During the three-year double-blind trial, participants took a placebo or 800 mcg of folic acid per day. Doctors tested hearing in the range of sound frequencies of the normal speaking voice. By the end of the three-year period, those who had taken folic acid could hear low frequencies—such as the voice of a bass or baritone singer—better than those who had taken the placebo. Hearing in both groups declined, but the placebo group needed a louder noise—13.4 decibels on average—before being able to hear low-frequency sound, compared to those in the folic acid group, who on average could hear low frequencies at 12.7 decibels or 0.7 decibels softer than the placebo group.
At the beginning of the study, all participants heard low-frequency sound at an average of 11.7 decibels. By the end of the study, the placebo group needed a 15% increase in sound volume compared to a 9% increase in the folic acid group. Folic acid did not affect the decline in hearing high-frequency sounds, such as those of a tenor or soprano singer, which participants heard at an average of 34.2 decibels at the beginning of the study. Researchers noted that some countries, including the Netherlands, do not require food manufacturers to fortify foods with folic acid, and that the average blood level of folate at the beginning of the study was 50% lower than the average level in the U.S., which does fortify foods with folic acid.