Last week I came down with the flu. My sinuses became inflamed and blocked; my throat burned; my breathing was reduced to a dry, irritated wheeze; I routinely broke into hot sweats, only to freeze a moment later. Among the most debilitating effects of my illness, however, was my inability to taste. Despite my best efforts to find a favorable food item, nothing I tried satisfied my palette, not even the staples of my daily diet. Everything in the cabinet tasted different, alien, bland. This disability served to further my discomfort; my body was denying me the pleasure of a satisfying, flavorful meal. When we touched on the molecular biology of taste in class, my interest was piqued. How does the body interpret molecular structure into taste? And how do the changes in the body during illness affect the perception of flavor? In the following paragraphs I explore the mechanisms of taste and the ways in which these mechanisms become disrupted during illness.
The perception of a flavor is the result of a multi-tiered, complex mechanism. It involves the orchestration of the senses of taste, olfaction, and touch. For the sake of this exploration, we will focus on taste and olfaction. Our journey begins at the origin of flavor stimulation: the mouth. Chemoreceptors called papillary cells populate the tongue, throat, and soft palette (6). These papillae cluster into regions we know as “taste buds” (2). Food molecules bind loosely with specific taste bud receptors [proteins] according to their chemical composition and structure [this specificity of molecular interaction accounts for the differentiation of one flavor from another]. As a result, certain gated ion channels are activated in the papillary cells, eliciting a change in cell composition. This change results in a chemical message to the nerves that connect the tongue with the brain (1, 3, 4, 6). In the same way, food molecules will bind to specific receptors in the nasal cavity and elicit a neural response (3). This process of olfaction occurs concurrently with mastication as the breakdown of food results in the release of certain aromas through the back of the mouth.
The tongue is innervated by three main facial nerves. The primary nerve is the chorda tympani. It is directly connected with the cluster of receptors located at the apex of the tongue. The glossopharyngeal nerve transmits sensory information received by the back of the tongue and parts of the throat. Finally, the trigeminal nerve accounts for sensations of pain and temperature in the mouth (1). The electrical impulses transduced by each of these nerves are synthesized with neural messages from the nose in the brainstem (3). The brain interprets these messages into five general categories of flavor: sour, sweet, salty, bitter, and umami (or savory). These flavors are marked by certain nutritional qualities: a sour taste typically corresponds to foods that are acidic; sweet tastes indicate energy rich nutrients; salty foods allow the body to balance the amount of electrolytes; bitter tastes will indicate the presence of toxins or other harmful materials; the taste of umami corresponds to amino acids, or foods high in protein. The body will adapt its taste preferences according to the materials it needs (4). In other words, if your body needs energy, you might begin craving a piece of buttered toast or a chocolate bar.
The question now is how is this system interrupted during illness? The most obvious answer to the question involves the disruption of the olfactory system. As I mentioned toward the beginning of my exploration, my sinuses were inflamed and backed up during my bout with the flu. One could hypothesize that the taste disturbance I experienced was a function of the blockage of the nasal passages. The aromatic chemicals associated with food could not reach the nasal cavity due to the build up of mucous in the back of the throat. If this is the case, olfaction may play a more significant role in flavor perception than taste buds. Indeed, this would be consistent with general experience; if you have to take a poor-tasting medication, what do you do? You hold your nose. This stops flavor perception instantly.
Another potential source of my taste disturbance could have been the malfunction of my chorda tympani. As previously noted, this nerve connects the apex of the tongue [or the tip] to the brainstem. In the process, it passes directly under “the eardrum, the tympanic membrane.” As a result, any ear infection or upper respiratory illness affecting the ear poses a threat to the nerve; it can become infected by various viruses, knocking out the nerve until the body clears the infection (1). However, the dysfunction of the chorda tympani does not result in total loss of taste. This is because the glossopharyngeal nerve running from the back of the tongue will compensate for the loss of taste in the front (1). This means that my taste disturbance could have been in part a function of an infection in my chorda tympani, but that it was most likely a combination of infection and olfactory disruption.
If my taste disturbance were to continue [and I am happy to say it did not], I might be diagnosed with a taste disorder. These disorders, which run the gamut from reduced sensitivity to certain flavors to total loss of taste perception, are uncommon, with about 200,00 patients in the US per year (7). Disorders are often rooted in the malfunction of the chorda tympani. For example, if a viral infection is severe enough, the nerve could be damaged permanently. Also, the residual effects of middle ear problems that require surgery sometimes include dysfunction of the chorda tympani. Often patients will report the total loss of taste after ear surgery for a short period. This type of disturbance is usually overcome in a matter of days with the reactivation of the nerve, though not all patients are so lucky (7).
Due to the compensatory nature of the taste nerves in the mouth, disorders often go unnoticed for long periods of time. This is dangerous because taste malfunction can affect other areas of general health significantly. Our sense of taste functions as an important warning system for the body. For example, the potential ingestion of toxins is signaled by the taste of bitterness. If we cannot perceive the bitterness of bad milk we may become ill from its harmful contents. Also, taste disturbances can aversely affect people suffering from diabetes or heart disease. People who experience taste loss are more likely to change their dietary habits. An inconsistent diet of varying nutritional value could be devastating for a patient of diabetes (7).
This investigation has sparked some questions unrelated to my original interest [concerning the relationship between taste and illness]. First and foremost, if taste is a function of receptor proteins, and proteins are a function of our genes, then it follows that taste is largely hereditary. Obesity, which has become a significant cause for concern, is also thought to be somewhat hereditary. Could the inheritance of a particular set of taste preferences be the cause for obesity? Also, the fact that olfaction is highly involved with flavor perception is significant due to the close link between smell and memory. This link implies a relationship between taste and memory [i.e. where you first tried a specific food, who you were with, etc.]. My question is how much of taste preference has to do with this kind of association? I suppose the question boils down to our ongoing debate of nature versus nurture. Is it our genes that dictate taste or is it experience that informs our palette?
Overall, this exploration of taste has served to further confirm the notion that reality is a function of the mind. The chemical interaction of food molecules with taste receptors would be totally meaningless if the resulting signals were not sent to the brain. Using chemical messages and the bank of our memories the brain concocts the intricate and often pleasurable story of flavor. It is essential to realize that we taste with our brains, not with our mouths. In this way, taste, like all other sensations, is personal. Through the knowledge of the senses we learn of the brain’s capacity to stitch together the patchwork of our existence, and it is of this patchwork that meaning is borne.
1. "Tourist in a Taste Lab." Discover: Science, Technology, and The Future 1 July 2000.
2. “Taste bud.” Wikipedia: The Free Encyclopedia. 26 Oct 2009. 8 Nov 2009 <wikipedia.org>.
3. The Taste Science Laboratory. Web. 08 Nov. 2009. <http://www.tastescience.com>.
4. Bowen, R. "Physiology of Taste." Arbl.cvmbs.colostate.edu. 10 Dec. 2006. Web. 08 Nov. 2009. <http://www.vivo.colostate.edu>.
5, Monell Chemical Senses Center. Web. 08 Nov. 2009. <http://www.monell.org>.
6. This, Hervé. Molecular gastronomy exploring the science of flavor. New York: Columbia UP, 2005. Print.
7. National Institute on Deafness and Other Communication Disorders [NIDCD]. Taste Disorders. Web. 09 Nov. 2009. <http://www.nidcd.nih.gov>.