Introduction

In older adults, a decline in swallowing function is caused by anatomical1,2 and functional3,4 changes in the muscles of the oral cavity, pharynx, larynx, and upper esophagus. Increased thresholds of sensory receptors5,6 and decreased nerve conduction velocity7 also occur.

Presbyphagia is an age-related decline in swallowing function in older adults8. It differs from dysphagia in that the latter returns to normal with proper nutritional management and exercise guidance. Sarcopenic dysphagia is a comorbidity of decreased overall muscle strength, muscle mass, and swallowing function and is related to systemic sarcopenia9. The prevalence of systemic sarcopenia in the Japanese population increases with age, affecting approximately 22% of men and women aged 75–79 years, as well as 32% of men and 48% of women aged 80 + years10. The estimated prevalence of dysphagia in Japanese community-dwelling older adults is 16–23%, increasing to 27% in those aged > 75 years. However, studies on dysphagia in this population are lacking and the exact prevalence of sarcopenic dysphagia remains unknown11. However, the exact prevalence of sarcopenic dysphagia remains unknown.

Sarcopenia is related to nutritional status, and both nutritional and exercise interventions have been used to prevent it12. Nutritional management and exercises to strengthen swallowing-related muscles are also important in preventing sarcopenia and dysphagia9. Takeuchi et al.13 reported higher electromyography activity in the masseter and suprahyoid muscles when swallowing carbonated water than when swallowing still water. Min et al.14 reported that as carbonate concentration increased, the onset times of the orbicularis oris, masseter, submental muscle complex, and infrahyoid muscle contractions during swallowing decreased significantly. In contrast, the mean submental electromyography activity amplitude increased significantly. Krival and Bates15 reported that linguopalatal swallowing pressure significantly increased with the combination of carbonated and zingiber drinks compared to carbonated water and still water.

Thus, increased muscle activity and lingual swallowing pressure have been reported during the swallowing of carbonated drinks. It is possible that residents who habitually consume carbonated drinks routinely engage in higher oral and pharyngeal muscle activity than those who do not. Although there is no unified view on the intensity and frequency of resistance training for swallowing-related muscles among older adults, habitual intake of carbonated drinks may have a similar ability to resistance training to prevent sarcopenic dysphagia. We hypothesized that sarcopenic dysphagia and subjective difficulty in swallowing are related because the diagnostic criteria for sarcopenic dysphagia include impaired swallowing function and tongue pressure. In this study, we examined the association between the habitual intake of carbonated drinks and subjective swallowing difficulty in community-dwelling older adults.

Results

Characteristics of study participants

Of the 351 participants who received the questionnaires, 325 (48 men and 277 women, mean age 77.6 ± 6.9 years) were included in the analysis after excluding participants with incomplete questionnaire entries. Of the 325 participants, 172 (52%) habitually consumed carbonated drinks. Table 1 lists the characteristics of the participants who consumed carbonated drinks (consumers) and those who did not (non-consumers). Among the participants, 44 (13.5%) had the Eating Assessment Tool-10 (EAT-10) score above the cutoff and were classified as having subjective difficulty in swallowing. Based on the Mini Nutritional Assessment Short Form (MNA-SF), 96 participants (29.5%) were classified as being “at risk of malnutrition” and 3 (1.0%) as “malnourished”. The consumers were significantly younger (p = 0.019) and had a higher proportion of male participants (p = 0.007) than the non-consumers. There was no difference in nutritional status according to the MNA-SF or in the degree of systemic sarcopenia according to the Strength, Assistance in walking, Rise from a chair, Climb stairs, and Falls (SARC-F) questionnaire, which was better than the cutoff value of the mean. Similarly, the percentages of consumers and non-consumers who exceeded the cutoff value did not differ. Based on the EAT-10, subjective difficulty in swallowing was lower in the consumer group (p = 0.006); however, the mean of consumer group and non-consumer group were below the cutoff value. The percentage of participants above the cutoff was higher among non-consumers (p = 0.007). Among consumers, the can intake was 2.1 ± 2.7, which means they consume 2.1 cans (2.1*350 mL) per week. Fifteen participants had a habitual intake of carbonated drinks in the past but no current consumption. These participants had an EAT-10 score of 0.

Table 1 Characteristics of study participants.
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Comparison of participants with and without subjective difficulty swallowing

The characteristics of participants with and without subjective difficulty in swallowing are shown in Table 2. Participants with subjective difficulty in swallowing were significantly older (p < 0.001), had a lower mean MNA-SF score (p < 0.001), had a higher mean SARC-F score (p < 0.001), had fewer remaining teeth (p = 0.002), and had higher denture use (p = 0.031). Significantly fewer participants had subjective difficulty in swallowing (p = 0.007). Sex, number of carbonated drinks consumed, and type of carbonated drink did not differ significantly between consumers.

Table 2 Comparison of participants with and without subjective difficulty in swallowing.
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Factors associated with subjective difficulty in swallowing

First, we examined whether subjective difficulty in swallowing was independently associated with age, nutritional status, degree of systemic sarcopenia, and carbonated drinks intake. To examine the factors associated with subjective difficulty in swallowing, we used a model with the independent variables set as age, total MNA-SF score, total SARC-F score, and current habitual intake of carbonated drinks (Table 3). In this model, age (odds ratio [OR], 1.077; p = 0.011), MNA-SF (OR, 0.807; p = 0.040), SARC-F (OR, 1.753; p < 0.001), and carbonated drink intake (consumer) (OR, 0.455; p = 0.039) were significantly associated with subjective difficulty in swallowing according to the EAT-10.

Table 3 Factors associated with subjective difficulty in swallowing (association with habitual intake of carbonated drinks).
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Next, we examined whether subjective difficulty in swallowing was independently related to age, nutritional status, degree of systemic sarcopenia, and number of carbonated drinks consumed. To examine the factors associated with subjective difficulty in swallowing, we used a model with age, total MNA-SF score, total SARC-F score, and current cans of carbonated drink intake as independent variables (Table 4). In this model, age (OR, 1.078; p = 0.010), MNA-SF (OR, 0.798; p = 0.033), and SARC-F (OR, 1.723; p < 0.001) were significantly associated with subjective difficulty of swallowing according to the EAT-10, but not with the amount of carbonated drink intake (intake cans).

Table 4 Factors associated with subjective difficulty of swallowing (association with habitual intake amounts of carbonated drinks).
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Discussion

The results of this study indicate that the habitual consumption of carbonated drinks is associated with subjective difficulty in swallowing in community-dwelling older adults; however, this relationship does not depend on the amount of carbonated drinks consumed.

In our study, 52% of participants habitually consumed carbonated drinks. To our knowledge, no previous study has examined the proportion of older adults in Japan who prefer carbonated drinks. Sweetened carbonated drinks are popular among United States older adults16. In these study revealed that carbonated drinks are the most commonly consumed drinks in United States, which is also likely to be true in Japan. Thickened carbonated drinks are preferred over non-carbonated liquids by Japanese patients with dysphagia and have also been reported to improve swallowing behavior17,18.

Age, nutritional status, sarcopenia, and carbonated drink consumption were identified as factors associated with subjective difficulty in swallowing. The proportion of patients with dysphagia increases with age11, and sarcopenic dysphagia is an independent risk factor of dysphagia in older adults in older adults19. Furthermore, in community-dwelling older adults, grip strength is associated with tongue pressure and bite strength, whereas walking speed is associated with lip movement and swallowing ability20. Thus, sarcopenia and dysphagia are interrelated. Sarcopenic dysphagia occurs when older adults with presbyphagia are exposed to low nutrition and invasive procedures that increase sarcopenia21. Furthermore, dysphagia can lead to undernutrition in older adults because of reduced variation in the foods they can eat22. Thus, nutritional status and dysphagia are also related.

Regarding the association between carbonated drink intake and the subjective difficulty in swallowing, we initially hypothesized that it was associated with the activity of swallowing-related muscles during carbonated drinks intake. However, the current study analyzed participants who consumed carbonated drinks at least once per week. Furthermore, no association was observed between carbonated drink intake and subjective difficulty swallowing. Among the studies on strengthening swallowing-related muscles, there are no reports on the effect of strengthening muscles less than once a week. No studies have directly compared muscle activity during the swallowing of carbonated drinks with conventional swallow training. However, a comparison of studies of swallowing-related muscle activity during carbonated drinks intake13,14 with studies of muscle activity during head-raising training23 and chin tuck exercises24 showed that muscle activity during swallowing of a carbonated drink was markedly lower than that during head-raising training and chin tuck exercises. These facts make it difficult to conclude that consuming carbonated drinks directly affects the subjective difficulty in swallowing.

We also considered that older adults with impaired swallowing tended to avoid the intake of carbonated drinks because it has been reported that carbonated drinks are difficult to swallow owing to the oral irritation caused by carbonation13. However, all participants who habitually consumed carbonated drinks but did not currently consume them had an EAT-10 score of 0. Therefore, it was ruled out that participants who developed subjective difficulty in swallowing discontinued the intake of carbonated drinks. To clarify the association between carbonated drink intake and subjective swallowing difficulty, the characteristics of older adults who consume carbonated drinks may be confounding factors.

This study aimed to clarify the attributes of older adults who consume carbonated drinks. Although activity levels and social interactions were not examined, older adults who habitually consume carbonated drinks may be more active than those who do not. Carbonated drinks may be consumed while being alone or in with the company of neighbors. Community residents aged > 40 years who interacted with neighbors and participated in leisure activities more frequently had higher tongue pressure25, as did older adults with a broader range of living arrangements26. These results suggest that maintaining and increasing speech and physical activity through social interaction may prevent sarcopenic dysphagia.

Our results do not indicate that carbonated drinks directly affect the swallowing function because the frequency of carbonated drink consumption is so low that carbonated drinks do not help maintain swallowing function. Additionally, the sugar in carbonated drinks can worsen oral health and increase the number of oral pathogens27, and aspiration of oral pathogens along with saliva may increase the risk of aspiration pneumonia27. The intake of carbonated drinks also increases an individual’s risk of diabetes and metabolic syndrome28. Even the ingestion of artificially sweetened beverages can risk an individual’s health29. Our findings do not support a high intake of carbonated drinks. Future research should identify the characteristics of older adults who prefer carbonated drinks and determine their effects on swallowing function. This study provides preliminary findings on the effect of carbonated drinks consumption on swallowing function.

Conclusion

Previous studies have reported that carbonated30 and thickened carbonated drinks18 immediately change swallowing dynamics in older patients with and without dysphagia. However, no reports have examined the long-term changes in swallowing dynamics after drinking carbonated drinks. The current study hypothesized that habitual intake of carbonated drinks would decrease the prevalence of subjective difficulty in swallowing among community-dwelling older adults. Our study is the first to report that habitual carbonated drink intake is associated with subjective difficulty in swallowing. Older age, malnutrition, sarcopenia, and lack of habitual intake of carbonated drinks are associated with subjective difficulty in swallowing in community-dwelling older adults. However, because older adults who consumed carbonated drinks less frequently were included in this study, we cannot conclude whether muscle activity during carbonated drink intake impacts swallowing function. Difficulty in swallowing in community-dwelling older adults may be related to the amount of physical activity and social interactions in daily life, which may be confounding factors for older adults who consume carbonated drinks. Future studies should identify the characteristics of older adults who prefer to consume carbonated drinks.

Methods

Study design

This was a cross-sectional study based on the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) statement.

Participants

We targeted community-dwelling older adults attending care prevention activities at the Tokyo Metropolis and Okayama prefectures between 2021 and 2022. Among them, we included older adults who understood the study and did not have neurological diseases. The exclusion criteria comprised: older adults with cognitive impairment who had difficulty understanding the study content; those with neurological diseases such as stroke; and those who routinely use thickening agents or eat texture-modified diets. Because age, systemic sarcopenia, and nutritional status are associated with sarcopenic dysphagia in community-dwelling older adults31, we decided to use four factors as independent variables in the logistic regression analysis, along with carbonated drinks consumption. Peduzzi et al.32 demonstrated that bias, precision, model fit, and other problems with the analyses may occur when the size of any fewer categories of the dependent variable in a logistic regression analysis is less than ten times that of the explanatory variable. Therefore, we computed our sample size with the goal that the size of the smaller categories was at least 40 participants. The researchers asked participants to complete a questionnaire. We explained the purpose of the study, its content, how the results would be published, and the protection of participants’ privacy in writing. We also explained that we considered the participants’ questionnaire completion as informed consent for the study. The questionnaire was distributed to those individuals who indicated a willingness to cooperate (n = 351) and were collected on the same day. We conducted this study according to the Declaration of Helsinki and included in the questionnaire a document confirming that informed consent was obtained from all participants. We also obtained confirmation of informed consent and agreement to participate in the study from all participants. This study was approved by the Institutional Review Board of Kibi International University (approval No. 21-25).

Research instruments

The initial questionnaire included questions regarding age, sex, degree of dysphagia, nutritional status, degree of sarcopenia, number of remaining teeth, denture use, and the frequency of carbonated drink intake. We also investigated the presence of diabetes and liver disease, which may influence the consumption of carbonated drinks and beer. It is challenging to evaluate community-dwelling older adults using the standard diagnostic tools for dysphagia, such as swallowing videofluorography and fiberoptic endoscopic evaluation of swallowing (FEES). Therefore, the EAT-1033, a subjective assessment of swallowing function, was used for evaluation. The EAT-10 has been shown to correlate with FEES results in patients admitted to rehabilitation hospitals34, and those with head and neck cancer35 and chronic obstructive pulmonary disease36. Furthermore, the reliability and validity of the Japanese version of the EAT-10 have been previously reported37. The EAT-10 consists of 10 questions about swallowing problems, each of which is scored on a scale from 0 (“no problem”) to 4 (“severe problem”), and the total score is calculated. A total score of 3 points was considered “suspected dysphagia” . We defined subjective difficulty in swallowing as a total score of > 3. The MNA-SF38 was used to assess nutritional status. The MNA-SF consists of six items: weight loss, food deprivation, neuropsychological problems, mobility, psychological distress or acute disease, and body mass index. A total score of < 8 is considered “malnourished”, while a score of 8–11 indicates “at risk of malnutrition”. The reliability and validity of the Japanese version of the MNA-SF have been previously reported39. The Strength, Assistance in walking, Rise from a chair, Climb stairs, and Falls (SARC-F) questionnaire40 was used to assess sarcopenia severity. The SARC-F comprises five questions to be answered on a scale from 0 to 2––with 0 indicating “none,” 1 “a lot,” and 2 “unable”––for strength, assistance walking, rising from a chair, climbing stairs, and falls, with a total score of > 4 being “predictive of sarcopenia.” The reliability and validity of the Japanese version of the SARC-F have been previously reported41.

The survey on carbonated drink intake asked participants about the type of carbonated drink most frequently consumed, the average number of days consumed per week, and the amount consumed daily. Additionally, participants who had habitual intake in the past, even if they did not currently consume it, were asked to indicate the age at which they had habitual intake.

Statistical analysis

Habitual intake of carbonated drinks was defined as consumption at least once per week. We chose this criterion with consideration of our survey of participants, who reported habitually consuming carbonated drinks approximately once a week. If the frequency was less than that, we assumed the effect on swallowing function would be too small. The number of carbonated drinks consumed daily was not considered. There are no reports of alcohol content in drinks affecting swallowing function. Therefore, we counted and analyzed alcoholic and soft carbonated drinks together. The intake of carbonated drinks was calculated as an intake of at least 350 mL can per week based on the average number of days per week and the number of carbonated drinks consumed daily. For example, if they ingested 350 mL on 3 days per week, three cans would be recorded.

Student’s t test and the χ-squared test were performed to examine differences in the participants’ characteristics and respective total scores with versus without the current habitual intake of carbonated drinks. These tests were also used to examine differences in attribution due to subjective swallowing difficulty, with EAT-10 scores being ≥ 3 or < 3.

First, logistic regression analysis was performed with the presence or absence of subjective difficulty in swallowing on the EAT-10 as the dependent variable and age, total MNA-SF score, total SARC-F score, and current habitual intake of carbonated drinks as independent variables. Next, logistic regression analysis was performed with the presence or absence of subjective difficulty in swallowing on the EAT-10 as the dependent variable, age, total MNA-SF score, total SARC-F score, and number of cans of carbonated drink intake as independent variables. If the participant had previous habitual intake but was not currently consuming them, they were identified as “no habitual intake.”

All statistical analyses were performed using SPSS version 25 (IBM Corp., Armonk, NY, USA). Statistical significance was set at p < 0.05.