Background: Diminished taste acuity (hypogeusia) has been linked to zinc deficiency in humans and animals. This phenomenon has been exploited in the Zinc Taste Test (ZTT), a taste acuity test commonly employed by Australian naturopaths. However, its validity has not yet been firmly established.
Methods: A systematic search of several key databases was conducted. Only studies in which there were full reports of clinical trials comparing the ZTT to at least one other zinc test within the same sample population were included.
Results: Three (3) studies matched the criteria for inclusion. Study I compared the ZTT with sweat zinc in patients with food intolerance, reporting moderate correlation. Study II recruited pregnant women using the ZTT and serum zinc to assess zinc status, with above 70% congruence between the two tests at the start of the trial and 100% congruence at the end. Study III also recruited pregnant women at three stages during gestation, assessing ZTT and leukocyte zinc initially, later adding dietary zinc intake and at delivery cord blood zinc. No significant correlation was found between the results of these different methods; however, statistically significant differences in the ZTT responders (tasters and nontasters) were found for pregnancy outcomes.
Discussion: The methodology of the three studies is critically discussed. Although depletion of zinc leads to decreased taste acuity, it does not explain all cases of hypogeusia. Various other influences on taste perception are discussed in relation to the validity of the ZTT. Stringent exclusion criteria are therefore mandatory to increase specificity. Large variations from the original test design have been identified. The laboratory assays of zinc in these studies are also lacking sensitivity to accurately assess zinc status.
Conclusions: To date, there are no tests that are both sensitive and specific that accurately assess marginal zinc status in humans. The ZTT, albeit widely used, does not fill this void, and further research is needed.