The study, published in the new issue of the Annals of Internal Medicine, followed 728 men and women between the ages of 50 and 70 randomly assigned to receive either a folic acid supplement (800 micrograms per day) or placebo for three years.
Lead author Jane Durga and her colleagues from Wageningen University and Wageningen Centre for Food Sciences, and University Hospital Maastricht, reported that the folic acid-supplemented group exhibited lower age-related hearing loss in the low frequency region.
"Considering that the folate status of older adults is generally low in countries without folic acid fortification programs, our findings suggest a possible way to diminish the public health burden of hearing loss in those countries," wrote Durga.
However, it is not known if such benefits would be observable in countries like the US and Canada where mandatory fortification occurs.
Folate is found in foods such as green leafy vegetables, chick peas and lentils, and an overwhelming body of evidence links has linked folate deficiency in early pregnancy to increased risk of neural tube defects (NTD) - most commonly spina bifida and anencephaly - in infants.
This connection led to the 1998 introduction of public health measures in the US and Canada, where all grain products are fortified with folic acid - the synthetic, bioavailable form of folate.
While preliminary evidence indicates that the measure is having an effect with a reported 15 to 50 per cent reduction in NTD incidence, parallel measures in European countries, including the UK and Ireland, are still on the table.
The researchers report that, at the start of the study, the average threshold for hearing in the low frequency range (0.5 to 2 kHz) was 11.7 decibels (dB), and 34.2 dB in the high frequency range (4 to 8 kHz). At the end of the study the thresholds had increased for both folic acid and placebo groups, meaning a louder noise was required before the participants registered the noise. However, the increase was lower in the supplemented group in the low frequency range (1.0 versus 1.7 dB increase for folic acid versus placebo groups, respectively).
No significant difference in threshold decline in the higher frequency region was observed.
The researchers suggest that hearing loss may be linked to homocysteine levels, which could be reduced by folic acid supplementation.
Durga, currently with the Nestle Research Center in Lausanne, focussed on older men and women with average plasma total homocysteine concentrations of 13 micromoles per litre or more and vitamin B12 levels of at least 200 picomoles per litre.
Normal homocysteine levels are defined as between five and 15 micromoles per litre of blood, placing the study participants at the upper end of the 'normal' bracket.
However the focus of the study on people with these homocysteine levels limited the researchers from extending the findings to a general population. The authors also note that no mandatory folic acid fortification was present in the Netherlands during the study, and that the baseline levels of folate in the blood was about 50 per cent that of the US population, where mandatory folic acid fortification is present.
"Folic acid supplementation slowed the decline in hearing of the speech frequencies associated with aging in a population from a country without folic acid fortification of food. The effect requires confirmation, especially in populations from countries with folic acid fortification programs," concluded the researchers.
In an accompanying editorial, Robert Dobie from the University of California, Davis said that if such a benefit could be applied generally then a five-decibel decrease in age-related hearing loss might be observed over a 20-year period. This would lead to a subsequent significant reduction in the need for hearing aids.
The research was supported by grants from the Netherlands Organisation for Health Research and Development, Wageningen University, and Wageningen Centre for Food Sciences.
Source: Annals of Internal Medicine 2 January 2007, Volume 146, Issue 1, Pages 1-9 "Effects of Folic Acid Supplementation on Hearing in Older Adults - A Randomized, Controlled Trial" Authors: J. Durga, P. Verhoef, L.J.C. Anteunis, E. Schouten, and F.J. Kok
Editorial: Annals of Internal Medicine 2 January 2007, Volume 146, Issue 1, Pages 63-64 "Folate Supplementation and Age-Related Hearing Loss" Author: R.A. Dobie