Cross-national differences in the association between estimated cardiorespiratory fitness and depressive symptoms among older adults: findings from three nationwide cohort studies.

Background: Cardiorespiratory fitness (CRF) is a modifiable risk factor for chronic diseases, but its association with depressive symptoms remains unclear, particularly across different populations. We aimed to investigate the link between estimated CRF (eCRF) and incident depressive symptoms among individuals over 50 years old, and to explore potential cross-country variations in this association.

Methods: Data were retrieved from three national cohorts: the Health and Retirement Study (HRS, United States), the English Longitudinal Study of Ageing (ELSA, England), and the China Health and Retirement Longitudinal Study (CHARLS, China). eCRF was estimated using sex-specific algorithms and categorized into low (quintiles 1), moderate (quintiles 2-3), and high (quintiles 4-5) levels. Depressive symptoms were measured using the 8-item Center for Epidemiological Studies Depression Scale (CESD-8) (cutoff ≥ 3) in HRS and ELSA, and the 10-item version (CESD-10) (cutoff ≥ 10) in CHARLS. Cox proportional hazard models estimated hazard ratios (HRs) and 95% confidence intervals (CIs), adjusting for potential confounders.

Results: This study included 13,680 participants (4195 from HRS, 5421 from ELSA, and 4064 from CHARLS) with median follow-ups of 9.78, 12.11, and 5.73 years, respectively. Depressive symptoms incidence was 22.79% in HRS, 22.15% in ELSA, and 40.58% in CHARLS. Per 1-SD increase in eCRF was associated with 9% lower risk of depressive symptoms in HRS (HR = 0.91; 95% CI, 0.87-0.96), 8% lower in ELSA (HR = 0.92; 95% CI, 0.87-0.97), but 6% higher in CHARLS (HR = 1.06; 95% CI, 1.01-1.16). Compared with the low eCRF level group, high eCRF level was associated with decreased risk of depressive symptoms in HRS (HR = 0.69; 95% CI, 0.55-0.85) and ELSA (HR = 0.62; 95% CI, 0.48-0.79), but increased risk in CHARLS (HR = 1.27; 95% CI, 1.01-1.61). Subgroup analyses revealed that the associations were modified by smoking status (HRS), by gender and presence of diabetes (ELSA), and by the presence of hypertension (CHARLS) (P for interaction < 0.05).

Conclusions: A higher level of eCRF was associated with reduced depressive symptoms risk in the US (HRS) and England (ELSA) older adults but with increased risk in China (CHARLS), emphasizing the need for nation-specific public health strategies.