By Shiyi Liang
Our sense of taste is important in our daily life. The main organs involved in this are the tongue and brain. After food is ingested, it is broken down and mixed with saliva. The tastant, dissolved in saliva, will act as a signal molecule and bind to the taste receptor cells within the taste buds which will recognize them. Taste buds are found in papillae; each type of papillae houses a different number of taste buds (Kinnamon, 2000). Tastants alter the cell’s ionic pathway, producing a depolarizing receptor potential. The signal is then transported to the cortical gustatory area in the parietal lobe through both the hypothalamus and limbic system (Sherwood, 2013). The tongue can differentiate between many taste sensations but there are five primary tastes: salty, sour, sweet, bitter and umami. It was once believed that the sensation of specific tastes was separated into regions, however, after extensive research, it has become known that every taste is recognized in all regions of the tongue. Although the transduction mechanism is not always the same according to regions and species (Kinnamon, 2000).
Taste disorders can be simply categorised into three main types: the loss or reduced sense of taste and alteration in the perception of taste referred to as ageusia, hypogeusia and dysgeusia (Daly, Daly, Minniti and Daly, 2012). There are five different methods used to measure taste sensitivity in ingestive behaviour or chemo sensational studies: Detection threshold (DT), recognition threshold (RT), suprathreshold intensity, propylthiouracil (PROP) bitterness, and the quantification of fungiform papillae (FP) anatomy. DT and RT provide an estimate of the ability to perceive and identify a substance by increasing the concentration of the target substance from the lowest chemical concentration that can be detected. In contrast, suprathreshold intensity measures the intensity detected above the highest chemical concentration that can be detected (terminal threshold). PROP is a substance for which the bitterness perceived varies between people and is used to identify supertasters, who can have stronger ability to sense oral stimulation (Eldeghaidy et al., 2018). The quantification of FP is the fifth method, where the number of FP is related to the intensity of taste perception; the more FP, the higher the intensity of taste perception (Webb et al., 2015). The counting of FP is processed by staining the tongue and then taking a high-quality digital picture of a small region of the tongue. It was counted manually in the past, but recently automated methods were developed (Eldeghaidy et al., 2018).
Causes of taste disorders can vary. The common causes are exposure to toxin or medicine, ageing, infections of the upper respiratory tract. Furthermore, head trauma or other neurological issues are regarded as a cause of disorder in taste to some extent. (Taste Disorders, 2020).
Taste disorders caused by drugs is common in actual practice, although it can be induced through different mechanisms. Dysfunction of the ion channel involved in taste buds or neurons is the usual case, resulting in an unfavourable metallic taste. Acetazolamide and tetracyclines are examples of drugs that are responsible for the above-mentioned mechanism. Other drugs, like penicillin and methimazole, may lead to a taste disorder by inducing zinc and copper deficiency. More research shows that calcium channels can also be blocked by drugs like verapamil, so that transmitters of taste are affected (Naik, Shetty and Maben, 2010). Another group of scientists suggested that patients have different sensitivity to drug-related taste disorders depending on various factors, including age, sex and even genetic variation (Doty, Shah and Bromley, 2008).
The taste disorders caused by ageing are more likely to manifest as an increase in taste threshold and discrete taste loss, instead of a complete loss of taste in the whole mouth (BARTOSHUK, 1989). For example, a study on a population in Turkey found that the threshold of bitter taste may be elevated in older people. They also took the quantity of circumvallate papillae into account, but there was no direct relation (Mavi and Ceyhan, 1999). There is speculation that the reduction in saliva production may affect the tasting process since less tastants can be dissolved (Boyce, 2006). Diseases related to ageing, like Alzheimer’s Disease are also being studied. A Japanese study suggested that the neuronal loss in the brain due to the disease could result in a failed transmission of taste stimuli. Their experiment with some Japanese Alzheimer’s patients shows a decrease in detecting and recognising all basic tastes except sweet taste. Nevertheless, the results of impairment of gustatory function in practice vary between many researches (Ogawa et al., 2017 ).
Recently, taste loss has been regarded as a symptom of COVID-19 which indicates that infections of the upper respiratory tract may lead to taste disorder. It is believed that the ACE2 receptors on taste buds are attacked by SARS-CoV and cause dysfunction in the salivary gland (Lozada-Nur, Chainani-Wu, Fortuna and Sroussi, 2020).
Scientists have also analysed cases of post-traumatic taste disorder. Trauma in peripheral and or central regions of the brain could lead to gustatory dysfunction. A study measured the taste acuity of 29 patients after head trauma with their result of incidence of taste abnormalities are slightly higher than expected and showed that symptoms can appear both immediately after injury or after several months. (Schechter and Henkin, 1974). However, some other scientists believed these cases to be rare and hard to explore due to a confusion between smell and taste, as well as the widely spread nerves and receptors. (Cecchini et al., 2018).
Taste influences our thoughts to decide whether to ingest food or not, acting as a detector of toxin or other inedible substances, and is related to the processes after food ingestion, dominating our physiological response (Breslin, 2013). Taste also takes part in food choice and is often a factor that influences people’s diet (Kourouniotis et al., 2016). So, taste disorders can create negative consequences for people. Individuals could add more sugar or salt than the normal amount in order to have a sense of taste, and a high take in of these condiments could be at risk of having a multiple illness like obesity (Taste Disorders, 2020).
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