28.13: Taste acuity (24c.13)

Diminished taste acuity (hypogeusia) is a non-specific charac­teristic of marginal zinc defi­ciency and has been investigated as a functional bioindicator of zinc status in both cross-sectional and experimentally controlled studies in children and adults (Hambidge et al., 1972a; Buzina et al., 1980; Gibson et al., 1989a; Wright et al., 1981). For example, in a study of boys 5–7y with low height percentiles and low hair zinc concen­trations, impaired taste acuity was reported (Gibson et al., 1989a), but in a zinc depletion study of young men (0.25mg Zn/d for 4–9 weeks), there was a significant decline in their ability to discriminate differ­ences in saltiness com­pared with baseline levels. This change was unrelated to changes in zinc concen­trations in parotid saliva or plasma (Wright et al., 1981).

An age-related decline in taste acuity is well established in the elderly, but whether it is associated with a decrease in zinc status is uncertain. Some cross-sectional studies have reported positive associations between taste acuity for salt and erythrocyte zinc in the elderly (Stewart-Knox et al., 2005). Moreover, taste acuity for salt (but not for sweet, sour, or bitter taste) reportedly increased in older adults from Grenoble but not from Rome in response to 30mg Zn/d com­pared to placebo during a six-month double-blind randomized controlled trial (Stewart-Knox et al., 2008). Loss of taste during irradiation or chemotherapy has been restored with zinc supple­mentation in some (Yamagata et al., 2003; Najafizade et al., 2013), but not all (Halyard et al., 2007; Khan et al., 2019) studies.

Measurement of taste acuity

Several methods for testing taste acuity are available. Usually, evaluation is based on assessment of both the detection and recognition thresholds for each of four taste qualities (e.g., sweet, salt, sour, bitter), and sometimes umami (monosodium glutamate). The detection threshold is defined as “the lowest concen­tration at which a taste can just be detected”; the recognition threshold is “the lowest concen­tration at which the quality of the taste stimulus can be recognized”. The two thresholds are each deter­mined for one taste quality before proceeding to the next. Several investigators have used this technique on young children (Hambidge et al., 1972a; Buzina et al., 1980).

An alternative technique that assesses only recognition thresholds and is better suited to children who are easily distracted and have short attention spans, has been developed (Desor and Maller, 1975). In some of these studies on children, recognition thresholds for only one taste quality (often salt) have been deter­mined. Salt is selected because taste perception to moderately salty solutions changed significantly during a zinc-depletion study of young adult men (Wright et al., 1981). Recognition thresholds for salt have been measured in a study of Canadian school boys with low height percentiles and in Guatemalan school children. In these two investigations, school children with low hair zinc concen­trations had higher recognition thresholds for salt than children with hair zinc concen­trations indicative of “normal” zinc status (Gibson et al., 1989a; Cavan et al., 1993a). Sodium chloride concen­trations used for testing recognition thresholds in these school children were 10, 15, 20, and 25mmol/L (Gibson et al., 1989a). Participants rinse a small amount (10mL) of each solution around in their mouths, expectorate it, and are then asked to identify the presented sample as salty or plain water. Salt solutions of increasing or decreasing concen­trations are used, where appropriate, until the sub­jects correctly identify salt at one concen­tration and fail to do so at the next lower concen­tration. Only 2 out of 10 judgments are allowed to be incorrect before moving to the next higher or lower salt concen­tration. The midpoint between these two concen­trations is used as the recognition threshold for salt for each participant.

Some studies have used an electrogustometer to measure taste thresholds by applying a weak electric current to the tongue (Prosser et al., 2010). A threshold value is deter­mined by alternately lowering and raising the current todetermine the smallest stimulus that can be discriminated correctly (Grant et al., 1987). For a review of the inherent strengths and limitations of electrogustometry, see Stillman et al. (2003). Automated-com­puter-controlled electrogustometry can now be used to estimate taste detection thresholds (Stillman et al., 2000), although so far, this method has not been applied in studies of zinc status. For all taste acuity methods, it is preferable to perform the tests midmorning, at least 2 hours after a meal, with the same person administering the test on each occasion. Many other factors affect taste function. Hence, taste acuity should be used in conjunction with other biomarkers to assess zinc status. The BOND Zinc Expert Panel classified taste acuity as an “emerging” zinc biomarker.