Introduction

Decline in sensory functions, including vision and hearing, in older adults has garnered increasing attention in recent years owing to its significant impact on quality of life and health. The World Health Organization Guideline recommends regular visual and auditory examinations, as well as appropriate care, to help older individuals maintain their physical and mental health and independence [1]. Declining sensory function has been shown to cause difficulties in daily activities, exacerbate feelings of isolation, and increase the risk of developing depressive symptoms [2–4]. Notably, visual and hearing impairments are linked to reduced cognitive function and increased risk of dementia [5].

Furthermore, olfactory and gustatory disturbances, frequently reported as sequelae of severe acute respiratory syndrome infection, have been shown to have a substantial impact on quality of life, particularly in older adults [6,7]. Decline in sensory functions can lead to reduction in meal enjoyment and negatively affect dietary quality. Additionally, tactile senses such as touch and warmth play a crucial role in defensive responses and self-awareness. Their decline has been associated with elevated risk of physical injuries, including falls and burns [8,9].

Despite the proliferation of research in this area, most studies on sensory function and health in older adults have been focused on individual senses, with few being conducted on comprehensive assessments of all five senses (vision, hearing, olfaction, taste, and tactile). In addition, although a discrepancy between objective sensory impairment and subjective symptoms has been reported in previous studies, the relationship between subjective assessments of sensory impairment and oral function involving many senses has not been examined in detail. The objective of this study was to investigate the relationship between sensory and oral functions by subjectively assessing all five sensory functions, as part of a cohort study conducted in a mountainous region. The aim was to provide valuable insights into the impact of sensory decline on the health and quality of life of older adults, contributing to the development of potential strategies for prevention and intervention.

Materials and methods

Questionnaire for subjective assessment of sensory perception and eating enjoyment

The questionnaire consisted of 10 items evaluating participants’ current subjective perceptions of vision, hearing, olfaction, taste, and tactile capacities, as well as a retrospective assessment comparing current sensory abilities with those in the past (Table 1). The examiner showed each question to the participants, read the questions aloud, and confirmed their answers. Responses were rated on a four-point scale: 1 = Not applicable, 2 = Tends to be the case, 3 = Sometimes the case, and 4 = Fully applicable. Given the inherent complexities in the subjective evaluation of tactile sensations, and recognizing the heterogeneity in cognitive function and comprehension among the older adult participants in this study, the retrospective assessment employed simplified and easily memorable expressions—such as the common experience of feeling “hot or cold.” In the current assessment, more direct and readily identifiable sensory descriptors, including “discomfort” or “numbness,” were introduced to better capture immediate tactile abnormalities. This modification aimed to enhance the accuracy of participant responses by accounting for potential variations in understanding. Eating enjoyment using all five senses was also assessed on the same four-point scale.

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Table 1. Questionnaire for subjective assessment of sensory perception and eating enjoyment.

https://doi.org/10.1371/journal.pone.0326788.t001

Results

Forty females (mean age 81.4 ± 6.14 years) and 22 males (mean age 80.0 ± 5.85 years) were included in the analysis.

The means and standard deviations of the scores for the sensory questions are presented in Table 2. Among these, only the score for hearing assessments fell within the 2-point range (2.3 ± 1.28 for females, 3.1 ± 1.17 for males, and 2.6 ± 1.30 overall); all other scores were within the 1-point range, with particularly low scores for taste and tactile sensations, indicating that respondents experienced minimal sensory issues. For vision, hearing, and olfaction, the scores for current inconvenience were significantly lower than those for retrospective assessments, suggesting that sensory assessments may be influenced by habituation. Comparisons between male and female participants revealed that males tended to have higher scores than did females, particularly for hearing, for which their score was significantly higher. Additionally, significant correlations were found between taste and olfaction (ρ = 0.306; p < 0.05) and between tactile and vision (ρ = 0.469; p < 0.01).

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Table 2. Results of subjective assessment.

https://doi.org/10.1371/journal.pone.0326788.t002

The means and standard deviations of the basic attributes of the participants and the measurements of oral function are shown in Table 3. Males had significantly higher scores than did females for maximum occlusal force and RSST. No sex differences were observed for other measurement items. The measurement values themselves were not considered to be a significant problematic older group, considering the age of the participants. Subjective sensory assessment was not related to age; however, factors related to oral function, except for TCI and RSST, were significantly negatively correlated with age.

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Table 3. Basic attributes and measurements on oral function factors.

https://doi.org/10.1371/journal.pone.0326788.t003

The mean and standard deviation of eating enjoyment scores were 3.3 ± 0.92, indicating high values. Eating enjoyment showed significant negative correlations with olfactory (ρ = −0.292; p < 0.05) and taste (ρ = −0.314; p < 0.05) assessments but no significant correlation with oral factors.

Binomial logistic analysis of the factors affecting each sensory problem, with sex, age, and oral function as independent variables, yielded the results shown in Table 4. Number of remaining teeth was determined to be relevant in hearing and taste assessments, and oral dryness was associated with olfaction.

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Table 4. Relationship of subjective assessments of five senses with oral function factors.

https://doi.org/10.1371/journal.pone.0326788.t004

Discussion

Decline in sensory functions and impairments in sensory organs, including teeth, in older adults not only directly reduces the quality of life but is also linked to various diseases. For example, hearing loss is the highest-ranking risk factor for dementia [5]. Furthermore, reduced social connectedness has been shown to be associated with increased mortality and functional disability [12]. It will be reasonable to infer that a decline in sensory function and sensory organ impairment contributes to reduced social connectedness.

In terms of aging sensory functions, particularly hearing, the prevalence of hearing loss has been reported to be 71.4% in males and 67.3% in females aged 75–79 years, and 84.3% in males and 73.3% in females aged 80 years and older [13]. Decline in high-frequency hearing is particularly pronounced with aging [14]. However, only 34.4% of older adults aged 75 years or older are aware of their hearing loss [15], highlighting the low level of awareness of this condition. With regard to vision, 47.3% of male and 42.6% of female aged 70 years and older reported having visual impairments [16]. Visual impairment is considered a factor that significantly reduces quality of life and independence, and it has been pointed out that it may limit daily activities (reading, driving, housework, etc.) and increase the risks of social isolation and depression. In addition, recent studies have suggested that reduced vision is associated with reduced cognitive function, possibly due to reduced sensory stimulation of the brain caused by visual impairment and the effects of psychological and social factors caused by loss of vision [1,3,17,18]. Regarding olfaction, the Beaver Dam Offspring Study revealed that 13.9% of adults aged 65 years and older were aware of their olfactory impairment, and the prevalence increased with age [19]. Among individuals aged 80–97 years, 62.5% had olfactory impairment [20]. Olfactory dysfunction has been identified as an early indicator of neurodegenerative diseases, including Alzheimer’s and Parkinson’s diseases. This condition is thought to arise from degenerative changes in the olfactory nerves, abnormal neurotransmission in the brain, or other physiological factors [21,22]. Taste sense also declines with age, and in particular, a decrease in saltiness sensitivity has been noted. These changes are thought to be due not only to a decrease in the number of taste buds, which are sensors of taste perception, but also to changes in saliva properties, dryness of the oral cavity, and other factors [22–24]. The effects of aging on tactile sense has been a subject of significant interest. Various factors, such as alterations in saliva composition, oral dryness, and changes in the mechanoreceptors distributed on the tongue surface, have been implicated in age-related changes in tongue tactile sensation [25,26].

In this study, although many participants were likely to have sensory impairments owing to their age, few older adults were aware of these impairments, particularly those related to oral sensory function. In contrast, they were relatively more aware of auditory and visual issues. A comparison of retrospective subjective assessments of sensory function with current assessments showed that the latter were significantly lower. While oral motor functions, such as lip and tongue movements and occlusal force, showed significant correlations with age, which is consistent with the results of previous studies [27,28], no correlation was found between subjective sensory assessments and age. The subjective evaluation of difficulty in chewing is related to both age and oral factors in older adults [29], and a decline in oral function may lead to a decrease in eating enjoyment due to difficulty in chewing. However, in this study, no direct impact was observed. Furthermore, although subjective assessment of olfaction was shown to be associated with oral dryness, no significant associations were found between subjective assessments of sensory function and oral factors. These findings suggest that the older adults are accustomed to declining sensory function and accept sensory decline like “Gero-transcendence [30,31].” This “acceptance” or “adaptation” mechanism may be part of the psychological adaptability of older adults. However, when asked whether they enjoy eating every day, subjective problems with taste and smell were shown to have an impact, confirming the influence of sensory function on quality of life with regard to food enjoyment. Regarding the relationship between subjective assessments of sensory functions, the correlation between olfactory and taste problems may be because these senses cannot be clearly distinguished and may be considered as problems related to food intake. The reason for the significant correlation between vision and tactile senses is unclear, but it may be related to a kind of “cross-modal plasticity,” in which the lack of visual information increases anxiety about what one eats [32]. The observation that our study participants demonstrated a potential effect of geriatric transcendence on sensory decline may be attributable to the characteristics of the mountainous region in which the study was conducted. In this area, access to essential aspects of daily life—such as medical care, nursing services, and shopping—is limited. As a result, a culture of mutual support and community interdependence may continue to thrive. The positive subjective sensory assessments observed in our study suggest that these close-knit community relationships could have a beneficial impact on both sensory function and the overall well-being of older adults.

Additionally, the limited access to dental care in this region warrants attention. The scarcity of professional dental services may lead to fewer opportunities for older adults to receive objective evaluations and appropriate management of their declining oral function. This lack of professional input could, in turn, contribute to reduced awareness of their actual oral health status, potentially explaining the discrepancy observed between subjective sensory assessments and objective oral function in our findings.

This study has several limitations. It was conducted in a single region with a relatively small sample size, and therefore caution is warranted when generalizing the results. A total of 62 participants aged 65 years or older were included. A post hoc power analysis was performed using G*Power software (version 3.1.9.7, Heinrich-Heine-University, Düsseldorf, Germany). With an alpha level (α) of 0.05 and an effect size of 0.52 (calculated from the observed means and standard deviations), the statistical power of the study was 0.98, indicating that the sample size was adequate to detect the observed effect. However, this study utilized binomial logistic regression analysis. Previous research has suggested that when the sample size of the smaller category of the dependent variable is less than ten times the number of explanatory variables, issues such as estimation bias, reduced precision, and poor model fit may arise [33]. In our analysis, the sample sizes of the smaller outcome categories ranged from 4 to 32, suggesting a potential for such limitations. Therefore, we acknowledge the risk of bias and reduced precision in our findings due to these small subgroup sizes. Nonetheless, the results of this study may still be meaningful. The study population comprised nearly all older adults who could be examined at the only clinic in a mountainous region with a population aging rate exceeding 60%. This unique regional context may offer valuable insights into aging and oral health in underserved areas. Future studies should aim to include a broader range of geographical regions with similar demographic characteristics to validate and expand upon these findings.

Furthermore, this study relied solely on subjective sensory evaluation, rather than objective assessments. This methodological choice stemmed from two primary considerations within the context of our older study population and the Mima-SONGS project’s aims. First, the practical challenges of implementing comprehensive objective sensory testing across all five senses in an epidemiological study involving older adults in a rural mountainous region were substantial. Objective assessments, particularly for tactile, thermal, taste, and olfactory functions, are often time-intensive and require specialized equipment and trained personnel, posing logistical difficulties for a cohort study with potential access limitations and participant burden. Conversely, basic oral function tests were more readily integrated into our existing protocol due to their shorter administration times. Second, our focus on subjective evaluation was also informed by existing literature, as noted in the Introduction, which frequently reports discrepancies between objective sensory impairment and subjective awareness in older adults. The relationship between subjective sensory impairment and a comprehensive evaluation of all five senses remains understudied. Consequently, we addressed this gap by concentrating on the subjective evaluation of the five senses in this study. However, it is important to acknowledge potential communication issues that might have influenced the subjective evaluations, given the age of our participants and the lack of formal validation for the questionnaire.

It was conducted in an isolated mountainous area, the study revealed the state of awareness regarding sensory function among older adults living in a highly independent mountainous area. This study provides valuable data for developing future support measures for older adults. To further build on these findings, follow-up surveys should be conducted in diverse regions and environments to gather additional evidence.

Conclusions

Most older adults in an isolated mountain community were not subjectively aware of sensory problems, especially olfaction, taste, and tactile impairments. Subjective assessments of sensory functions showed a moderate but meaningful association with oral health conditions and age.