Can age-related CNS taste differences be detected as early as middle age? Evidence from fMRI

Does presbygeusia really exist? An updated narrative review

This review critically assessed the existence of presbygeusia, i.e., the impairment in taste perception occurring in the elderly, as a natural part of the aging process and its potential clinical implications. Several factors might contribute to age-related taste alterations (TAs), including structural changes in taste buds, alterations in saliva composition, central nervous system changes, and oral microbiota dysbiosis. A comprehensive literature review was conducted to disentangle the effects of age from those of the several age-related diseases or conditions promoting TAs. Most of the included studies reported TAs in healthy elderly people, suggesting that presbygeusia is a relatively frequent condition associated with age-related changes in the absence of pathological conditions. However, the impact of TAs on dietary preferences and food choices among the elderly seems to be less relevant when compared to other factors, such as cultural, psychological, and social influences. In conclusion, presbygeusia exists even in the absence of comorbidities or drug side effects, but its impact on dietary choices in the elderly is likely modest.

The Effect of Genetic Taste Status on Swallowing: A Literature Review

PURPOSE

Swallowing and taste share innervation pathways and are crucial to nutritive intake. Individuals vary in their perception of taste due to factors such as genetics; however, it is unclear to what extent genetic taste status influences swallowing physiology and function. The purpose of this review article is to provide background on genetic taste status, review the evidence on the association between genetic taste status and swallowing, and discuss research and clinical implications.

METHOD

A comprehensive literature review was conducted using search terms related to swallowing and genetic taste status. Studies were included if they investigated the main effect of genetic taste status on swallowing or the interaction of genetic taste status with other variables. Studies were grouped by participant population (healthy participants or persons with a swallowing disorder), swallowing-related outcome measure, and method of genetic taste status measurement.

RESULTS

The results were mixed, with five of 10 reviewed studies reporting a statistically significant main or interaction effect on swallowing. Most studies included healthy participants, with only one study investigating participants with dysphagia. Additionally, swallowing-related outcome measures and methods of determining genetic taste status varied greatly between studies conducted on separate cohorts.

CONCLUSIONS

Few studies have incorporated genetic taste status as a variable in swallowing research, and results are mixed. Future research on sensation and swallowing should consider the potential effect of genetic taste status and follow standardized procedures for its determination. Despite the limited evidence, clinicians may consider how individual differences in perception shape swallowing outcomes.

The effect of taste on swallowing: A scoping and systematic review

Consuming foods and liquids for nutrition requires the coordination of several muscles. Swallowing is triggered and modified by sensory inputs from the aerodigestive tract. Taste has recently received attention as a potential modulator of swallowing physiology, function, and neural activation; additionally, taste impairment is a sequela of COVID-19. This review presents factors impacting taste and swallowing, systematically summarizes the existing literature, and assesses the quality of included studies. A search was conducted for original research including taste stimulation, deglutition-related measure(s), and human participants. Study design, independent and dependent variables, and participant characteristics were coded; included studies were assessed for quality and risk of bias. Forty-eight articles were included after abstract and full-text review. Synthesis was complicated by variable sensory components of stimuli (taste category and intensity, pure taste vs. flavor, chemesthesis, volume/amount, consistency, temperature), participant characteristics, confounding variables such as genetic taster status, and methods of measurement. Most studies had a high risk of at least one type of bias and were of fair or poor quality. Interpretation is limited by wide variability in methods, taste stimulation, confounding factors, and lower-quality evidence. Existing studies suggest that taste can modulate swallowing, but more rigorous and standardized research is needed.

Differential Effects of BMI on Brain Response to Odor in Olfactory, Reward and Memory Regions: Evidence from fMRI

：Obesity has reached epidemic proportions, motivating research into the underlying mechanisms. Olfaction is a powerful mediator of food consumption, and obesity has been associated with altered olfactory sensitivity. The current study used an event-related functional magnetic resonance imaging (fMRI) to examine the central processing of odor in humans to gain insight into the effect of the body mass index (BMI) on the neural processes involved in rating the pleasantness of a food odor during a hunger state and in a satiety state. We hypothesized that, during the hedonic evaluation of food odor, BMI would be associated with differences in brain activation within olfactory and higher order processing areas important for perception, reward, and memory. We report novel findings of a dissociation between the relationship between BMI and activation in reward areas and in olfactory and odor memory areas, i.e., activation in reward areas decreased as BMI increased, whereas activation in primary olfactory and memory regions increased as BMI increased. A greater BMI is associated with decreased activation in the reward and frontal regions, supporting a blunted reward response in obesity. These findings have important potential implications for decision making, response inhibition, and reward-based behaviors that may play key roles as causal and maintenance factors in obesity. In contrast, a greater BMI is associated with an increased activation in the primary olfactory and memory areas, which was observed during a hunger state. These results raise the speculative hypothesis that high BMI may be associated with hyperactivation in the olfactory and memory areas, and that over time, the resulting excitotoxic effects may contribute to neurodegenerative changes in these areas.

Age-Related Deficits in Taste and Smell

Disturbances in both the ability to smell and to taste are common in older persons. Such disturbances influence nutrition, safety, quality of life, and psychological and physical health. The anatomic and physiologic causes of age-related disturbances are multiple and interacting, and depend on genetic and environmental factors. Frank losses of function, distortions, and hallucinations are common. Most distortions resolve over time, although this can take months or even years. Olfactory dysfunction occurs during the earliest stages of several neurologic disorders, most notably Alzheimer's disease and Parkinson's disease, likely heralding the onset of the underlying pathologies.

Affective value, intensity and quality of liquid tastants/food discernment in the human brain: An activation likelihood estimation meta-analysis

The primary dimensions of taste are affective value, intensity and quality. Numerous studies have reported the role of the insula in evaluating these dimensions of taste; however, the results were inconsistent. Therefore, in the current study, we performed meta-analyses of published data to identify locations consistently activated across studies and evaluate whether different regions of the human brain could be responsible for processing different dimensions of taste. Meta-analyses were performed on 39 experiments, with 846 total healthy subjects (without psychiatric/neurological disorders) in 34 studies reporting whole-brain results. The aim was to establish the activation likelihood estimation (ALE) of taste-mediated regional activation across the whole brain. Apart from one meta-analysis for all studies in general, three analyses were performed to reveal the clusters of activation that were attributable to processing the affective value (data from 323 foci), intensity (data from 43 foci) and quality (data from 45 foci) of taste. The ALE revealed eight clusters of activation outside the insula for processing affective value, covering the middle and posterior cingulate, pre-/post-central gyrus, caudate and thalamus. The affective value had four clusters of activation (two in each hemisphere) in the insula. The intensity and quality activated only the insula, each with one cluster on the right. The concurrence between studies was moderate; at best, 53% of the experiments contributed to the significant clusters attributable to the affective value, 60% to intensity and 50% to quality. The affective value was processed bilaterally in the anterior to middle insula, whereas intensity was processed in the right antero-middle insula, and quality was processed in the right middle insula. The right middle dorsal insula was responsible for processing both the affective value and quality of taste. The exploratory analysis on taste quality did not have a significant result if the studies using liquid food stimuli were excluded. Results from the meta-analyses on studies involving the oral delivery of liquid tastants or liquid food stimuli confirmed that the insula is involved in processing all three dimensions of taste. More experimental studies are required to investigate whether brain activations differ between liquid tastants and food. The coordinates of activated brain areas and brain maps are provided to serve as references for future taste/food studies.

Multisensory influence on eating behavior: Hedonic consumption

Research in obesity has traditionally focused on prevention strategies and treatments aimed at changing lifestyle habits. However, recent research suggests that eating behavior is a habit regulated not only by homeostatic mechanisms, but also by the hedonic pathway that controls appetite and satiety processes. Cognitive, emotional, social, economic, and cultural factors, as well as organoleptic properties of food, are basic aspects to consider in order to understand eating behavior and its impact on health. This review presents a multisensory integrative view of food at both the homeostatic and non-homeostatic levels. This information will be of scientific interest to determine behavior drivers leading to overeating and, thus, to propose effective measures, at both the individual and population levels, for the prevention of obesity and associated metabolic diseases.

Sensory Functioning in Older Adults: Relevance for Food Preference

The world's population is aging and older adults represent the fastest growing segment of the consumer market. Changes in sensory, perceptual and cognitive function in this segment of the population have been described psychophysically, however, more is known about the young-old than the old-old or oldest-old. Only now are we exploring the potential for neuroimaging tools to probe the changes in central nervous system function related to taste and smell that are relevant to sensory perception, reward value, anticipation of and consumption of food stimuli. There is real potential for brain imaging to provide a greater understanding of older adults' consumer behavior.

Basic taste processing recruits bilateral anteroventral and middle dorsal insulae: An activation likelihood estimation meta-analysis of fMRI studies

BACKGROUND AND PURPOSE

Numerous task-based functional magnetic resonance imaging (fMRI) studies have reported the locations of basic taste representations in the human brain, but they usually employed a limited number of subjects (<20) with different methodologies and stimuli. Moreover, the reported brain regions were sometimes inconsistent. Thus, we aimed at performing a meta-analysis of the published data to identify locations consistently activated across studies, and performed a connectivity analysis to reveal how these taste processing regions connect with other brain regions.

MATERIALS AND METHODS

A meta-analysis was performed based on 34 experiments, with 238 total participants in 16 studies, to establish the activation likelihood estimation (ALE) of taste-mediated regional activation. Meta-analytic connectivity modeling (MACM) and data stored in BrainMap database were employed to reveal the functional connectivity of the regions identified by ALE with other brain regions, across all types of experiments that caused activation among healthy subjects.

RESULTS

ALE identified nine activated clusters in bilateral anteroventral and middle dorsal insulae, bilateral thalamus and caudate, bilateral pre-/postcentral gyrus, and right hippocampus. The concurrence between studies was moderate, with at best 38% of experiments contributed to the significant clusters activated by taste stimulation. Sweet taste was the predominant contributing taste. MACM revealed that at least 50% of the nine clusters coactivated with the middle cingulate cortex, medial frontal gyrus, inferior parietal lobule, and putamen.

CONCLUSION

Results suggested that fMRI studies have reported reproducible patterns of activations across studies. The basic taste stimulations resulted in activations in a mostly bilateral network. Moreover, they were connected with cognitive and emotional relevant brain regions.

Age-Related Changes in Gustatory, Homeostatic, Reward, and Memory Processing of Sweet Taste in the Metabolic Syndrome: An fMRI Study

Age affects the human taste system at peripheral and central levels. Metabolic syndrome is a constellation of risk factors (e.g., abdominal obesity and hypertension) that co-occur, increase with age, and heighten risk for cardiovascular disease, diabetes, and cognitive decline. Little is known about how age, metabolic syndrome, and hunger state interact to influence how the brain processes information about taste. We investigated brain activation during the hedonic evaluation of a pleasant, nutritive stimulus (sucrose) within regions critical for taste, homeostatic energy regulation, and reward, as a function of the interactions among age, metabolic syndrome, and hunger condition. We scanned young and elderly adults, half with risk factors associated with metabolic syndrome twice: Once fasted overnight and once after a preload. Functional magnetic resonance imaging data indicated significant effects of age as well as interactive effects with metabolic syndrome and hunger condition. Age-related differences in activation were dependent on the hunger state in regions critical for homoeostatic energy regulation and basic as well as higher order sensory processing and integration. The effects of age and metabolic syndrome on activation in the insula, orbital frontal cortex, caudate, and the hypothalamus may have particularly important implications for taste processing, energy regulation, and dietary choices.

Age differences in the brain mechanisms of good taste

There is strong evidence demonstrating age-related differences in the acceptability of foods and beverages. To examine the neural foundations underlying these age-related differences in the acceptability of different flavors and foods, we performed an fMRI study to investigate brain and hedonic responses to orange juice, orange soda, and vegetable juice in three different age groups: Young (22), Middle (40) and Elderly (60 years). Orange juice and orange soda were found to be liked by all age groups, while vegetable juice was disliked by the Young, but liked by the Elderly. In the insular primary taste cortex, the activations to these stimuli were similar in the 3 age groups, indicating that the differences in liking for these stimuli between the 3 groups were not represented in this first stage of cortical taste processing. In the agranular insula (anterior to the insular primary taste cortex) where flavor is represented, the activations to the stimuli were similar in the Elderly, but in the Young the activations were larger to the vegetable juice than to the orange drinks; and the activations here were correlated with the unpleasantness of the stimuli. In the anterior midcingulate cortex, investigated as a site where the activations were correlated with the unpleasantness of the stimuli, there was again a greater activation to the vegetable than to the orange stimuli in the Young but not in the Elderly. In the amygdala (and orbitofrontal cortex), investigated as sites where the activations were correlated with the pleasantness of the stimuli, there was a smaller activation to the vegetable than to the orange stimuli in the Young but not in the Elderly. The Middle group was intermediate with respect to the separation of their activations to the stimuli in the brain areas that represent the pleasantness or unpleasantness of flavors. Thus age differences in the activations to different flavors can in some brain areas be related to, and probably cause, the differences in pleasantness of foods as they differ for people of different ages. This novel work provides a foundation for understanding the underlying neural bases for differences in food acceptability between age groups.

Neural correlates of taste and pleasantness evaluation in the metabolic syndrome

Metabolic syndrome (MetS) is a constellation of cardiometabolic abnormalities that commonly occur together and increase risk for cardiovascular disease and type II diabetes. Having MetS, especially during middle-age, increases the risk for dementia in later life. Abdominal obesity is a central feature of MetS; therefore, increased efforts to prevent obesity and identify predictors of weight gain are of extreme importance. Altered processing of food reward in the brain of obese individuals has been suggested to be a possible mechanism related to overeating. We scanned fifteen healthy middle-aged controls (aged 44-54) and sixteen middle-aged adults with MetS after a fast (hungry) and after a preload (sated), while they rated the pleasantness of sucrose (sweet) and caffeine (bitter) solutions. Data were analyzed using voxelwise linear mixed-effects modeling, and a region of interest analysis to examine associations between hypothalamic activation to sweet taste and BMI during hunger and satiety. The results indicate that middle-aged individuals with MetS respond with significantly less brain activation than controls without MetS during pleasantness evaluation of sweet and bitter tastes in regions involved in sensory and higher-level taste processing. Participants with higher BMI had greater hypothalamic response during pleasantness evaluation of sucrose in the sated condition. Importantly, this study is the first to document differential brain circuitry in middle-aged adults with MetS, a population at risk for poor physical and cognitive outcomes. Future research aimed at better understanding relationships among MetS, obesity, and brain function is warranted to better conceptualize and develop interventions for overeating in these disorders.