di EMINE MERAL INELMEN
Dipartimento di Medicina- DIMED
Cattedra di Geriatria-Direttore: Prof. Enzo Manzato
Università degli Studi di Padova
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
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
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
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
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).
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
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).
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
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
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.
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).
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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1: Causes of taste disorders in the elderly. Source
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IN THE ORAL CAVITY
↓ thickness, dryness
glands: ↓ acini, ↑ fibrous adipose
↓ density of taste buds
periodontal diseases, candidiasis, stomatitis, dental-alveolar
infections, xerostomia, tumors, mechanical trauma
stroke, mild cognitive impairment, Alzheimer’s disease,
Parkinson’s disease, major depression
diabetes mellitus types 1 and 2, hypothyroidism,
of the lung, breast, head and neck, esophagus,
chronic renal failure
acute and chronic liver diseases, cirrhosis
tract: irritable bowel syndrome, gastroesophageal
reflux disease, Crohn’s disease
and viral diseases: chronic obstructive pulmonary
disease, post-influenza conditions
middle ear surgery, tonsillectomy
alcohol consumption, poor hygiene of the oral
cavity and dental prosthetic devices
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