Dipartimento di Medicina- DIMED
Cattedra di Geriatria-Direttore: Prof. Enzo Manzato
Università degli Studi di Padova
email: eminemeral.inelmen@unipd.it

Introduction
The aim of this article is to highlight the importance of the sense of taste, and the consequences of taste loss, a common condition in advanced age. The treatment should be of help to enhance the quality of life in older adults.

Importance of taste

The taste sense is one of the five human senses. It is essential to our survival because it enables the individual the choice of correct food, which, in turn, is crucial for one's existence, maintenance and function. Taste helps all of us recognize when food is good or bad.
The loss of sense of taste significantly affects a person's enjoyment of the external environment and overall life. Taste is implicated in the loss of appetite through a perceived decline in the pleasantness of food, and is also an important part of the cephalic phase response that prepares the body for digestion. It helps modulate food choice and meal size by increasing satiety and the pleasure of eating. Of concern is the potential for poor nutritional intake. Any change in nutrient intake can lead to malnutrition with its potentially serious consequences.
There are five basic tastes: bitter, sweet, sour, salty and umami, each of which has a role in food selection, being responsible for the recognition of certain chemicals, which may be either necessary or dangerous to our body.
The following terms are commonly used in this topic: "ageusia" (Greek for "no taste") is absence of one or more of the four basic tastes; "parageusia" is the perception of foul or spoiled food instead of a normal sense of taste; "dysgeusia" is the distortion or misinterpretation of a taste while eating.

A "new" basic taste : umami

The biological significance of this basic taste, discovered about 100 years ago, is high, comparable perhaps to that of sweet taste. "Umami", a term derived from the Japanese umai (delicious, savory), designed a pleasant taste sensation which is qualitatively different from sweet, salty, sour and bitter (1). Umami is a dominant taste of food containing L-glutamate, like chicken-broth, meat extracts and ageing cheese (1). The umami substances (MSG monosodium glutamate, GMP dysodium gluanylate and IMP disodium inositate) that are often added to food as a flavor enhancer, are contained in various food, including vegetables (tomato, potato, cabbage, mushrooms, carrot, soybean and green tea), seafood (fish, kelp, seaweed, oyster, prawn, crab, sea urchin, clam and scallop), meat (beef, pork and chicken) and cheese and contribute to the characteristic tastes of these food (2). Nevertheless, researchers still debate the existence of a fifth class of basic taste-umami.

Physiology of taste

The sense of taste is mediated by taste receptor cells which are bundled in clusters called taste buds. The taste buds are situated in the tongue- called papillae- oral cavity and the proximal third of the esophagus. This translates the chemical signal of tastants in food to electrical stimulation that transfers the signal to higher processing centers in the brain, in a process called transduction. Taste is associated with three nerves, CN IX, CN VII, and the vagus nerve (CN X), innervating specific locations in the tongue and palate. Once taste signals are transmitted to the brain, the efferent neural pathways are activated that are important to digestive function. For example, tasting food is followed rapidly by increased salivation and by low level secretory activity in the stomach. Enjoyment of food and the impetus to eat to satiety depend on normal functioning of this chemosensory area. In addition to signal transduction by taste receptor cells, it is also clear that the sense of smell profoundly affects the sensation of taste.

Alteration of taste

Alteration in the sense of taste may be due various central (involvement of the "Taste area" in the temporal lobe) or peripheral (changes in the receptor cell taste buds) function as in xerostomia or damage to the gustatory afferents in the facial or the hypoglossal nerve (3). Bitter taste is the first to be affected and is the last to come back while sweet goes later and is the first to come back; a possible explanation could be that fewer fibers carry bitter taste signals while a larger number of fibers carry the sweet sensation signals (3). However, true gustatory disorders are rare; because up to 80% of a meal's flavor is a result of olfactory input, patients frequently interpret a loss of smell as a loss of taste (4). For most patients who complain of decreased ability to perceive food flavor or who experience "unpleasant taste", the sense of taste is intact, and likely are suffering from olfactory dysfunction (4).

Evaluation of taste

Evaluation of chemosensory ability should assess the sense the quality and intensity of taste. Typically, the following stimuli are used to test four taste qualities: sodium chloride (salty), sucrose (sweet), citric acid (sour), and quinine hydrochloride or coffee (bitter). To assess taste intensities, patients are used as their own control and the differences between the right and the left sides of the mouth are evaluated (4).
The evaluation of taste disorders measures detection or recognition thresholds. A high threshold will result in the patient perceiving that food is tasteless, whereas food that tastes different than its original taste may also result in reduce intake. However, the tests are extremely variable and are unable to determinate the cause and give neither prognostic information nor therapeutic guidance (4).
An instrument to test the gustatory function named "electrogustometry" which is based on a weak electrical stimulus producing a sour taste when applied to taste receptors, is currently available (4). This method has several strengths, such as the ability to provide quantitative control of the intensity of stimulation, the short time required for testing, and ease of administration, but electrogustometry is inappropriate for evaluation of taste qualities other than sour.

Taste and aging

Loss of the sense of taste is common among older people (5), but the cause of taste loss is not fully understood. A common complaint voiced by older people especially living in nursing homes, is that the food lacks taste. Possible theories include a decline in gustatory function due to physiological decline in the density of the taste buds and papillae (6). In fact, several studies on taste dysfunction have concluded that changes in taste bud density with increasing age may affect taste function differently on different regions of the tongue (7). The findings of a recent systematic review (8) suggest that taste perception declines during healthy ageing process, although the extent of decline varies between studies. A Chinese study (9) had shown that after about 70 years of age, taste threshold begins to increase, and that diseases lead to dysgeusia, while another recent study (10) has focused the importance of hyposalivation as a risk factor for taste disturbances in older people.
With regard to gender differences, studies report the higher incidence of hypogeusia among older women of all races compared with men of similar ages, likely because older women are more commonly treated with long-term prescription drugs than older men (11)

Causes of taste disorders in aging

In addition to physiological changes associated with the aging process, the most common causes of taste disorders are: oral and systemic diseases (7.4 and 6.4%, respectively), drug use (21.7%) and zinc deficiency (14.5%)(12).

Table 1 shows the most frequent causes of taste disorders in aging.

a) Oral diseases
Oral health and dentition have been shown to significantly affect food intake, gustatory function, and generally deteriorate with aging. Older people sometimes report that food tastes unpleasant or bitter. This reaction may be an oral condition. Oral conditions affecting composition and amount of saliva also affect flavor perception. It has been shown that older people have less of their own teeth; 59% of people aged 65-74 years were dentate, but only 35% of people aged 75 years or over (13). Furthermore, edentate people reported greater difficulty with eating a range of foods, more chewing problems occurred, and mouth dryness was more common (13).
The most common diseases of the oral cavity in the older people are dental caries and gingival and periodontal diseases (14). Oral candidiasis and stomatitis can alter the sense of taste (14) as well, ad so can the noxious bacterial products of dental -alveolar infections (15).Tumors of the oral cavity can also cause taste changes; the main risk factors are smoking and alcohol abuse (15).
Finally poor oral hygiene results in a higher risk of taste disorders. Older people who may have difficulty in maintaining oral hygiene, particularly those who use prosthetic devices, can be helped by brushing the tongue and rinsing the mouth.

b) Systemic diseases
Changes in the sense of taste can occur in several diseases common in aging as stroke, Mild Cognitive Impairment (MCI), Alzheimer's disease, Parkinson's disease, major depression, diabetes, hypothyroidism, hyperthyroidism, cancer (lung, breast, head and neck, esophagus, stomach), chronic renal failure, acute and chronic liver diseases, cirrhosis, hypertension, Sjögren's syndrome, irritable bowel syndrome, gastroesophageal reflux disease, Crohn's disease, chronic obstructive pulmonary disease, post-influenza conditions (5). Particularly renal disease may produce a phantom taste (metallic or bitter), likely secondary to accumulation of uremic toxins because improvement occurs after dialysis (4).

c) Drugs
Loss of taste in aging and can be exacerbated drugs (5). Studies show how important taste alterations can be; for example, an elderly person (with one or more medical conditions, and who takes an average of three medications) needs 11 times as much salt and almost three times as much sugar to detect these tastes in food compared with younger people (16).
Several drugs with sulfhydryl groups, such as propylthiouracil, methimazole, captopril, and penicillamine, may induce hypogeusia or bitter or metallic dysgeusia (17). Dysgeusia is a common but often ignored adverse effect that is associated with angiotensin-converting enzyme (ACE) inhibitors; for these patients a metallic, bitter, or sweet taste overrides any other taste sensation caused by food (17). The angiotensin II receptor antagonist losartan has also been associated with ageusia (17). Lipid lowering drugs, anti-infiammatories, metabolic agents (biguanide, thiamazole), anti-depressants, diuretics, antiarrhythmic agents, anticoagulants (clopidogrel), are also involved in taste disturbances (5).
" Xerostomia", or dry mouth, is a common side effect of agents with anticholinergic action and is associated with decreased taste sensation; xerostomia interferes with proper mastication of food, makes swallowing food difficult, and increases the risk of mucosal infections (17). Nevertheless, the mechanism by which these drugs affect taste remains unknown (13).

d) Tobacco and Tobacco products
Substances with pharmacologic action such as tobacco products alter chemosensation. Smokers have reduced pleasure while eating foods such as chocolate, which have a high fat content (18). Reduced intake of fatty foods may explain the reduced average weight of smokers compared with nonsmokers (18).

e) Treatments
Taste alterations can frequently be observed in oncological patients undergoing chemotherapy and are reported as being among the most distressing side effects, along with fatigue, nausea, vomiting, and hair loss (19,20). Despite their frequent occurrence, the literature on this issue is scarse.
Taste alterations often start at the beginning of chemotherapy and do not always cease with its termination, but may persist for weeks or even months beyond active therapy (21,22). Both radiotherapy and chemotherapy cause taste disturbances and 78% of patients with taste disturbances receiving palliative care have also a positive culture of Candida spp. (23): there is a close relationship between candidiasis and taste disturbances (14).
Nevertheless, although direct radionecrosis of the salivary glands and the taste buds might explain the chemosensory problems after radiotherapy, the gustatory complaints seen after chemotherapy remained unexplained.

Consequences of taste loss in aging

At this point, it may be suggested the close connection between taste function and total health. The main danger of taste decline and disturbance in the old is food-anhedonia (inability to experience pleasure), causing loss of body weight via decreased calorie and nutrient intake. Therefore, disturbed taste sensation should be considered whenever unexplained nutritional decline is present. Any change in nutrient intake can lead to malnutrition with its potentially serious consequences.
Malnutrition can be defined as the state of being poorly nourished. It may be caused by the lack of one or more nutrients (under-nutrition), or an excess of nutrients (over-nutrition). Many studies have found a direct relation between the degree of malnutrition and increased length of stay, treatment costs, return to usual life, and re-admission to hospital rates. On the other hand, the increase in taste threshold with age leads older people to prefer food with strong tastes such as very salty or very sweet foods. Increased consumption of salt or sugar is not desirable because excessive salt consumption is related to hypertension, a common condition in aging, and excessive sugar consumption may result in excessive calories (over-nutrition), promoting undesirable weight gain and diabetes.

Treatment

The treatment and prevention of malnutrition which can be a consequence of taste loss, is an important challenge for the health care system. Because many factors contribute to taste disorder in aging, successful treatment depends on accurate assessment of the cause of the disorder. As serum data show decreased levels of zinc or ferrum, it should be recommended to administrate these microelements, because zinc and/or ferrum is known to be effective in treating taste disorder (24).
It has been suggested that the use of food enhancers or flavors would be appropriate in improving dietary intake. Herbs, spices, and flavoring can be added to food during cooking or directly at the table. Making food colorful adds interest and makes it more attractive to eat (25). Umami (glutamate) might be an effective remedy for hypogeusia based on the improvement of hyposalivation without side effects (24).Therefore, food enhancers using chemical substitutes with no adverse health consequences, or use of flavors that are primarily odors, would be appropriate in improving dietary intake, particularly among those at risk for malnutrition, such as elderly residents of old age homes or inpatients in hospitals.
Lastly the observation that older adults eat more and with more pleasure food with a varied meal, is important.
Despite the importance of this topic, a few studies have been conducted that show that improving the flavor of the foods can improve nutritional intake and increase body weight in hospital and nursing home patients, as well as the healthy older people (13).

Conclusion

Taste loss might not be as deadly as other diseases of aging, such as heart disease, stroke, diabetes, but it can profoundly diminish quality of life. A resulting dampened appetite can lead to poor nutrition and unhealthy weight loss. In addition, with the expanding indications of some drugs (ACE inhibitors, dihydropyridine calcium channel blockers), scrupulous drug screening is mandatory, so physicians should ask questions concerning loss of taste early in therapy in order to protect older patients' quality of life.
In summary, older people should continue to experience the same pleasure from food; therefore more robust large scale and longitudinal studies monitoring the impact of ageing on the sensory system, and how this influences the perception of foods and beverages, are needed to keep old people enjoying the spice of life.

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