Objectives. We aimed to critically appraise scientific, peer-reviewed articles, published in the past 10 years on the effects of hydration on voice quality in adults. Study design. This is a systematic review. Methods. Five databases were searched using the key words "vocal fold hydration", "voice quality", "vocal fold de-hydration", and "hygienic voice therapy". The Preferred Reporting Items for Systematic Review and Meta-Analyses (PRISMA) guidelines were followed. The included studies were scored based on American Speech-Language-Hearing Association's levels of evidence and quality indicators, as well as the Cochrane Collaboration's risk of bias tool. Results. Systemic dehydration as a result of fasting and not ingesting fluids significantly negatively affected the parameters of noise-to-harmonics ratio (NHR), shimmer, jitter, frequency, and the s/z ratio. Water ingestion led to significant improvements in shimmer, jitter, frequency, and maximum phonation time values. Caffeine intake does not appear to negatively affect voice production. Laryngeal desiccation challenges by oral breathing led to surface dehydration which negatively affected jitter, shimmer, NHR, phonation threshold pressure, and perceived phonatory effort. Steam inhalation significantly improved NHR, shimmer, and jitter. Only nebulization of isotonic solution decreased phonation threshold pressure and showed some indication of a potential positive effect of nebulization substances. Treatments in high humidity environments prove to be effective and adaptations of low humidity environments should be encouraged. Conclusions. Recent literature regarding vocal hydration is high quality evidence. Systemic hydration is the easiest and most cost-effective solution to improve voice quality. Recent evidence therefore supports the inclusion of hydration in a vocal hygiene program.
