Level 1: Likely reliable evidence
In an elderly population with low dietary folate intake at baseline, folic-acid supplementation slows the progression of age-related hearing loss at low frequencies, based on a randomized trial of 819 adults aged 50-70 years in the Netherlands (Ann Intern Med. 2007;146:1-9). Enrollment criteria were plasma total homocysteine levels 13-26 µmol/L and serum vitamin B12 at least 200 pmol/L.
Those who qualified were randomized to folic acid 800 µg or placebo by mouth once daily for three years. Ninety-one participants were excluded due to a middle-ear problem or asymmetric hearing loss at baseline. Intention-to-treat analysis included 728 participants, of whom 12 died, four were lost to follow-up, and five stopped taking their pills.
Participants taking folic acid had an increase in median low-frequency hearing thresholds from 11.7 dB to 12.5 dB, with a median difference of 1 dB. The placebo group had an increase from 11.7 dB to 13.3 dB, with a median worsening of 1.7 dB. The difference between groups was statistically significant (P =.02). There was no difference between groups in median baseline hearing threshold at high frequencies (33.3 dB with folic acid vs. 35 dB with placebo) or at three years (40 dB in both groups). An accompanying editorial notes that changes in alertness and attention may account for differences in responses to audiologic testing and that authors should control for folate effects on cognitive function (Ann Intern Med. 2007;146:63-64). Compared with placebo, folic acid also improved cognitive function (memory, information-processing speed, sensorimotor speed) (Lancet. 2007;369:208-216). These study results may not be applicable to the general population because the study was conducted in an area without folic-acid fortification of food and was limited to persons with elevated homocysteine and vitamin B12 levels.