Taste Reactivity

The discrimination ability of taste seems to pale in comparison to those of the other senses. Taste can only distinguish between a few qualities: bitterness, saltiness, sweetness, sourness, and possibly a few others, including umami or savoriness. Despite its seeming simplicity, the sense of taste interacts strongly with the central nervous system (CNS) to regulate appetite for certain foods, and there is a relationship between the need for a specific nutrient and an increased desire for foods with a specific taste. This relationship can be quantified by the taste reactivity test.

The taste reactivity test is performed by injecting liquids with a specific taste through a surgically implanted intraoral catheter in animals (usually rats) and grading the animal's response as either ingestive or aversive. The value of the taste reactivity test in understanding how taste and nutrient need interact is best illustrated by specific experiments.

A simple experiment to demonstrate this interaction compares sodium-replete and sodium-deficient rats when a taste sensitivity test is performed using a solution of sodium chloride and water. Both groups will demonstrate aversive responses to intraoral quinine injection, a strongly bitter stimulus. Sodium-deficient rats, however, will demonstrate a strong ingestive response to salt water administration, whereas sodium-replete rats demonstrate the opposite reaction.

Taste sensitivity experiments have demonstrated a sophisticated interaction between the nutritive needs of animals and taste preferences. Fat-deprived rats will demonstrate stronger ingestive responses to normally avoided, nonnutritive oils, such as mineral oil, than to solutions containing artificial sweeteners. Evidence in both rats and humans suggests that taste receptors may be able to distinguish fatty acids as having a distinct taste and that this is sensitized in fat-deprived individuals. There is also evidence to suggest interindividual variability in the sensitivity for detection of fatty acids by these receptors. Responses to nonessential nutritive substances, such as ethanol, can also be influenced by taste preferences. For example, in rats, there is a correlation between a preference for sweet solutions and greater ethanol intake that has also been observed in human alcoholics.

The mechanisms that link taste and food-seeking behavior likely involve multiple pathways in the CNS. Modulation of neurotransmitters can lead to changes in taste sensitivity test preferences. For instance, rats given naltrexone, a drug that blocks opioid receptors, demonstrate decreased responsiveness to sweet solutions during taste sensitivity testing. This property may be exploited to reduce positive stimulatory feedback in individuals with addictive patterns of food ingestion or excessive cravings for specific nonnutritive tastes (e.g., ethanol). Improved understanding of the interplay between specific tastes and patterns of food ingestion will hopefully allow better prescription of dietary modifications for individuals with certain taste preferences.