Temporal change in the association between life satisfaction and functional decline with gender differences: an age-specific prospective cohort study

BACKGROUND: Although life satisfaction (LS) has been shown to predict mortality, research studying the relationship between LS and functional decline is scarce. This study examined the association between LS and functional decline across four time points in young-old Japanese adults. METHODS: We analysed 1,899 community-dwelling 65-year-olds in this age-specific cohort study conducted between 2000 and 2005. The Life Satisfaction Index K was used to evaluate LS and was classified into quartiles. Functional decline was determined using the Japanese Long-Term Care Insurance (LTCI) system: 1) mild disability; 2) severe disability; 3) all-cause mortality; 4) mild or severe disability; 5) severe disability or death; 6) mild or severe disability, or death. Adjusted hazard ratios (HR) with 95% confidence intervals (CI) were calculated using the Cox proportional hazard model. The analyses were conducted in the 8(th), 10(th), 12(th), and 14(th) years to assess the effect of LS on functional decline across time points. RESULTS: The impact of LS gradually weakened over time. In the 8(th) year (aged 72–73), a higher LS was associated with a lower risk of mild or severe disability among the women participants (adjusted HR [95% CI] = 0.30 [0.11–0.81]). However, the effect disappeared gradually (adjusted HR [95% CI] = 0.55 [0.27–1.14]) in the 10(th) year (aged 74–75), 0.72 (0.41–1.26) in the 12(th) year (aged 76–77), and 0.68 (0.41–1.14) in the 14(th) year (aged 78–79). This trend continued in severe disability or death (adjusted HR [95% CI] = 0.24 [0.06–0.70], 0.31 [0.11–0.76], 0.57 [0.28–1.14], and 0.60 [0.32–1.12]) and mild or severe disability, or death (adjusted HR [95% CI] = 0.30 [0.14–0.68], 0.46 [0.24–0.87], 0.67 [0.41–1.10], and 0.65 [0.42–1.02]) in the 8(th), 10(th), 12(th), and 14(th) years, respectively. No statistically significant association was found among men at any time points or in any classification of outcomes. CONCLUSIONS: Higher LS scores in 65-year-old women were associated with a lower risk for functional decline in any combination of mild disability, severe disability, or death. Additionally, the effect of LS was observed to weaken over time. TRIAL REGISTRATION: This is not an intervention survey and does not require registration.