Population aging is progressing worldwide, and the increasing number of older adults has become a common social challenge across many countries. Japan is one of the most rapidly aging societies with a growing number of older adults requiring long-term care under the Japanese Long-term Care Insurance (LTCI) system.¹⁾ These individuals tend to be functionally vulnerable because of declines in physical function and activities of daily living, highlighting the need for strategies to prevent further functional deterioration and progression of care dependency.^1,2) Approximately half of this population has sarcopenia,^3,4) suggesting that interventions targeting sarcopenia are crucial for preventing functional decline.

In recent years, subjective well-being has gained increasing attention as an important health indicator among older adults. Diener⁵⁾ defined subjective well-being as a concept based on individuals’ subjective evaluations rather than external criteria such as income or social status and proposed that it consists of both cognitive and affective components. The cognitive component refers to life satisfaction, which reflects a global evaluation of one’s life as a whole, whereas the affective component comprises positive and negative affect experienced in daily life. In this way, subjective well-being provides a multidimensional assessment of older adults’ health from their own perspective and reflects aspects that differ from objective indicators, such as physical function and disease status.

Previous studies reported that lower subjective well-being is associated with adverse health outcomes including mortality, disease onset, incident need for long-term care, and falls.^6-9) Furthermore, a decline in subjective well-being has been shown to be associated with low skeletal muscle mass, which is a core component of sarcopenia.¹⁰⁾ This association is thought to involve both biological and behavioral mechanisms. Indeed, older adults with higher subjective well-being exhibit lower levels of the inflammatory marker interleukin-6,¹¹⁾ higher levels of physical activity,¹²⁾ and greater adherence to healthy dietary patterns.^13,14) Because these factors play important roles in the maintenance of skeletal muscle mass, subjective well-being may contribute to changes in skeletal muscle mass. Altogether, the accumulating evidence suggests that subjective well-being is an important indicator of longevity.⁷⁾ However, research focusing on the subjective well-being of older adults requiring support or care in an LTCI system is scarce. Among community-dwelling older adults requiring support or care, many outing purposes had been reported to be associated with higher subjective well-being among those who prefer going out,¹⁵⁾ whereas among those who do not favor going out, a greater number of in-home activities have been shown to be related to subjective well-being.¹⁶⁾ In addition, satisfaction with interpersonal relationships has been shown to be associated with subjective well-being among users of daycare rehabilitation services,¹⁷⁾ whereas respect for individuality has been reported to be related to subjective well-being among residents of long-term care facilities.¹⁸⁾ However, studies on subjective well-being among older adults requiring support or care have primarily been limited to exploring factors associated solely with subjective well-being. To our knowledge, no studies have examined its relationship with sarcopenia among older adults requiring support or care, which is a major issue in contemporary geriatric medicine. Based on the above, there is a need to build on the accumulated evidence on the association between subjective well-being and adverse outcomes in the general older population by examining this relationship among older adults requiring support or care in Japan, where this population exceeds 6 million,¹⁹⁾ and by focusing on its relationship with sarcopenia-related indicators to address this evidence gap.

Therefore, we hypothesized that the pattern of change in sarcopenia-related measures would differ according to subjective well-being status among older adults requiring support or care under the LTCI system, and that these patterns would differ between males and females, given sex differences in skeletal muscle mass and grip strength.²⁰⁾ This study aimed to evaluate subjective well-being from two aspects—life satisfaction and happiness—and to clarify its association with 1-year longitudinal changes in sarcopenia-related indicators among older adults requiring support or care.

The 101 participants included in this study had a mean age of 80.3±7.0 years and a mean Barthel Index score of 86.1±18.3. Based on life satisfaction, the participants were classified into the satisfied (n=74) and non-satisfied (n=27) groups. Based on happiness, participants were classified into happy (n=61) and non-happy (n=40) groups. Comparisons of baseline characteristics between the satisfied and non-satisfied groups are shown in Table 1, and comparisons between the happy and non-happy groups are shown in Table 2. At baseline, sarcopenia was present in 39.4%. At follow-up, sarcopenia was observed in 43.9% of participants.

The results of the two-way repeated-measures ANCOVA adjusted for age, Barthel Index, LTCI level, and orthopedic disease, which showed significant baseline differences between groups, as well as baseline SMI/handgrip strength, are presented in Tables 3 and 4. Regarding changes in SMI, a significant group-by-time interaction was observed for the classification based on life satisfaction in the overall sample, between the satisfied (baseline 6.29±1.07 kg/m²; 1-year follow-up 6.24±1.12 kg/m²) and non-satisfied (baseline 6.54±1.00 kg/m²; 1-year follow-up 6.31±1.07 kg/m²) groups (p=0.048, ηp2=0.041), with an effect size slightly below the medium range.²⁸⁾ The estimated marginal means of change were −0.05 kg/m² (95% confidence interval [CI] −0.14 to 0.03) in the satisfied group and −0.22 kg/m² (95% CI −0.37 to −0.08) in the non-satisfied group. Similarly, among females, a significant group-by-time interaction was observed between the satisfied (baseline 5.60±0.77 kg/m²; 1-year follow-up 5.59±0.89 kg/m²) and non-satisfied (baseline 6.16±1.07 kg/m²; 1-year follow-up 5.84±0.99 kg/m²) groups (p=0.0498, ηp2=0.095), with a medium effect size.²⁸⁾ The estimated marginal means of change were −0.01 kg/m² (95% CI −0.15 to 0.13) in the satisfied group and −0.31 kg/m² (95% CI −0.56 to −0.05) in the non-satisfied group. In contrast, no significant group-by-time interaction was observed among males (p=0.665). For the classification based on happiness, no significant group-by-time interaction in SMI changes was observed. With respect to changes in handgrip strength, no significant group-by-time interaction was observed for either life satisfaction- or happiness-based classifications. For the sensitivity analysis, the results of the two-way repeated-measures ANCOVA adjusted for age only are shown in Supplementary Tables S1 and S2. Comparisons of the baseline characteristics between participants who completed and were lost to follow-up revealed no significant differences in any variables (Table 5).

This is the first study to examine the effects of life satisfaction and happiness—the core components of subjective well-being—on the changes in sarcopenia-related indicators in older adults requiring support or care under the Japanese LTCI system. In addition, a distinctive feature of this study is that it separately examined the effects of life satisfaction, which reflects accumulated and relatively stable evaluations of life, and happiness, which is more susceptible to short-term fluctuations. The results showed that higher life satisfaction was associated with less decline in skeletal muscle mass, particularly among females. These findings indicate that, alongside conventional physical interventions, approaches focusing on life satisfaction from an individual’s perspective may represent a clinically meaningful strategy for sarcopenia prevention. A U-shaped relationship between subjective well-being and age has been suggested,⁷⁾ with relatively higher levels in older adults than in middle-aged individuals. Given that older adults requiring support or care may have limited potential for improvement in physical or cognitive function,¹⁶⁾ this population may be a relevant target for approaches focusing on subjective well-being in rehabilitation.

In this study, decline in skeletal muscle mass was associated with life satisfaction rather than with happiness. Life satisfaction is a cognitive component of subjective well-being that reflects a global evaluation of one’s overall life circumstances, including daily activities, social roles, and living conditions, and is relatively stable over time.^5,29) It captures an individual’s overall appraisal of life based on accumulated experiences and ongoing living conditions and is therefore closely linked to habitual behaviors and long-term lifestyle patterns.^5,29) In contrast, happiness represents an affective component that reflects transient emotional states and is more susceptible to short-term fluctuations.^5,29) From this perspective, responses to happiness measures may be influenced by momentary mood or situational factors, which could introduce variability and attenuate its association with longer-term health outcomes. Given these differences, life satisfaction may be more closely related to the maintenance of health-related behaviors and daily functioning, which in turn may be associated with changes in skeletal muscle mass. Among factors that determine life satisfaction in older adults, those that can be targeted through rehabilitation approaches include objective health status, subjective health perception, social participation, psychological factors, depression, loneliness, self-esteem, and physical activity.^5,30,31) In addition, spiritual aspects, such as a sense of meaning in life, role awareness, and ikigai have been suggested to be associated with subjective well-being.³²⁾ In this context, subjective well-being is considered a multidimensional construct,⁷⁾ and these diverse factors should be comprehensively assessed with attention to the individuality of each older adult, including their values and life circumstances.³³⁾ Based on such individualized assessments, support aimed at optimizing life satisfaction may be important in rehabilitation.

Furthermore, the association between higher life satisfaction and a reduced decline in skeletal muscle mass was more pronounced in females. Previous studies have shown that women are more likely to receive emotional support through social interactions and that this support has a greater impact on psychological and physical health than in men.³⁴⁾ In contrast, males tend to have social networks centered on their spouses.³⁵⁾ Consistent with this, a study of day-care rehabilitation users reported that among females, greater satisfaction with interpersonal relationships was associated with higher subjective well-being.¹⁷⁾ Based on this background, social interactions through participation in day-care rehabilitation may have had a greater impact on females in the present study and may have contributed to the association between life satisfaction and a reduced decline in skeletal muscle mass. Among older adults requiring support or care, opportunities for social interaction in the community are often substantially restricted by physical limitations and environmental barriers. In this context, attendance-based long-term care services may play a particularly important role as accessible and well-organized settings that facilitate regular social interaction.

In this study, life satisfaction was associated with changes in skeletal muscle mass rather than with handgrip strength. Handgrip strength measurements inevitably involve some degree of measurement error due to factors such as measurement technique and participant effort.³⁶⁾ In addition, measurement error in physical function assessments has been shown to be greater in older adults requiring long-term care or support than in those without LTCI certification.³⁷⁾ Accordingly, some measurement errors may have occurred between the baseline and follow-up assessments in this study. In contrast, skeletal muscle mass is measured using instruments that minimize observer and subject variability, which represents a methodological advantage. Furthermore, we examined the clinical significance of the between-group difference in skeletal muscle mass identified in this study. The adjusted between-group differences in the change in SMI were 0.17 kg/m² in the overall population and 0.30 kg/m² among females. To date, reports on the minimal clinically important difference (MCID) for SMI are limited. Therefore, as a reference, we estimated an equivalent MCID for SMI by converting the reported MCID of 0.015 for appendicular skeletal muscle mass adjusted for body mass index (BMI) using the mean BMI (22.8 kg/m²) and height (1.588 m) of the participants in this study, yielding an estimated value of approximately 0.14 kg/m².³⁸⁾ The observed between-group differences exceeded this estimated threshold, indicating that the differences in SMI observed herein may represent clinically meaningful changes, rather than merely measurement error. In addition, body composition compensates for the limitations of BMI and reflects the cumulative effects of lifelong behaviors such as nutrition and physical activity.³⁹⁾ Therefore, skeletal muscle mass may be more sensitive than handgrip strength in capturing the association with life satisfaction.

This study had several limitations. First, life satisfaction and happiness were assessed using single-item measures, although previous studies used this approach.^40,41) Additionally, the use of a dichotomous response format may have resulted in a loss of sensitivity in the information obtained. Second, unmeasured confounding factors such as socioeconomic status may have influenced the results. Third, because the numbers of participants with incident sarcopenia and those who recovered from sarcopenia were limited, it was difficult to use changes in sarcopenia status itself as the primary outcome; therefore, changes in related indicators were analyzed as the primary outcome. In future studies, an increased sample size will be required to examine sarcopenia status itself as the primary outcome. Despite these limitations, this 1-year longitudinal study of 101 older adults requiring support or care under the LTCI system, a population that is generally difficult to follow longitudinally, provides valuable insights into the association between subjective well-being and sarcopenia progression.

In conclusion, among older adults requiring support or care under the LTCI system, higher life satisfaction was associated with less decline in skeletal muscle mass over one year, particularly among females. These findings suggest a potential association between life satisfaction and changes in skeletal muscle mass in this population.