Taste and Smell

Animals in the wild depend more than humans on the chemical senses of gustation (taste) and olfaction (smell), and have more acute tasting and smelling abilities. Vision and hearing are the major human senses, but taste and smell also provide important information on the environment and make our lives safer and more enjoyable.

The receptors for taste are located in the taste buds, which are concentrated on the tip, sides, and back of the tongue. Of the four taste modalities— sweet, bitter, sour, and salty—the front of the tongue is more sensitive to sweet, the back to bitter, the sides to sour, and the front and sides to salty. The receptors for smell are located in the olfactory epithelium at the top of the nasal passageway near the base of the brain. Substances must be in solution to be tasted, and can be tasted more effectively when they are swished around in the mouth. In order to be smelled, however, a substance must follow an air route; the olfactory experience can be hastened and enhanced by sniffing the aromatic substance.

Everyday observations and folklore point to declines in the acuity of taste and smell with age. For example, the saying "too old to cut the mustard" originated in a time when mustard was ground at home and "cut" by adding vinegar to it. Because older adults had a tendency to put too much vinegar into the mixture, an older person who performed any task ineffectively came to be referred to as "too old to cut the mustard." Another source of reported age decrements in taste sensitivity are complaints from older adults that "everything tastes flat" or is "boring." To compensate for the flat taste, they tend to use greater amounts of salt, pepper, and other seasonings in their foods. Food complaints among older adults may also be the results of attitude, personal problems, and feelings of isolation or abandonment.

With respect to changes in the sense of smell, the observation that older people have a greater tolerance for foul odors has caused them to be the objects of numerous jokes and labels. Less humorous is the research finding that they are less adept at identifying the odor of mercaptan, a substance placed in natural gas to make it more detectable. The fact that older adults can sense the odor of roses as well as younger adults has prompted the suggestion that it might be better to replace the mercaptan in natural gas with rose extract.

What can be concluded from research on age-related declines in taste and smell sensitivity? Because taste and smell interact, it is difficult to separate the effects of one sense from the other. In addition, both senses are affected by experience, disease, psychological factors, and sociocultural factors, which must be controlled to determine whether they decline with age. In general, the number of taste buds and smell receptors is somewhat less in older than in younger adults (Miller, 1988; Spitzer, 1988). However, the loss of receptors is not closely correlated with sensitivity. Although there is some age-related decrement in taste sensitivity, compared with sensitivity to odors it remains fairly stable (Bartoshuk & Weiffenbach, 1990; Weiffenbach, Cowart, & Baum, 1986). Sensitivity to sweetness and saltiness is fairly stable across age groups, but there is some decline in sensitivity to bitterness (Bartoshuk & Weiffen-bach, 1990; Spitzer, 1988).

Aging appears to have a greater effect on smell than on taste, although at least some of the decline appears to be caused by a lifetime of living in a particular odor environment. In any event, the finding that the ability to detect various odors is poorer in older than in younger adults has received wide research support (Murphy, 1986; Stevens & Cain, 1985, 1986, 1987). Differences between children and adults in odor preferences have also been found; children prefer fruity odors such as strawberry, and adults prefer flowery smells such as lavender (Engen, 1977).

Even when age-related changes in sensitivity to flavors and odors are observed, it is possible that they are due to disease or other variables associated with aging. For example, the ability to identify odors declines in Alzheimer's disease, and the ability to both detect and identify odors declines in Parkinson's disease (Doty, Deems, & Stellar, 1988; Koss, Weiffenbach, Haxby, & Friedland, 1988). The sense of smell can also be damaged by viral and bacterial infections and head trauma (Bartoshuk & Weiffenbach, 1990).

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