5. DETERMINE (13–15)
5.2 TASTE
The sense of taste is an oropharyngeal chemical sense that plays a critical role in food selection and food safety. Taste signals, along with odor signals, trigger cephalic phase secretions (e.g., salivary, gastric, pancreatic) that prepare for the digestion of food before it reaches the stomach. In addition, learned association of taste sensations with the metabolic consequences of food enables meal size and food choices to be modulated in anticipation of nutritional needs. Taste sensations are initiated when chemical stimuli interact with receptors and ion channels located on taste cells that are clustered into buds on the tongue surface and other discrete areas of the oral cavity(2,5). Taste signals from taste buds are carried by the seventh, ninth, and tenth cranial nerves to the nucleus of the solitary tract (NST) in the medulla of the brainstem that projects to the ventroposteromedial nucleus of the thalamus and finally the insular-opercular cortex. The NST receives information not only from the taste system but also from visceral sensory fibers that originate in the esophagus, stomach, intestines, and liver. Information from the olfactory nerve (cranial nerve I) that transmits information about smell also converges in the NST. This convergence of neural input in the NST enables taste and odor signals to impact ingestive and digestive activity by producing gastric and pancreatic secretions. The qualitative range of taste includes sweet, sour, salty, and bitter as well as other less familiar sensations that are also carried by taste nerves including umami (the taste of gluta- mate salts, brothy, savory, or meat-like), fatty, metallic, starchy/polysaccharide, chalky, and astringent(9).
The sense of taste gradually declines with aging, with differential losses depend- ing on the chemical structure of individual taste stimuli. The decrements in taste sensitivity that occur during normal aging are exacerbated by certain disease states, pharmacologic and surgical interventions, radiation, and environmental exposure
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(2,5,7). The cause of taste losses in normal aging independent of disease or medications is not known; some researchers have found losses in the number of taste buds in older individuals while others have not. Whatever the cause, taste losses reduce the motiva- tion to eat, interfere with the ability to modulate appetite and food choices, impair quality of life, and can lead to inadequate nutritional status especially in the sick or malnourished older persons. When taste and smell losses no longer play a major role in initiating, sustaining, and terminating ingestion, the quantity of food that is eaten and the size of meals can be affected. Cephalic phase responses including salivary, gastric, pancreatic, and intestinal secretions can be blunted which can affect digestion of food and absorption of nutrients.
5.2.1 Taste Losses at Threshold Levels
Older adults have losses in the ability to detect and recognize all taste qualities as well as other oral stimuli(4,8,9). The detection thresholds (DT) for tastes are elevated in older persons, and hence they require the presence of more molecules (or ions) for a sensation to be perceived compared to a younger cohort. The recognition thresholds (RT) are also elevated so a greater concentration of a tastant is required to correctly recognize its quality. Table 5.1 compares mean DTs and RTs for older persons with those for the young persons for a broad range of compounds including sodium salts with different anions, bitter compounds, sweeteners, acids, astringent compounds, amino acids including glutamate salts, metallic compounds, fats, gums, and astringent compounds(4,9). The older adult subjects in these studies took an average of 3.4 medications but otherwise led active, normal lives. For detection thresholds (DTs), the ratio of DT (older)/DT (young) revealed that DTs in older persons were higher by the following amounts: 11.6 times higher for sodium salts; 7.0 times higher for bitter compounds; 2.7 times higher for sweeteners; 4.3 times higher for acids; 2.8 for astringent compounds; 2.5 times higher for amino acids; 5.0 times higher for glutamate salts; 3.1 times higher for fats/oils; 3.7 times higher for polysaccharides/gums; and 2.2 times higher for metallic compounds.
For recognition thresholds (RTs), the ratio of RT (older)/RT (young) revealed that RTs in older persons were higher by the following amounts: 5.8 times higher for sodium salts; 7.5 times higher for bitter compounds; 2.1 times higher for sweeteners; 6.8 times higher for acids with sour tastes; 3.0 for astringent com- pounds; 3.0 times higher for polysaccharides/gums; and 2.0 times higher for metallic compounds.
Examination of Table 5.1 reveals that age-related decrements as determined by the ratio DT (older)/DT (young) varied widely over the different compounds tested.
For sodium salts, the losses at the threshold level (i.e., elevated thresholds) were greatest for anions with the largest molar conductivity (Na sulfate, Na tartrate, Na citrate, and Na succinate). Molar conductivity (!) is a measure of the electrical charge carried by the anion per unit time. That is, age-related losses in sensitivity to sodium salts were greatest for anions with the highest charge mobility. For bitter compounds, the greatest losses for older adults at the threshold level were for the least lipophilic compounds, i.e., MgNO3, MgSO4, and KNO3. For sour acids, the greatest loss in sensitivity in older adults was for HCl, the acid with the lowest
Chapter 5/ Sensory Impairment 79
Table5.1 Comparisonoftastedetectionandrecognitionthresholdsforolderandyoungsubjectsforabroadrangeofstimuli SodiumsaltsDetectionthresholdsRecognitionthresholdsfor saltiness Older(O)Young(Y)O/YOlder(O)Young(Y)O/Y MSG(monosodiumglutamate)0.00638M0.00126M5.060.0091M0.00207M4.41 Naacetate0.0190M0.00242M7.840.0229M0.00952M2.41 Naascorbate0.0250M0.00404M6.190.0265M0.00809M3.28 Nacarbonate0.00829M0.00218M3.790.0234M0.00425M5.51 Nachloride0.01850M0.00238M7.760.0227M0.00815M2.79 Nacitrate0.0130M0.000531M24.50.0187M0.00190M9.84 Naphosphatemonobasic0.0160M0.00307M5.210.0253M0.01140M2.22 Nasuccinate0.0138M0.000854M16.20.0167M0.00217M7.71 Nasulfate0.0283M0.000981M28.80.0349M0.00322M10.86 Natartrate0.0159M0.00151M10.50.0277M0.00295M9.39 BittercompoundsDetectionthresholdsRecognitionthresholdsfor bitterness Older(O)Young(Y)O/YOlder(O)Young(Y)O/Y Caffeine1.99mM1.30mM1.536.74mM1.87mM3.60 Denatoniumbenzoate0.0323mM0.0115mM2.810.0387mM0.0123mM3.14 KNO332.7mM1.91mM17.1271mM5.97mM45.4 MgCl25.20mM1.02mM5.1021.8mM20.3mM1.07 MgNO333.3mM1.40mM23.8191mM14.8mM12.9 MgSO46.08mM0.323mM18.814.8mM2.59mM5.71 Naringin0.138mM0.0427mM3.230.195mM0.0561mM3.48 Phenylthiocarbamide1.26mM0.591mM2.131.74mM1.21mM1.44 QuinineHCl8.07mM3.99mM2.0212.3mM4.75mM2.59 (continued)
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Table5.1 (continued) Quininesulfate8.75mM2.04mM4.2912.3mM2.53mM4.86 Sucroseoctaacetate5.32mM3.89mM1.3722.8mM5.30mM4.30 Urea0.116M0.103M1.120.245M0.134M1.83 SweetenersDetectionthresholdsRecognitionthresholdsfor sweetness Older(O)Young(Y)O/YOlder(O)Young(Y)O/Y Acesulfame-K74.7mM44.4mM1.68239mM161mM1.48 Aspartame91.3mM22.4mM4.07124mM44.9mM2.76 Calciumcyclamate0.412mM0.266mM1.551.69mM1.33mM1.27 Fructose10.1mM4.39mM2.3026.4mM16.6mM1.59 Monellin0.0913mM0.0195mM4.670.0676mM Neohesperidin dihydrochalcone4.60mM2.20mM2.095.28mM Rebaudioside13.0mM4.61mM2.8213.6mM Sodiumsaccharin42.4mM14.7mM2.88137mM49.7mM2.76 Stevioside16.0mM5.31mM3.0223.7mM Thaumatin0.133mM0.0716mM1.860.201mM D-tryptophan0.322mM0.109mM2.951.45mM0.546mM2.66 SourcompoundsDetectionthresholdsRecognitionthresholdsfor sourness Older(O)Young(Y)O/YOlder(O)Young(Y)O/Y Aceticacid0.273mM0.106mM2.580.819mM0.294mM2.79 Ascorbicacid0.725mM0.281mM2.582.190mM0.396mM5.53 Citricacid0.375mM0.0498mM7.530.816mM0.131mM6.23 Glutamicacid0.463mM0.0920mM5.031.500mM0.309mM4.85 Hydrochloricacid0.200mM0.0179mM11.170.477mM0.0226mM21.11 (continued)
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Table5.1 (continued) Succinicacid0.188mM0.132mM1.421.330mM0.174mM7.64 Sulfuricacid0.100mM0.0468mM2.140.170mM0.0468mM3.63 Tartaricacid0.163mM0.0864mM1.890.297mM0.131mM2.27 AstringentcompoundsDetectionthresholdsRecognitionthresholdsfor astringency Older(O)Young(Y)O/YOlder(O)Young(Y)O/Y Gallicacid0.780mM0.250mM3.122.07mM1.10mM1.88 Tartaricacid0.220mM0.0549mM4.010.324mM0.0689mM4.70 Tannicacid0.072mM0.0271mM2.660.295mM0.0528mM5.59 Catechin1.48mM1.18mM1.252.500mM1.56mM1.60 Ammoniumalum0.172mM0.0780mM2.210.487mM0.244mM2.00 Potassiumalum0.454mM0.120mM3.781.380mM0.723mM1.91 AminoacidsDetectionthresholds Older(O)Young(Y)O/Y L-alanine19.5mM16.2mM1.20 L-arginine1.12mM1.20mM0.93 L-arginineHCl2.39mM1.23mM1.94 L-asparagine9.33mM1.62mM5.75 L-asparticacid0.501mM0.182mM2.75 L-cysteine0.390mM0.0630mM6.19 L-cysteineHCl20.0mM16.0mM1.25 L-glutamicacid0.100mM0.0630mM1.59 L-glutamine26.9mM9.77mM2.75 L-glycine0.0617M0.0309M2.00 L-histidine6.45mM1.23mM5.24 L-histidineHCl0.389mM0.0794mM4.90 (continued)
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Table5.1 (continued) L-isoleucine12.0mM7.41mM1.62 L-leucine12.9mM6.45mM2.00 L-lysine2.24mM0.708mM3.16 L-lysineHCl2.09mM0.447mM4.68 L-methionine2.63mM3.72mM0.71 L-phenylalanine19.1mM6.61mM2.89 L-proline0.0372M0.0151M2.46 L-serine0.0263M0.0209M1.26 L-threonine0.020M0.0257M0.78 L-tryptophan2.88mM2.29mM1.26 L-valine0.0115M0.00416M2.76 Glutamatesalts(with andwithouttheenhancerIMP1 ) DetectionthresholdsRecognitionthresholds forumami Older(O)Young(Y)O/YOlder(O)Young(Y)O/Y Sodiumglutamate2.83mM0.902mM3.145.24mM2.55mM2.05 Sodiumglutamate with0.1mMIMP10.888mM0.113mM7.861.82mM0.183mM9.94 Sodiumglutamate with1mMIMP10.145mM0.0480mM3.020.328mM0.0964mM3.40 Potassiumglutamate7.69mM0.902mM8.5310.1mM5.13mM1.97 Potassiumglutamate with0.1mMIMP0.549mM0.106mM5.180.205mM0.189mM1.08 Potassiumglutamate with1mMIMP0.0928mM0.0108mM8.590.231mM0.0378mM6.11 Ammonium glutamate4.26mM1.08mM3.948.70mM2.75mM3.16 Ammoniumglutamate with0.1mMIMP0.458mM0.139mM3.290.581mM0.252mM2.30 (continued)
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Table5.1 (continued) Ammoniumglutamate with1mMIMP0.129mM0.0343mM3.760.274mM0.065mM4.22 Calciumdiglutamate1.09mM0.292mM3.731.06mM1.29mM0.82 Calciumdiglutamate with0.1mMIMP0.327mM0.0606mM5.400.409mM0.0848mM4.82 Calciumdiglutamate with1mMIMP0.0692mM0.0190mM3.640.0692mM0.033mM2.09 Magnesium diglutamate1.86mM0.253mM7.353.15mM0.854mM3.69 Magnesium diglutamate with0.1mMIMP
0.289mM0.0421mM6.860.795mM0.0674mM11.8 Magnesium diglutamate with1mMIMP
0.0452mM0.0257mM1.760.109mM0.0524mM2.08 IMP(inosine50- monophosphate)1.99mM0.430mM4.632.12mM1.07mM1.98 Oilsinfourdifferent emulsifiers2 Detectionthresholds Older(O)Young(Y)O/Y MCT2 (inacacia)10.1%2.85%3.54 Soybean(inacacia)12.9%4.02%3.20 Mineral(inacacia)9.77%4.43%2.20 MCT(inEmplex)25.0%3.93%6.37 Soybean(inEmplex)14.9%6.52%2.28 Mineral(inEmplex)20.0%8.85%2.26 MCT(inTween-80)19.3%5.35%3.60 Soybean(inTween-80)17.7%5.85%3.02 Mineral(inTween-80)19.9%5.77%3.44 (continued)
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Table5.1 (continued) MCT(inNacaseinate)13.6%6.18%2.20 Soybean(inNa caseinate)13.0%5.35%2.43 Mineral(inNa caseinate)13.4%4.27%3.13 Polysaccharides/gumsDetectionthresholdsRecognitionthresholdsfor thickness Older(O)Young(Y)O/YOlder(O)Young(Y)O/Y Acaciagum1.02%0.644%1.583.12%1.44%2.17 Guargum0.116%0.057%2.040.42%0.22%1.91 Locustbeangum0.43%0.061%7.050.74%0.22%3.36 Xanthangum0.238%0.0396%6.010.41%0.071%5.77 Algin0.115%0.0605%1.900.26%0.15% 1.73 MetalliccompoundsDetectionthresholdsRecognitionthresholdsformetallic taste FeSO40.343mM0.143mM2.42.14mM1.07mM2.0 FeCl21.663mM0.875mM1.91.76mM0.924mM1.9 ZnSO41.050mM0.456mM2.31.46mM0.695mM2.1 ZnCl20.774mM0.387mM2.00.890mM0.445mM2.0 1 Thresholdswereobtainedforfiveglutamatesalts(sodiumglutamate,potassiumglutamate,ammoniumglutamate,calciumdiglutamate,and magnesiumdiglutamate).Theeffectofinosine5’-monophosphate(IMP),atasteenhancer,attwoconcentrations(0.1mMIMPand1.0mM IMP)ontastethresholdsoftheseglutamatecompoundswasalsoinvestigated.While0.1mMIMPloweredthresholdsforallfivesaltsforyoung butnotoldersubjects,1mMIMPloweredthresholdsinbothyoungandoldergroups. 2 Theoilswererefined,bleached,deodorizedsoybeanoil(LCT);MCT(mediumchaintriglyceride)oil;andlightmineraloil.Thesefatsare long-chaintriglycerides,medium-chaintriglycerides,andamixtureofliquidhydrocarbonsfrompetroleum,respectively.OiL-in-wateremulsions weremadewitheachoilusingoneoffourdifferentemulsifiers:Polysorbate-80(Tween-80),sodiumstearoyllactylate(Emplex),sodiumcaseinate, andacaciagum.
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molecular weight. For amino acids, age-related losses tended to be higher for two amino acids with side chains containing basic groups (L-histidine andL-lysine) and their monohydrochloride derivations.
Table 5.1 reveals two very important points about taste perception. First, the relative differences in loss for individual compounds with age contribute to the distortions of taste (called dysgeusia) experienced by many persons in later life.
Foods are a mixture of many different compounds, and that mixture will taste different to an older person than a younger one because the relative sensory salience of the individual compounds will differ based on age. Second, compounds with high caloric or nutritional value such as sugars, fats, and amino acids tend to have higher detection thresholds than certain noxious bitter compounds that can be detected in minute amounts. A possible explanation for higher concentrations required to detect sugars, amino acids, and fats is that too much taste at low concentrations could inhibit intake of adequate calories.
5.2.2 Suprathreshold Taste Perception
Suprathreshold taste studies that relate perceived intensity to concentration indicate that tastes are less intense for older persons compared with the young.
Like threshold measurements, the degree of loss is not uniform across com- pounds but rather depends on the chemical structure of the tastant(2–9); the lack of uniform loss plays a role in taste distortions (dysgeusia) experienced by older persons for foods comprised of mixtures of numerous compounds. For 23 amino acids, there was loss in perceived intensity with age but the losses forL- aspartic acid andL-glutamic acid were far greater than forL-lysine andL-proline.
For sweeteners, the suprathreshold losses were greater for large sweetener mole- cules such as thaumatin, rebaudioside, and neohesperidin dihydrochalcone than for sweeteners with lower molecular weights. Furthermore, the ability to dis- criminate between different suprathreshold intensities of the same stimulus is also impaired with age. For example, while young subjects required a 34%
difference in concentration to perceive a perceptible difference in the bitterness of caffeine, older subjects required an increment of 74%. Decrements in the ability to perceive suprathreshold concentrations of NaCl (salty) or sucrose (sweet) can tempt older individuals with hypertension (who must comply with sodium-restricted diets) or those with diabetes (who must monitor their carbo- hydrate intake) to use too much salt or sugar to improve the taste of their food.
Reduced ability to perceive the oral component of fats (taste and mouth-feel) makes it difficult for older adults to comply with a low-fat diet and thus can potentially increase the risk from medical conditions such as cardiovascular disease, diabetes, and hypertension in which high-fat intake is contraindicated.
Many older individuals unknowingly consume large amounts of fat without being able to really perceive it.
5.2.3 Medications and Medical Conditions Associated with Taste Alterations While the actual incidence and prevalence of drug-induced taste disorders are not well documented, several community and longitudinal studies of older persons suggest that between 11 and 33% experience medication-related alterations in
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taste (10). Medicated older individuals are more likely to complain of ‘‘loss of taste’’, ‘‘altered taste’’, and ‘‘metallic taste’’ than either age-matched non-medicated controls or younger medicated and non-medicated controls(5–9). Hundreds of medications, including most major drug classes, have been associated clinically with taste complaints(11). However, there is individual variability in predisposition to adverse taste side effects experienced from use of medications. Genetic testing for variability in genes such as the cytochrome P450 2D6 (CYP2D6) that determine the rate of drug metabolism may ultimately be helpful in predicting whether an indivi- dual is vulnerable to taste disorders from drugs. Genetic tests for variations in genes (genotypes) associated with drug metabolism are available from medical providers as well as directly to consumers(12).
Neither the sites of action nor cascade of cellular events by which medications induce taste complaints is well understood. Medications can potentially alter taste perception by affecting the peripheral receptors, neural pathways, and/or the brainstem and brain. At the periphery, drugs in the saliva can generate a taste of their own or modify transduction mechanisms in taste receptor cells. For some drugs, the plasma concentrations are high enough to stimulate taste receptors on the basolateral side of taste cells (called intravascular taste). Even when the salivary or plasma concentrations of medications are lower than the taste thresh- old values, drugs or their metabolites can accumulate in taste buds over time to reach (especially lipophilic bitter-tasting drugs) concentrations that are greater than taste detection thresholds. Drugs can also alter neurotransmitter levels along neural pathways or interfere with taste signals in the brainstem and brain. Drugs that cause a substantial percentage of taste disorders (such as the antifungal agent terbinafine) tend to be highly lipophilic and are thus readily distributed into the brain and brainstem.
The metabolism and absorption of drugs can be modified by dietary constituents, and thus food choices can play a role in potential taste disorders. When lipophilic medications are ingested with a fatty meal, absorption of these drugs can increase.
Ingestion of dietary protein with methyl-dopa andL-dopa (used to treat Parkinson’s disease) can affect the metabolism and absorption of these medications because they are amino acid derivatives. Intestinal transit time, which affects the absorption of drugs, can be modified by ingestion of spicy foods, dietary fiber, and other food components(13).
A vast range of medical conditions have been reported to alter the sense of taste including infectious and parasitic diseases; cancer; endocrine, nutritional, and metabolic diseases; as well as diseases of the nervous, circulatory, digestive, respira- tory, and musculoskeletal systems. Cancer is an example of a medical condition in which patients are especially vulnerable to taste disorders. Taste alterations occur in both untreated cancer patients and those receiving radiation therapy or chemother- apy. Some complaints, such as taste aversions in cancer patients, are not due to altered sensory physiology per se but to learned aversions in which the taste of foods is associated with the noxious effects of treatment. Clinical observations indicate that inflammatory conditions and wasting also predispose sick older individuals to taste disorders.
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