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The Journals of Gerontology Series A: Biological Sciences and Medical Sciences 60:109-113 (2005)
© 2005 The Gerontological Society of America

Influences of Aging on Taste Perception and Oral Somatic Sensation

Akiko Fukunaga1, Hiroshi Uematsu1 and Kumiko Sugimoto2,

1 Section of Gerodontology, Division of Gerontology and Gerodontology, Graduate School
2 Section of Fundamental Oral Health Care Science, School of Oral Health Care Sciences, Faculty of Dentistry, Tokyo Medical and Dental University, Japan.

Address correspondence to Kumiko Sugimoto, Section of Fundamental Oral Health Care Science, Department of Fundamental Oral Health Care Science, School of Oral Health Care Sciences, Faculty of Dentistry, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan. E-mail: k-sugimoto.fohc{at}tmd.ac.jp


   Abstract
Top
 Abstract
Methods
 Results
 Discussion
 References
 
Background. Many elderly persons report reduced taste perception of the foods they eat. Any disturbance of taste and oral somatic sensations can induce this phenomenon. To determine the cause of decreased taste perception in older persons, the authors investigated age-related changes in taste perception and somatic sensations in the anterior tongue.

Methods. Thirty healthy young and elderly persons participated in this study. Recognition thresholds of four basic tastes (sweet, salty, sour, and bitter) and the threshold of irritating sensation induced by capsaicin were measured by local stimulation using filter-paper disks. Two-point discrimination thresholds were measured to evaluate the spatial acuity of touch sensation. All measurements were carried out at the tip of the dorsal tongue.

Results. The recognition thresholds of all four basic tastes of elderly participants were significantly higher than those of young participants. On the contrary, age-associated changes were not observed in 2-point discrimination and capsaicin sensation.

Conclusions. Significant age-associated deterioration was observed in taste but not somatic sensations such as touch and burning pain in the tongue, showing that aging affects taste perception and oral somatic sensations differently. This suggests that decreased taste perception of foods in elderly people may be caused primarily by perceptual loss of taste among oral sensations.

GOOD meals are essential for the maintenance of physical and mental health. Intact oral sensation is especially important for elderly persons, because disturbances of taste perception may cause decreased appetite, resulting in emotional problems and considerable weight loss. However, decreased taste perception with aging has been reported (1–3). Because perception of foods relies on information transmitted from oral somatosensory receptors and taste and olfactory receptors, age-related decreases in any of these sensations can cause decreased taste perception of foods.

Several studies of age-associated changes in taste sensitivities have shown that taste sensitivity decreases with age (4–8), although the extent of loss varied depending on taste qualities. Most of these studies compared the detection thresholds among participants of various ages using a whole-mouth method that stimulates the entire oral cavity. When the whole-mouth method is used in elderly persons, taste thresholds might be influenced by oral conditions that vary widely depending on whether the participant has residual teeth, wears dentures, or has decreased or increased saliva secretion. In addition, decreased taste sensitivity of one oral region might be masked by higher sensitivity of other regions.

Therefore, in the current study, we used a local stimulation method with filter-paper disks that enabled us to maintain fixed amounts and concentrations of taste solutions and stimulated areas. In addition, we focused on recognition thresholds, because recognition thresholds, rather than detection thresholds, better reflect the actual feeling of foods. Some previous studies included elderly persons who had chronic diseases and were taking medications that could cause taste disorders. To study changes in taste sensitivities solely by physiologic aging, we selected healthy persons according to their anamneses.

Touch sensation of the tongue plays an important role in perception of food texture. To evaluate the spatial acuity of touch sensation, many studies have measured 2-point discrimination thresholds, the least 2-point separation at which the participant feels 2 points (9–15). In studies of the influence of aging on 2-point discrimination, decreased acuities with age have been reported in the hand and the forearm (10,12). In the orofacial regions, age-related deterioration was reported in the cheeks and lips but not in the anterior tongue and palate (13). On the contrary, Stevens and Choo (16) investigated the influence of age on the 2-point gap recognition threshold and found decreased spatial acuity in the tongue. These studies changed the distance between 2 points with relatively large steps (0.5–1.0 mm), although several other studies (9,11,13,14) have reported that the anterior tongue has the best spatial acuity (e.g., {approx}2 mm of the 2-point discrimination threshold) among various body regions including the hands and legs. Therefore, more precise measurements are needed to evaluate the effect of aging on spatial acuity in the anterior tongue. We used 0.2-mm steps to measure the 2-point discrimination threshold in our current investigation.

Pungent spices are often added to foods to stimulate the appetite, which is useful especially for elderly persons with decreased appetite. Capsaicin, the pungent ingredient in hot chili peppers, is a popular spice known to activate a class of nociceptors that produce a sensation of burning pain. Recently, cloned capsaicin receptor (vanilloid receptor 1) (17) was found to be expressed in the tongue and various body tissues (18). The thresholds of burning pain induced by capsaicin have been measured by whole-mouth stimulation in young persons (19), but the influence of age on these thresholds has not been investigated.

In the current study, to elucidate the influence of aging on oral sensations and the cause of decreased taste perception, we evaluated the thresholds of taste, 2-point discrimination, and capsaicin in the tongue using the previously noted methods in young and old persons.


   METHODS
Top
 Abstract
 Methods
Results
 Discussion
 References
 
Participants
All participants understood the purpose of the study and gave their informed consent before being examined. The Ethical Committee for Human Research of Tokyo Medical and Dental University approved the study.

We screened the participants using a questionnaire about their physical condition, medical history, and habits. We excluded any potential participants if they reported any problem with taste or touch sensation, any disease or medication known to influence these sensations, and history of heavy smoking or alcohol consumption. Consequently, 30 young persons (ages 18–29 years; mean age ± SD, 24.1 ± 3.1; 14 men and 16 women) and 30 elderly persons (ages 65 to 85 years; mean age ± SD, 75.2 ± 4.1; 12 men and 18 women) participated. There were no significant differences in composition of sex between the 2 groups (chi-squared test, p >.1) or in ages between the men and women in each group (Student's t test, p >.1). All participants were asked to refrain from eating spicy foods within 24 hours and any foods for at least 2 hours before the test.

Measurement of Sensitivity
We measured the acuity of oral sensations at the dorsal tip of the tongue, because this region has a high density of taste buds (20) and high sensitivities to taste (21) and tactile stimuli (9).

Taste threshold.-- We prepared the series of taste solutions representing four basic tastes (sweet, salty, sour, and bitter) using sucrose, sodium chloride, tartaric acid, and quinine hydrochloride, respectively. We numbered the solutions of four basic tastes by their concentrations, with the lowest designated 1 (Table 1). For local application of taste stimuli, we prepared filter-paper disks (Toyo Roshi Ltd, Tokyo, Japan) with a diameter of 6 mm.


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  		Table 1. Concentrations of Four Basic Taste and Capsaicin Solutions.

 
At the beginning and the end of measurement of each taste quality, participants rinsed out their mouths with distilled water. A filter-paper disk soaked in a given taste solution was placed on the tip of the tongue for 3 seconds. Immediately afterward the experimenter removed the disk and washed the participant's tongue with distilled water using a plastic syringe. Taste solutions were applied in ascending order. The lowest concentration for which the participant reported the existence of stimuli (i.e., compared with distilled water) was taken as the detection threshold, and the lower concentration of 2 consecutive solutions that the participant correctly recognized the taste quality was considered the recognition threshold. Statistical analyses were conducted using one-way analysis of variance, and probability values of.05 or less were considered significant.

Two-point discrimination threshold.-- To measure 2-point discrimination thresholds, we made stimulus probes (2.5 g) that had 2 stimulus points using hard nylon thread with a diameter of 285 µm (Unitika Ltd, Osaka, Japan). The cut end of the nylon thread was flattened by heating and formed a disk-shaped stimulus point with a diameter of 400 µm. The interval of 2 stimulus points ranged from 0.6 mm to 1.8 mm, with 0.2 mm increments.

Two points of a probe were simultaneously contacted with the surface of the tip of the participant's tongue for 2 seconds in either the anteroposterior or mediolateral direction, and the participant indicated whether he or she felt the stimulation to be 1 point or 2 points. The space between the probe points was increased until the participant felt 2 points, and the minimum recognition interval was considered the 2-point discrimination threshold. The participants practiced these procedures at the tip of their index finger before actual testing. We used the Student t test to analyze the results, and probability values of.05 or less were considered significant.

Capsaicin threshold.-- To make a capsaicin stock solution of 0.3 mmol/L, 91 mg capsaicin (97%; Sigma Chemical, St. Louis, MO) was first dissolved in 0.5 ml ethanol and then made up to 100 ml with distilled water. The stock solution was later diluted with distilled water to yield seven concentration steps, which were numbered by their concentrations as shown in Table 1.

The thresholds of capsaicin were investigated according to the same method used to measure taste threshold. The disk soaked in capsaicin solution was held on the tip of the tongue for 10 seconds. Because capsaicin sensation is known to sensitize and desensitize even at a low concentration (22,23), we changed the location of a stimulation disk next to the previous stimulation spot within the tongue tip every time. The lowest concentration at which the participant could feel an irritating sensation before burning pain was regarded as the threshold. The method of data analysis was the same used for taste threshold.


   RESULTS
Top
 Abstract
 Methods
 Results
Discussion
 References
 
Taste
Figure 1 shows the cumulative curves of the recognition threshold for four basic tastes. The onset and the completion of the curves correspond to the lowest threshold and the highest threshold, respectively, and the steepness of slopes inversely relates to the range width of thresholds of each group. The curves for sucrose, sodium chloride, and tartaric acid had similar patterns; that is, the curves of elderly participants shifted to the right, indicating that both onset and completion levels were elevated in the elderly participants compared with the young participants. As seen in Figure 1D, the curves for quinine hydrochloride displayed a unique pattern in which the slope of the curve of the elderly participants was flatter than that of the young participants, showing that the thresholds of the elderly participants varied widely.



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  		Figure 1. The cumulative recognition threshold curves of sucrose, sodium chloride, tartaric acid, and quinine hydrochloride representing sweet, salty, sour, and bitter tastes, respectively, in young and elderly persons

 
Figure 2 shows the mean recognition thresholds of four basic tastes of young and elderly participants. When we converted solution step into absolute concentration, the mean recognition thresholds in the young and the elderly participants were 0.044 mol/L and 0.11 mol/L for sucrose, 0.045 mol/L and 0.17 mol/L for sodium chloride, 4.7 mmol/L and 12 mmol/L for tartaric acid, and 0.082 mmol/L and 1 mmol/L for quinine hydrochloride, respectively. We found significant increases in recognition thresholds with age (with all Fs [1, 58]) for sucrose (F = 19.04, p <.0001), sodium chloride (F = 27.73, p <.0001), tartaric acid (F = 13.22, p =.0006), and quinine hydrochloride (F = 32.01, p <.0001). Like the recognition thresholds, the detection thresholds for all basic tastes in the elderly participants were significantly higher than those in the young participants (p <.001 for sucrose and tartaric acid, and p <.0001 for sodium chloride and quinine hydrochloride). The mean detection thresholds in young and elderly participants were 0.017 mol/L and 0.045 mol/L for sucrose, 0.015 mol/L and 0.058 mol/L for sodium chloride, 1.6 mmol/L and 4.8 mmol/L for tartaric acid, and 0.017 mol/L and 0.3 mmol/L for quinine hydrochloride, respectively. We found no significant difference in the detection and recognition thresholds of all four tastes between the male and female participants in each age group (p >.1).



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  		Figure 2. The mean recognition thresholds of four basic tastes in young and elderly persons. The recognition thresholds for all four basic tastes of elderly persons were significantly higher than those of young persons (sucrose, sodium chloride, and quinine hydrochloride, p <.0001; tartaric acid, p =.0006)

 
Touch
Figure 3, A and B, shows the cumulative threshold curves and the mean threshold of 2-point discrimination in the anteroposterior and mediolateral directions. We observed no significant differences in 2-point discrimination thresholds between young and elderly participants in both directions (p >.1) and between two directions in both age groups (p >.1). We found no significant difference in the thresholds by sex in each age group (p >.1).



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  		Figure 3. (A) The cumulative threshold curves of 2-point discrimination in anteroposterior (AP) and mediolateral (MD) directions in two age groups. (B) The mean thresholds of 2-point discrimination in 2 groups. No statistically significant differences were observed between two age groups or two directions (p >.1). (C) The cumulative recognition threshold curves of burning-pain induced by capsaicin in two groups. (D) The mean threshold of capsaicin in two groups. No significant differences were observed between two age groups (p >.1)

 
Capsaicin
Figure 3C and 3D shows the cumulative threshold curves and the mean threshold of capsaicin. The mean recognition threshold for capsaicin of the elderly participants was not significantly different from that of young participants (p >.1). The mean threshold concentrations of capsaicin in the young and the elderly participants were 2.3 µmol/L and 2.1 µmol/L, respectively. We found no significant difference in capsaicin thresholds between male and female participants in each age group (p >.1).


   DISCUSSION
Top
 Abstract
 Methods
 Results
 Discussion
References
 
Several studies that have used the whole-mouth method consistently documented increased taste detection thresholds with age (4–8). In the current study, we used a local stimulation method to investigate the influence of aging on taste thresholds and found more remarkable elevation of the recognition thresholds and the detection thresholds for all four basic tastes with aging compared with the previous whole-mouth measurements. Although taste recognition involves cognitive processes such as language and memory, which are known to be impaired in elderly persons, impaired taste sensation may be a more likely cause than impaired cognition for increases in recognition thresholds in elderly people, because we also observed marked elevations of detection thresholds in the current experiment. As Mojet and colleagues (8) have reported, elderly persons show a large variability of thresholds, and they also found flatter cumulative threshold curves for bitterness in elderly persons. This feature may result from the presence of some elderly persons having extremely high thresholds for bitterness.

Taste receptor cells are packed in taste buds, which are specialized structures of tongue epithelium, and their apical processes are exposed to external oral circumstances. Decreasing taste sensitivity with age was once thought to result from a decrease in the number of taste buds (24,25), whereas more recent studies have found no significant decrease in the number with age (26,27). Other research has shown that taste receptor cells undergo continual turnover, with a life span of approximately 10 days in young animals (28), which may serve to maintain taste sensitivities. Thus, one possible mechanism of age-associated decline is the time lag of this turnover, resulting in deterioration of taste cell responses, as suggested by ultrastructural changes of taste bud cells in aging mice (29).

In the current study, we observed no age-related changes in 2-point discrimination thresholds at the tip of the tongue, a finding that corresponds with the report by Calhoun and colleagues (13), although they used rough scaling. Conversely, age-associated decreases in acuity of 2-point discrimination have been reported in the hands, forearms (10,12), and cheeks and lips (13). Two-point discrimination acuity depends on the density of mechanoreceptor units, because 2 touched points are felt as distinct points when each of 2 stimuli activates a distinct mechanoreceptor unit. As the morphologic basis of age-associated decrease in touch sensitivity, age-dependent decreases in the densities of Meissner corpuscles have been reported in human fingers and toes (30) and in mouse digital pads (31). In oral regions, encapsulated sensory corpuscles such as Krause's end-bulbs have been found in monkey fungiform papillae (32), but the influence of aging on these receptors has not been evaluated. This issue needs further histologic and physiologic investigation.

Capsaicin induces a persistent burning pain sensation. Sizer and Harris (19) evaluated the capsaicin threshold using the whole-mouth method in which participants swirled 10 ml capsaicin solution in their mouths for a maximum of 10 seconds. The authors reported that the concentration necessary for recognition of capsaicin-induced sensation in their young participants ranged from 0.295 µmol/L to 1.15 µmol/L. The mean threshold of irritating sensation induced by capsaicin obtained by topical stimulation in the current study was 2.3 µmol/L in the young participants, which is slightly greater than the previous value. This discrepancy may be due to spatial summation, a general property of sensation.

Thermal pain thresholds of the skin have been observed to increase in elderly people (33–36), with few exceptions (37,38). However, we found no age-related changes in thresholds of capsaicin sensation in the current study, indicating that the peripheral pain system responsive to capsaicin may be little affected by aging. Together with the results of 2-point discrimination, we found that the somatic sensations of the tongue were well retained in the elderly participants, unlike those of the skin.

Conclusion
We observed a significant decrease with aging in taste perception but not touch spatial sensation and irritating (burning pain) sensation, showing that aging affects taste perception and oral somatic sensations in different ways. In addition, smell also plays an important role in perception of food flavor. Because olfactory function decreases with aging (39,40), perceptual loss of both taste and smell may be involved in decreased perception of foods.


   Acknowledgments
 
Supported in part by Health Sciences Research grant H13-21EBM-018) from the Ministry of Health, Labour and Welfare, Tokyo, Japan.


   Footnotes
 
Decision Editor: John E. Morley, MB, BCh

Received May 22, 2003

Accepted September 19, 2003


   References
Top
 Abstract
 Methods
 Results
 Discussion
 References
 

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