, our study found no prospective association between vitamin C, E, beta carotene, B12 or folate intake and incident hearing loss. Higher intake of folate however, was associated with a 21% risk reduction for hearing loss in men 60 years and older. The relation between folate intake and incident hearing loss did not vary by alcohol intake. However, the lower risk of hearing loss among men with higher vitamin B12 intake was only observed in those with the highest alcohol intake.
Reactive oxygen metabolites may cause cochlear damage. Vitamins C, E, and beta carotene have shown promising otoprotection in animal studies, possibly due to their antioxidant properties.8
A 16 week randomized trial of vitamin C supplementation (600 mg/day) in 23 participants with presbycusis showed a small hearing threshold improvement.9
Our study, however, found no association between these vitamins and the overall incidence of hearing loss. The range of vitamin intake in our study included the ranges described in previous studies, and in the case of vitamin C, the 5th
quintile of intake in our cohort (1247mg/day) was considerably higher than that described in the previously mentioned trial. Nevertheless, we cannot exclude the possibility that an association may be seen with even higher levels of antioxidant intake. However, the negative results in our study are only indicative of the relation between specific vitamin intake in the ranges consumed and hearing loss in adult males.
The association between folate intake and incident hearing loss in our cohort is consistent with previous data showing that folic acid supplementation slowed the decline in low frequency hearing in older adults.13
A possible explanation for the impact of age on the relation between folate intake and hearing loss is the increased prevalence of folate malabsorption and folate depletion in the older age group.11
Higher folic acid intake may be necessary to meet the optimal folate needs in this age group. In men 60 years and older, the significant association with hearing loss risk was seen starting in the 4th
quintile of intake, which represented a total folate intake of at least 800 mcg/day. This level of folate intake is considerably higher than the current recommended minimum intake of 400 mcg/day for this age group.15
Although ethanol is known to impede the bioavailability of dietary folate as well as several folate-related biochemical reactions,16
it did not alter the relation between folate intake and hearing loss in our cohort.
Although there is no direct evidence in the literature that age-related hearing loss is associated with vitamin B12 deficiency, its role in neuronal and cellular metabolism may make vitamin B12 an important factor in cochlear pathology.7
Previous cross-sectional data have suggested a possible association between vitamin B12 status and hearing loss, as a study of 55 healthy women found that women with hearing impairment had a 38% lower serum vitamin B12 level than those with normal hearing.7
Our data did not support this finding, possibly due to the different methods used to measure vitamin B12 status. The SFFQ used in our study is a reliable measure of long term dietary intake, while serum biomarkers often provide information on the dietary status at a particular time point. While folate storage is limited, with deficiency occurring after just weeks of poor intake, vitamin B12 stores are often sufficient to prevent vitamin B12 deficiency for years in conditions of malabsorption.11
Ethanol, however, decreases hepatic B12 storage and can cause vitamin B12 deficiency.16
In these cases, a greater vitamin B12 intake is required to meet the physiologic requirements. This may explain the inverse association between vitamin B12 intake and incident hearing loss in men with higher alcohol intake.
Our study has limitations. Assessment of hearing loss was based on self-report of professionally diagnosed hearing loss and individuals who did not report hearing loss were considered not to be hearing impaired. Therefore, it is likely that cases of hearing loss were underreported. Although standard pure-tone audiometry is generally considered the gold standard of hearing loss evaluation, self-reported hearing loss has been demonstrated to be a reliable assessment.17
Nevertheless, the sensitivity of the questionnaire in identifying cases of hearing loss is likely to be lower than that of an audiogram. Given the high prevalence of hearing loss in this population,2
misclassification of outcome would likely bias the results toward the null.
The outcome analyzed in this study does not distinguish among different hearing loss etiologies. While sensorineural, age-related hearing loss is likely to be the dominant pathologic process in this cohort, we were not able to quantify other common entities such as noise induced hearing loss. We did not have information on noise exposure or reasons for vitamin use. Although noise exposure can cause both mechanical and metabolic cochlear damage,18
as well as increase the vulnerability to hearing loss related to age,19
there is no evidence that it is associated with variation in vitamin intake. It is therefore unlikely to be a confounder in this study.
The SFFQ has been validated and used to demonstrate numerous associations between our exposures of interest and other disease outcomes.14,20
This instrument quantifies vitamin intake from food as well as multivitamin supplement sources. This has allowed us to adjust the intake of each individual vitamin to the intakes of the other vitamins of interest in our multivariate analyses and to update the intake every four years. We cannot, however, exclude the possibility that one of the numerous possible combinations between the intakes of our vitamins of interest is associated with hearing loss risk. Since the increased folic acid supplementation of the food supply that began in the mid 1990’s, intake in our cohort has increased, and this change has been sufficiently quantified in the total folate intake calculation. While other hearing loss studies have used biomarkers as a method to quantify nutrient intake,13
a validated SFFQ is generally a more accurate method of assessing long-term dietary intake.
The participants in this cohort are not representative of the adult population in the U.S., but the follow-up rates are high and the information provided is reliable. The observed associations are likely to apply to other groups inasmuch as the underlying biologic and pharmacologic mechanisms are likely to be similar. However, additional studies are needed to examine these relations in women and younger men.
In conclusion, vitamin intake was not associated with the development of hearing loss. However, our study suggests that specific groups may benefit from increased folate or vitamin B12 intake to reduce the risk of hearing loss. Men aged 60 years and older may benefit from increased folate intake. Given the aging of the population and the high prevalence of hearing loss, identification of modifiable factors to reduce the burden of this common condition should be an important public health priority.