Vol. 27:02

The Taste and Smell in Medicine

If one were to conduct a survey on the importance of the senses, most people would respond that vision and hearing are far more important than taste and smell. Yet the ability to perceive odors and experience the taste of foods are very important sensory functions, and connect the brain to the outside world. Taste is a primary test in searching and identifying nutritional food substances throughout the animal kingdom. The nasal sensory organs of humans contain over ten million olfactory receptors, which play an important role in the enjoyment of food, attraction or repellent feelings toward other people, or instant recall of memories.¹ (In fact, the emotional components of this sensory function are the driving force behind the multi-billion dollar cosmetic and fragrance industry.)

Chemosensory stimulation triggers a cascade of molecular and cellular events that are translated into a signal for the nervous system. Interestingly, the sensory cells that detect odors and tastes have much in common with the rod and cone cells that detect light. The receptors of olfactory stimuli are ciliary transversal membrane proteins that react with any volatile compounds. The encoded information is then transmitted to the appropriate brain areas to initiate and maintain the sensory response.²

Is this sensory mechanism often utilized in Western medicine? It seems not. Most drugs are coated and masked to neutralize their taste and smell. Keeping the original taste and characteristic smell of the drug is considered a bad thing from a marketing standpoint.³

Traditional Eastern medicinal practices, however, consider taste to be a fundamental element of therapy. The idea is that taste can stimulate or turn off digestive processes, which means that food components are digested in a certain order according to their taste. Ayurvedic medicine distinguishes six tastes: sweet, sour, salty, pungent, bitter, and astringent. Traditional Chinese medicine has similar categories.^4,5 Western medicine generally agrees on four primary taste qualities: sweet, sour, salty, and bitter. Recently a fifth taste was introduced in Western medicine, with the help of the food industry, called "umami," which describes the taste of certain amino acids (glutamate, aspartate and related compounds).

Anatomy and Physiology of Taste
The taste sensation begins when ions or polar molecules of food taken into the mouth stimulate the cells of the taste buds. The taste buds are mainly found on the tongue, and are barrel-shaped organs made up of 40?0 cells each. They occur on the fungiform, foliate and circumvallate papillae, and are found on the soft palate, pharynx, epiglottis and larynx.^7,8

Sensory Mechanism: A salty taste is triggered by ions such as sodium chloride entering the cells via the appropriate ion channels, in this case Na⁺-channel. The entry of Na⁺ causes depolarization; transmitter release occurs and results in stimulation of the primary afferent nerve. A sour taste is the result of increased proton concentration H⁺ (higher than in the water). H⁺ ions block K⁺ channels that are responsible for maintaining proper hyperpolarized membrane potential (-85 mV). Blocking these channels causes depolarization and Ca²⁺ dependent transmitter release that results in increased firing of the primary afferent nerve. A sweet taste is sensed by receptors in the apical membrane that binds glucose and other carbohydrates. The receptors then activate gustducin and cAMP production by adenylate cyclase. The elevated levels of cAMP cause PKA-mediated phosphorylation of K⁺ channels, which inhibits the receptors. Depolarization occurs, followed by Ca²⁺ entry through the depolarized channels; transmitters are then released, which results in increased firing in the primary afferent nerve. A bitter taste is mediated via IP₃ second messenger action, and Ca²⁺ is released from internal stores. The elevated Ca²⁺ then triggers transmitter release and firing in the primary afferent nerve, as described above.^9,10

Traditional Approach: In traditional Chinese medicine, taste is considered the pharmacological force and action potential in and of itself. Sweet and pungent tastes are considered yang, which means they tend to move upward in the body, while other tastes are yin by nature and thus move downward. Each taste has a special relation to an organ in the body system that can develop to a tonifying, damaging or dispersing effect.¹¹ According to Ayurvedic medicine, each of the six tastes has its own specific set of therapeutic actions. Sweet taste builds and strengthens body tissues and has demulescent, mildly laxative and expectorant traits. Salty taste is a softening laxative and sedative. Sour taste has stimulant, carminative, nourishing, and thirst relieving qualities. Pungent taste is a stimulant and a carminative, yet has diaphoretic actions. Bitter taste is alterative, cleansing and detoxifying. Astringent taste stops bleeding and excess discharges, and promotes the healing of skin and mucous membrane.^4,12 Each body constitution needs certain amounts of each taste; the relative proportion depends on the actual humoral (dosha) balance. As in Chinese medicine, each taste in excess can damage the related organ. Sweet taste damages the spleen and pancreas, salty taste damages the kidneys, pungent taste dries the lungs, bitter taste damages the heart, and astringent damages the colon. The six tastes in combination have various therapeutic effects. For example, milk thistle, goldenseal and cayenne each have a drying, cleansing action.^4,12

Age, Gender, Disease and Medical Treatment Related Changes in Taste and Smell
Age-related declines in olfactory and gustatory function have been clearly demonstrated.^13-15 The poorer flavor discrimination and the decreased ability of an elderly person to identify taste of food in the mouth reflect the age-related decline of olfactory function. However, most of the studies completed have not separated the influence of aging from that of diseases and treatments.¹⁶ There is evidence that chemosensory disturbances and complaints are more likely the result of the decline of olfactory function rather than of taste.^17,18 In addition, elderly men suffer from chemosensory loss more frequently than women of that age group.¹⁶

Disease related changes: Smell and taste acuity were compared in 50 normal and 48 epileptic patients. The acid and salt sensation thresholds did not differ between the two groups but epileptic patients showed a lower detection threshold for sucrose (sweet) and urea (bitter) than the control subjects did. Also, epileptic patients had stronger smell acuity; this was most evident in patients with partial seizures.¹⁹ Some data suggest that an increase in Zn²⁺ concentration and/or turnover in the hippocampus and olfactory region reduces the convulsive and olfactory threshold.²⁰ Another piece of supporting evidence for the role that Zn²⁺ may play in taste and smell control is that Parkinson's disease patients, who suffer from vision loss as well as olfactory and taste loss, are Zn²⁺ deficient.²¹

Recent studies show that patients with chronic renal failure also develop taste and smell loss related to zinc deficiency that can be reversed by zinc supplementation. Although adequate maintenance of dialysis can help for a limited amount of time, the only lasting solution is renal transplantation. Most of the metabolic abnormalities of uremia are normalized following successful renal transplantation, but abnormalities of zinc metabolism may persist for a year. The recovery of the normal thresholds of taste also may take a year.²² The ability to detect a bitter taste of the chemical phenylthiocarbamide (also called phenylthiourea) has been used to indicate the presence of certain diseases. It has been observed that those who do not detect the bitter taste have a higher incidence of thyroid disease, glaucoma, tuberculosis, and carcinomas of the breast, cervix and ovary.²³

Taste and smell function were evaluated in 40 HIV-positive individuals and the same number of control subjects. This clinical study showed significant differences between HIV and control subjects when comparing glutamic acid and quinine hydrochloride taste detection threshold (p < 0.001).²⁴ The perception of taste and smell initiate cephalic phase reflexes which result in increased salivary and gastric secretion that is required for normal digestive and absorptive processes. Therefore the loss of chemosensory function may contribute to the impaired nutritional status of HIV patients. This theory seems to be supported by Schiffman and Warrick, who concluded that the addition of flavors to food can compensate for these losses. They observed elevated T and B lymphocyte numbers in elderly patients. The elevation of T and B lymphocyte levels may be due to increased nutrient absorption through improved cephalic reflexes or other psychological factors.²⁵

Both acute and chronic liver disease can alter smell and taste function. Increased detection and recognition thresholds for a variety of tastants have been reported in acute viral hepatitis, as well as a return to normal threshold levels after hepatic recovery.²⁶ In the case of chronic liver disease, the gustatory acuity was diminished compared to normal subjects.^26,27

A dramatic change in taste threshold was observed in patients following allogeneic bone marrow transplantation. Most patients complained of a loss of or change in taste function during the acute phase. The alterations lessen only after 30 to 50 days (or sometimes even after 100 days). In contrast, autologous transplantation does not cause late alterations of taste thresholds.^28,29

Patients receiving chemotherapy often complain about taste changes and less frequently decline in smell perception.^30,31 These side effects of the therapy have a negative effect on a patient's quality of life, and nurses and physicians rarely discuss taste changes with patients. The lack of information and awareness of this side effect leads to the patients changing their diets and seasoning their foods, which may increase the complications of the therapy.³⁰

Other drug therapies may also be followed by a decline of taste perception in the patients. Initiation of ACE inhibitor use to treat hypertension has also been associated with taste disturbances as a side effect.³² It is worth noting that the traditional functional view, and independent treatment of the nervous and immune system, is being challenged by a new view that the nervous system regulates the activity of the immune system.³³

Diet as disease prevention: Diets high in vegetables and fruit are assumed to be cancer preventive, although this generalization for all types of cancer is debated, and the clinical studies ended with variable conclusions. However, high vegetable and fruit consumption reduced the risk for cardiovascular disease by lowering both LDL levels and the oxidation of LDL because of the fruits' and vegetables' high content of phenolic acids, polymeric anthocyanins, catechins, and flavonols.³⁴ Most of these compounds are bitter, with diverse chemical structures that suggest the existence of multiple bitter-taste receptors.³⁵ Phenolic compounds may act as natural pesticides, protecting the plants from pathogens, insects, and other parasites.³⁶

Personalized diets can be tailored according to the need, body character, and the disease that the practitioner tries to prevent or treat (as described earlier in the Ayurvedic medicine section). The appropriate concentrations of phytonutrients, spices, herbal teas and other preparations that can be included in the patient's diet can act as antioxidants, cancer-preventive agents, etc. The idea of so-called functional foods is not new, and is part of both traditional Chinese medicine and Ayurvedic medicine, and has recently been adopted by the Japanese Ministry of Health meaning "processed foods [will contain] ingredients that aid specific bodily function in addition of nutrition."³⁸

Conclusion
It is widely accepted that the chemosensory functions of both taste and smell play an important role in appetite and food intake and also in the early detection of some diseases.²⁷

The meaning of "balanced diet" should exceed the quantitative measure of carbohydrate, protein and fat. Traditional Eastern medical practice believes that the balanced taste of a particular food is reflected in its chemical composition. Perhaps we should reconsider the combination of diet and medicine creating a functional food that can be included in the treatment of different chronic diseases. In addition, the general practice of masking the taste of drugs or "making them sweet" when the original taste is tolerable may need to be reconsidered.

Issue date: February 2002

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References

1. Vemuri V. Odor. http://my.engr.ucdavis.edu/~das/rao/Odour.html.

2. Lehninger Principles of Biochemistry Biosignaling Ch13. 463?465.

3. Schreiber WL, et al. Flavors and fragrances: The Chemistry Challenges. Chemtech 1997;27(3)58?62.

4. Swami Sada Shiva Tirtha. Ayurvedic Encyclopedia Pub Ayurveda Holistic Center 1997. 29?31.

5. Basic Theories of TCM (Academy Press) Vol.1. 3-?.

6. Chaudhari, et al. The taste of monosodium glutamate: Membrane receptors in taste buds. J Neurosci 16;3817-?826.

7. Gray's Anatomy. Running Press 1974. 824.

8. Yamamoto T. Taste responses of cortical neurons. Progress in Neurobiology 1984;23:273?315.

9. Herness MS and Gilbertson T. Cellular mechanisms of taste transduction. Annu Rev Physiol 1999;61:873-?00.

10. Jacob T. Taste http://www.cf.ac.uk/biosi/staff/jacob/teaching/sensory/taste.html.

11. Mills S, Bone K. Principles and Practice of Phytotherapy: Modern Herbal Medicine. Churchill Livingstone 2000 7-?0.

12. Frawley D. Ayurvedic Healing: A comprehensive guide. Lotus Press 1998.

13. Murphy C. Taste and smell in the elderly. In Meiselman HL, Rivlin, eds. Clinical measurement of taste and smell, New York: Macmillan, 1986:343?371.

14. Schiffman S. Changes in the taste and smell with age: Psychophysical aspects. In: Ordy JM, Brizzee K eds. Sensory system and communication in the elderly. (Aging, Vol;10) New York Raven Press, 1979: 227?246.

15. Doty RL et al Smell identification ability: Changes with age. Science 1984;226:1441?1443.

16. Ship JA, Weiffenbach JM. Age, gender, medical treatment, and medication effects on smell identification. J Gerontology: Medical Sciences 1993;48:M26--M32.

17. Goodspeed RB, et al. Clinical characteristics of patients with taste and smell disorders. In: Meiselman HL, Rivlin, eds. Clinical measurement of taste and smell, New York: Macmillan, 1986:451?466.

18. Deems DA, et al. Smell and taste disorder, a study of 750 patients from the University of Pennsylvania smell and taste center. Arch Otolaryngol Head Neck Surg 1991;117:519?528.

19. Campanella G, et al. Smell and taste acuity in epileptic syndromes. Eur Neurol 1978;17:136?141.

20. Schechter PJ, Henkin RI. Abnormalities of taste and smell after head trauma. J Neurol Neurosurg Psychiat 1974;37:802-?10.

21. Forsleff L, Schauss AG, et al. Evidence of functional zinc deficiency in parkinson disease. J of Alternative and Complementary Medicine 1999;5:57-?4.

22. Mahajan SK, Abraham J, et al. Effect of renal transplantation on zinc metabolism and taste acuity in uremia: A prospective study. Transplantation 1984;38(6):599?602.

23. Schlosberg A, Baruch I. Phenylthiocarbamide (PTC) tasting in paranoid and nonparanoid schizophrenic patients. Perceptual and Motor Skills 1992;74:383?386.

24. Graham CS, Graham BG, et al. Taste and smell losses in HIV infected patients. Physiology & Behavior 1995;58:287-?93.

25 Schiffman SS, Warwick ZS. Effect of flavor enhancement of foods for the elderly on nutritional status: Food intake, biochemical indices and anthropometric measures. Physiol Behav 1993;53:395-?02.

26. Smith FR, et al. Disordered gustatory acuity in liver disease. Gastroenterology 1976;70:568?571.

27. Bloomfeld RS, Graham BG, et al. Alteration of chemosensory function in end-stage liver disease. Physiology & Behavior 1999;66:203-?07.

28. Dahllof G, et al. Oral condition in children treated with bone marrow transplantation. Bone Marrow Transplantation 1988;3:43-?1.

29. Marinone GM, et al. Late taste disorders in bone marrow transplantation: Clinical evaluation with taste solutions in autologous and allogeneic bone marrow recipients. Haematologica 1991;76:519-?22.

30. Wickham RS et al. Taste changes experienced by patients receiving chemotherapy. Oncol Nurs Forum 1999;26(4):697-?06.

31. Lindley C et al. Perception of chemotherapy side effects cancer versus noncancer patients. Cancer Pract 1999;7(2):59-?5.

32. Rosenthal J et al. Analysis of adverse effects among patients with essential hypertension receiving an ACE inhibitor or a beta-blocker. Cardiology 1996;87(5):409?414.

33. Rodney W et al. Neural and Pavlovian influences on immunity. Pav J Biol Sci 1985;20(4):181?194.

34. Drewnowski A, Gomez-Carneros C. Bitter taste, phytonutrients, and the consumer: A review. Am J Clin Nutr 2000;72:1424?1435.

35. McBurney DH. Psychological dimensions and perceptual analyses of taste. In: Carterette EC. Friedman MP, eds Handbook of Perception: Tasting and Smelling. Vol 6A New York Academic Press 1978.

36. Ames BN, Profet M, et al. Dietary pesticides (99.99% all natural) Proc Natl Acad Sci USA 1990:87:777?781.

37. Hoffman D. Therapeutic Herbalism. A correspondence course in Phytotherapy. 1997 43?95.

38. Osawa T. Recent progress on functional food research in Japan. In: Shibamoto T, Tereao J, Osawa T. eds Functional Foods for Disease Prevention II. Medical Plants and Other Foods. Washington DC American Chemical Society. 1998:2?9.

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