Comment on “Association Between Serum Creatinine to Cystatin C Ratio and Activities of Daily Living Disability Trajectories: Evidence From the China Health and Retirement Longitudinal Study”

Dear Editors,

1.

Wu and colleagues [1] are to be congratulated for their longitudinal investigation of the association between the serum creatinine‐to‐cystatin C ratio (CCR) and trajectories of activities of daily living (ADL), basic ADL (BADL), and instrumental ADL (IADL) disability among middle‐aged and older Chinese adults. By combining a readily available biomarker with group‐based trajectory modeling, the study offers a practical approach to identifying individuals at risk of progressive functional decline. From a sport and exercise psychology perspective, four issues may warrant further consideration to strengthen the behavioral interpretation and intervention relevance of these findings.

First, habitual physical activity should be modeled as a potential behavioral pathway linking CCR with disability trajectories. The authors proposed reduced physical activity as one mechanism through which lower muscle reserve may contribute to impaired balance, strength, and independence, yet physical activity, sedentary behavior, and exercise history were not included in the adjusted models. This omission is important because lower CCR may limit activity through fatigue and reduced functional capacity, whereas physical inactivity may further accelerate muscle loss and disability. Future studies could combine repeated CCR measurements with accelerometry and distinguish leisure‐time exercise from occupational and household activity. Cross‐lagged or parallel‐process growth models may help clarify whether changes in physical activity precede, follow, or co‐develop with worsening ADL.

Second, self‐reported ADL difficulty reflects psychological appraisal as well as objective physical performance. Two individuals with similar functional capacity may report different levels of difficulty because of depressive symptoms, fear of falling, pain, low perceived competence, or reduced confidence in managing daily tasks. Fear of falling is common among older adults and is associated with activity restriction and poorer psychosocial functioning [2]. ADL disability and depressive symptoms may also reinforce one another over time [3]. Although the study adjusted for several clinical conditions, these psychological influences were not considered. Future assessments could supplement self‐reported ADL with gait speed, chair‐rise performance, grip strength, and direct observation while measuring depressive symptoms, fall‐related efficacy, and perceived functional competence.

Third, exercise self‐efficacy, positive exercise experiences, and psychological resilience may help explain heterogeneity in functional trajectories. Evidence from different populations indicates that physical activity is closely associated with regulatory emotional self‐efficacy, general self‐efficacy, and psychological resilience [4, 5, 6]. More directly, longitudinal research among older adults suggests that positive exercise experiences can predict subsequent behavioral outcomes [6]. These psychological resources may influence whether early functional difficulty leads to avoidance and activity withdrawal or encourages adaptive participation. Future trajectory studies could therefore examine exercise self‐efficacy, enjoyment, resilience, and emotion regulation as potential mediators or moderators of the association between CCR and disability progression.

Fourth, the interaction involving marital status should not be interpreted solely as evidence of a biological or household‐status difference. Marital status is a limited proxy for social support and does not indicate relationship quality, loneliness, caregiver availability, or exercise‐related encouragement. Social support is consistently associated with physical activity participation among older adults [7], while supportive feedback may strengthen self‐efficacy and wellbeing in physically or psychologically vulnerable populations. Measuring perceived support, living arrangements, loneliness, and participation in group‐based exercise could clarify whether the apparent marital‐status difference reflects interpersonal resources that facilitate sustained activity and independence.

In summary, Wu et al. [1] provide valuable evidence that CCR is associated with distinct ADL, BADL, and IADL disability trajectories. Incorporating objective physical activity, psychological appraisal, exercise self‐efficacy, positive exercise experiences, resilience, and social support would strengthen the interpretation of CCR as a functional‐risk marker. Such integration may help shift risk assessment from biomarker‐based identification alone toward personalized strategies that support confidence, sustained activity, and independence in later life.

Funding

The author has nothing to report.

Ethics Statement

The author has nothing to report.

Conflicts of Interest

The author declares no conflicts of interest.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.