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TASTE AND SMELL

The sense of taste influences food preferences and food choices. When people describe how food tastes, they are actually talking about food flavor, and not just the basic tastes of sweet, sour, salty and bitter. The range of flavor experiences also includes aroma, texture, and mouth ‘‘feel’’—and, some would say, even the pleasantness of foods.

Much of food flavor is perceived through taste and smell. The four taste qualities are perceived through receptors located on the tongue and elsewhere in the oral cavity. Some scientists count the taste of monosodium glutamate (MSG) as a fifth basic taste quality known as umami. Mouths also detect the texture and temperature of foods, and the chemical burn of chili peppers and other irritants. Other components of flavor (such as the aroma of vanilla or orange) are airborne particles that come directly through the nasal passages, or enter the nose from the back of the mouth through what is known as retronasal transport.

Scientists have always assumed that taste and smell would undergo dramatic declines with age, much as vision and hearing do. However, it now appears that the chemical senses are much more robust in older people than are vision, hearing, and even touch. Generally, age-related deficits in the sense of smell are more dramatic than taste deficits. These losses do not occur suddenly at any given age. Instead, a progressive decline begins by thirty or forty years of age and continues gradually in later life. Because of the very slow progression of sensory losses, an older person may not even be aware that a decline in taste or smell acuity has occurred.

Aging and the olfactory system

Odors are detected through some fifty million olfactory receptor cells. These cells die and are replaced every sixty days. They are located in the mucus membrane located at the top of the nasal cavities, each of which occupies about 1 square inch. Sniffing concentrates the odors, since less than 10 percent of the air entering the nasal cavity reaches the olfactory epithelium. Odor molecules must dissolve into the mucus membrane surrounding the olfactory receptors, which lines a piece of porous bone called the cribiform plate. Olfactory receptors send axons though the cribiform plate to the olfactory bulb, which lies at the base of the brain. Studies of olfactory epithelium indicate that it becomes scarred and abnormal with age. The scar tissue may block the pores in the cribiform plate, thereby preventing the olfactory receptors from sending axons to the olfactory bulb.

Older people need a higher concentration of a given substance in the air to detect a smell than do younger people. This rise in threshold may reduce the ability to react to the presence of harmful chemicals in the environment and thus may pose a danger to older adults. A study comparing the ability to detect ethyl mercaptan, an ingredient in liquid petroleum gas, showed that adults age seventy-four required ten times more of the chemical to detect it than did twenty-year-olds. However, the ability to detect odors varied across older adults, with some performing as well as younger subjects.

Older people find concentrated odors less intense. A 1986 survey conducted by the National Geographic Society involved a scratch-and-sniff test using six different odors. Respondents rated their intensity on a five-point scale. Intensity ratings declined with age, more so for some odors than for others. Studies normally find a broad loss in olfactory ability for a wide range of smells from pleasant to unpleasant. A more comprehensive scratch-and-sniff test using forty different odors, showed that a person’s ability to identify odors by name starts to decline around the age of sixty. Sometimes that may be due to cognitive impairments or memory loss. The ability to detect an odor—without being able to name it—may remain unchanged.

Losing the ability to smell may affect the flavor and the enjoyment of food. One study compared the ability of young and older subjects to detect the herb marjoram in carrot soup. Older adults performed less well than younger subjects, and some were unable to detect marjoram at all. The deficit was related to smell as opposed to taste. When tested using nose clips, young subjects also were unable to detect or identify marjoram in the soup. Middle-aged and older adults were also less able to detect a woody alcohol odor, and sometimes failed to recognize such common odors as baby powder, chocolate, cinnamon, coffee, and mothballs.

Smell and disease

Smell losses with age are most often linked to upper respiratory infections, and may represent accumulated damage over time. Head trauma can cause sudden loss of the ability to smell. This may occur in whiplash-like accidents that cause a tearing of olfactory filaments near the cribiform plate. More rarely, head trauma may damage nerves involved in taste perception.

Alzheimer’s disease is associated with a reduced sense of smell. This may result from a loss of nerve cells in the olfactory bulb that is sometimes observed prior to the onset of clinical dementia. Smell losses may therefore help in the early diagnosis of Alzheimer’s disease. Patients with Parkinson’s disease are also less able to perceive and identify odors.

Reduced odor perception through the back of the mouth has been observed in older subjects, even though odor perception through the nose is normal. This may be due to dentures that cover the roof of the mouth. Dentures may block the retronasal transport of odors from the mouth to the olfactory receptors, producing a decrease in the perceived flavor of food.

Aging and taste

Older people lose the ability to detect very low concentrations of bitter and salty substances. In contrast, the perception of sweet and sour is robust even in extreme old age. Not all bitter compounds are affected equally, however.

Age-related taste deficits are most pronounced when testing is localized to specific areas on the tongue. Instead of whole mouth tasting, the stimulus is applied locally to spots on the tongue by using a special apparatus or by applying the tastant with a cotton swab. Scientists believe that whole mouth perception may compensate for some of the regional losses of taste function with age.

Regional losses in taste might be expected, given the anatomy of the taste system. Tastes are detected in the mouth by specialized receptor cells located in the upper part of taste buds and near the taste pore. Taste buds are located not only on the tongue but also in the throat and on the roof of the mouth (soft palate). Taste buds are made up of thirty to fifty individual cells, which are organized into an oblong sphere, much like the segments of an orange. The entire taste bud is regenerated every two weeks. Individual taste cells live for only ten to twelve days, and new cells below them evolve to replace them as they die.

Taste buds in the front of the tongue are located in tiny, mushroom-shaped pieces of tissue known as fungiform papillae. These appear as small, round, pink bumps on the tongue surface. In humans each fungiform papilla contains a number of taste buds, each one opening to the outside through a tiny taste pore. Some people may have twenty-five or more taste buds per papilla. There are also regional differences in taste bud density. Fungiform papillae on the tip of the tongue have more taste buds than do those in the middle region of the tongue. A branch of the facial nerve innervates the fungiform papillae, and carries information about taste on the front of the tongue to the brain.

Papillae in the rear of the tongue, known as circumvallate papillae, are arranged in an inverted V shape and are level with the surface of the tongue. Information about taste in the rear of the tongue is carried to the brain by the glossopharyngeal nerve.

More taste buds are located in tissue folds on the sides of the tongue, just in front of the circumvallate papillae. These foliate papillae are mostly innervated by the glossopharyngeal nerve, but some appear to be innervated by the same nerve as papillae at the front of the tongue. Taste buds in the soft palate are innervated by a branch of the facial nerve, and taste buds in the throat are innervated by the glossopharyngeal nerve and the vagus nerve.

Because taste buds are found all over the oral cavity, the perceived taste of food appears to come from the entire mouth and not from isolated patches on the tongue, throat, and roof of the mouth. It is the sense of touch that serves to localize taste perception, such that the taste is perceived to be coming from the area that is stimulated by touch. Smelling chocolate odor while chewing on tasteless gum is interpreted by the brain as eating chocolate.

Taste and disease

It has always been assumed that the number of taste buds declines with age but that does not appear to be the case. There is no evidence that taste cells are no longer regenerated or that the structure of taste buds changes in healthy older adults. However, both disease and medication use may affect this process. For example, drugs that interfere with cell division or growth, such as cancer chemotherapy agents, can disrupt both taste and smell.

Some, if not all, of the observed taste losses with age may be caused by a cumulative history of disease or by the chronic use of medications. Nerve damage during the extraction of wisdom teeth has been found to reduce taste pore density (and presumably taste bud density) in the front of the tongue. Surgery to repair the nerve increases taste pore density and partially restores taste function.

Ear infections can also cause nerve damage. A history of middle ear infections has been associated with taste abnormalities. Damage from bacterial or viral infection may result in an enhanced perception of bitter taste or in taste phantoms. Taste phantoms are a taste sensation in the absence of a stimulus. Scientists believe that taste nerves mutually inhibit each other. A lessening of nerve input from one area of the tongue may cause another area to take over and thus maintain the level of taste sensation. This may explain why older people often do not perceive a loss in taste, even when damage to taste nerves is readily apparent to a medical professional.

The burning mouth syndrome

Some age-related taste abnormalities lead to clinical complaints. Patients with burning mouth syndrome report a persistent burning sensation, usually localized to the front of the tongue, as well as distorted and persistent bad tastes. This condition occurs most commonly in postmenopausal women. Patients with burning mouth syndrome are less sensitive to threshold concentrations of sweet and sour, and tend to rate high concentrations of sugar and salt as less intense than do healthy controls. Burning mouth syndrome is one of the very few conditions that affect taste response to sweet. Both taste and pain pathways may contribute to burning mouth syndrome. One suggested cause has been dry mouth associated with hormonal changes at menopause.

Taste perception is dramatically influenced by salivary function. Taste cells require the presence of gustin, a compound in saliva, in order to develop normally. One component of gustin is zinc. Therefore low dietary levels of zinc, disruption of salivation, or drugs that bind zinc and prevent its use by the body may contribute to taste loss. Patients with disruption in zinc metabolism often experience loss of both taste and smell. Circumvallate papillae of such patients show a disruption of taste buds and taste pores, with obvious cell death. Both taste and smell have been restored in some patients by oral zinc supplementation.

Taste and medication use

Medications may cause a pronounced taste loss or taste distortion. Many drugs prescribed for older adults, such as angiotensin converting enzyme (ACE) inhibitors, used in the treatment of high blood pressure, can bind to zinc. An unpleasant taste in the mouth is one of the main factors limiting use of the ACE-inhibitor captopril. Penicillamine, sometimes used to treat rheumatoid arthritis, also binds metals and interferes with taste perception. Diuretics, which reduce blood pressure by increasing urine output, have occasionally been associated with a loss of taste. Among the known side effects of antiglaucoma agents are changes in taste. Nicotine patches may interfere with taste perception, perhaps because they reduce salivary flow. Radiation treatment for oral cancer frequently directly damages salivary glands and thus can reduce taste function.

Diet and nutrition

There is no dramatic decline in taste or smell function in healthy aging. Rather, a subtle and gradual decline in sensitivity begins around middle age and continues in later life. Smell losses hinder the ability to identify familiar odors and reduce the perceived intensity of flavors in food. Taste losses, if present, are often localized and whole mouth taste sensation is often not affected.

The impact of such losses on diet, nutrition, and health has not always been clear. Some researchers have argued that irreversible changes in taste and smell lead directly to altered food preferences, reduced food consumption, and poor nutrition. However, very few studies have controlled for the subjects’ health status. As noted above, sensory deficits may result not from advanced age but from ill health. Moreover, very few studies have examined taste or smell function, dietary intakes, and nutritional status in the same persons.

For example, it is unclear whether reduced perception of saltiness leads to increased salt intakes among older adults. One study found that older subjects needed twice as much salt to detect its presence in tomato juice as did young subjects. More than half of middle-aged subjects and older subjects (age sixty-seven to eighty-nine) failed to detect the presence of salt in soup. However, there is no clear relationship between the detection of saltiness and preference for salt. Some studies on salt taste preferences reported that older subjects preferred saltier mashed potatoes and chicken broth than did young people, while others found no age-related increase in preference for salt in soup or in tomato juice. These studies did not speak to the key question of whether changes in salt taste perception affect intake of saltier foods on a regular basis.

Findings that older subjects sometimes prefer higher concentrations of sugar and salt in both water and other beverages were interpreted to mean that they would select sweeter and saltier foods. However, food consumption was not actually measured. Very few studies measured salt taste perception, salt taste preference and actual salt consumption among the same respondents. In one such study, young and older adults did not differ in their saltiness intensity ratings for chicken broth. Moreover, older adults generally preferred lower salt concentrations in chicken broth. The hedonic response to salt in soup was not related to daily sodium intakes as assessed by fifteen days of diet records. Sodium intakes (expressed as mg per 1,000 kcal) did not increase with age.

The ability to perceive bitter taste also declines with age. Age was also the strongest predictor of food preferences. Older women expressed increased liking (or reduced dislike) for cruciferous vegetables and bitter salad greens. The reduced response to bitter may increase the acceptance of some bitter foods by women.

The age-associated decline in the sense of smell is also thought to have nutritional consequences. Some scientists believe that olfactory deficits reduce the pleasantness of foods and are the direct cause of reduced food intake and malnutrition in the elderly. A study of smell in eighty older women (sixty-five to ninety-three years of age) showed that half had severe olfactory dysfunction. The dysfunction did not affect appetite, and was unrelated to total energy intake, body weight, or the body mass index (BMI). Body mass index, weight divided by the square of height (1kg/m2), is a measure of body mass. However, women with smell losses reported a lower interest in food-related activities (e.g., enjoying cooking, eating a wide variety of foods); lower preference for foods with sour/bitter taste, such as citrus fruits; higher intake of sweets; and higher intake of fats. Smell losses may lead people to select foods that are sweet or rich in fat, such that the taste and texture will contribute to sensory appeal.

Amplifying foods with noncaloric flavors is thought to be a promising approach in promoting energy intake by older persons and reversing age-related anorexia. In one such study, 75 percent of elderly subjects preferred foods that had been amplified or enhanced with added flavor. In some cases, carrots were amplified with carrot flavor; in other cases, peas were enhanced with bacon flavor or cauliflower with a cheese flavor. The results showed that flavor enhancement, as opposed to a simple flavor amplification, was the more effective method. The most effective enhancers of food intake among older persons were synthetic meat flavors: bacon, roast beef, and ham.

Data from nutritional surveys suggest that dietary variety, defined as the number of different foods consumed each day, often declines with age. This effect was not observed among healthy older people of means, but is reported to be common among institutionalized persons. Researchers believe that the mechanism of sensory-specific satiety encourages the consumption of a varied diet. Sensory specific satiety reduces preference for the just-consumed foods and foods much like them. In contrast, preferences for new foods are maintained at higher levels. In laboratory studies, sensory-specific satiety diminished with age and was lowest in persons over sixty-five. Though the reason for this reduction was unclear, it was unlikely to be caused by any deficits in sensory function.

Consumer studies show that food choices are very largely determined by how foods taste. However, economic factors and health concerns also play a role. Demographic and psychosocial factors have a major impact on the quality of the diet of older adults. When it comes to food choices, deficits in taste and smell, if present, can be compensated for by prior learning and experience.

ADAM DREWNOWSKI SUSAN E. COLDWELL BARBARA J. ROLLS

See also HEARING; MALNUTRITION; VISION.

BIBLIOGRAPHY

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