Commentary

The objective of this systematic review published in The Cochrane Library was to assess zinc supplementation or acupuncture therapy in the treatment of zinc-deficient/idiopathic taste disorders and taste disorders secondary to chronic renal failure in children and adults.

Our sense of smell and taste function to protect us from harmful substances but also substantially contribute to our quality of life.1 Therefore, it is paramount that clinicians understand the implications when these senses are compromised. Zinc deficiency has been associated with disturbances in the gustatory and olfactory systems.2 As such, clinical studies have attempted to determine the therapeutic effects of zinc supplementation in patients who suffer from taste impairment. While there is clearly an interest in the treatment of taste disturbances, there is a gap in the current evidence to guide clinicians in their decision-making.

Improvement in taste acuity and taste discrimination were the primary outcomes assessed in this review which included parallel and cross-over randomised controlled trials (RCTs) with either a pharmacological or non-pharmacological intervention. Following systematic searches in electronic databases, data were extracted from ten RCTs of which nine were included in the review, with a combined total of 566 participants. Methodological parameters including risk of bias, units of analysis, missing data, blinding and allocation were addressed as they related to each of the included studies. Importantly, individual study results were clearly displayed and meta-analysis was sub-divided by a common primary reported outcome, shared by two or three of the included trials. Taste acuity improvement was quantified as a patient-reported outcome (two trials), an objective outcome recorded as a continuous variable (three trials) and as a dichotomous variable (three trials), with corresponding meta-analysis demonstrated in figures 3, 4 and 5 respectively. While there appears to be statistical significance in improvement of taste acuity when measured as an objective outcome, there appears to be no improvement when the analysis was based on the patients' reported outcome. While the current evidence is weak, the methodology of the review was appropriate in that it extracted data from RCTs. The authors' conclusions are founded in the data analysis and do not overestimate the effect of zinc supplementation.

Four trials had high risk of bias which impacted on the value of their reported evidence. Furthermore, the evidence in the studies was determined to be of low quality by the GRADE approach.3 As aforementioned, the trials analysed here reported low quality evidence rendering this review inherently weak, despite its thorough methodology. The authors illuminate that there remains a lack of high quality evidence that corroborates improvement in taste perception with zinc supplementation or acupuncture. Further clinical trials with low risk of bias, superior quality evidence and comparable variables are necessary to define the role of these therapeutic interventions in the treatment of taste disturbance. Until then, clinicians should consider that zinc supplementation may marginally improve taste acuity and it may be considered as a treatment option, however side effects such as nausea and abdominal pain should be considered.