Descriptive Statistics
Of 5,664 participants, 9% lived alone, and 10.3% reported feeling lonely. Around 2.5% (n = 130) of the sample reported both living alone and feeling lonely. Table 1 presents descriptive and univariate statistics for the variables under consideration in the present study. The mean scores for PCS and MCS in our initial sample (n = 5,664) were 46.8 ± 8.9 and 59.3 ± 8.1, respectively. Prior to conducting inferential statistics, we excluded individuals who had missing value in any of the main variables of interest (Fig. 1; final n = 5,012). Older age, female, lower educational attainment, unemployment, unpartnered, residing in rural areas, and infrequent social contact were all associated with poorer PCS (ps < .0001). Correlations among PCS and an array of functional health variables (i.e., number of chronic diseases, handgrip strength SPPB, MMSE, ADL, and IADL) showed that poorer functional health was significantly related to lower PCS (ps < .0001). While a feeling of loneliness was associated with poorer PCS, older adults' PCS was not significantly different between adults who lived alone and those who lived with others (Ms = 46.8 ± 0.1 and 47.0 ± 0.5, respectively, p = .63). On the other hand, older age, female, unpartnered, infrequent social contact, lower functional health, as well as both of the main social isolation indicators (i.e., living alone and loneliness) were significantly associated with lower MCS, ps < .05.
Cross-sectional And Longitudinal Effects Of Living Alone And Loneliness On Health-related Quality Of Life
Multiple regression analysis was applied to examine the main effects of living alone and loneliness on PCS and MCS. The results from cross-sectional and longitudinal data are shown in Table 2 and Table 3, respectively. With cross-sectional data, the results shows that living alone was not significantly associated with PCS, unstandardized coefficient (B) = 0.26, 95% CI: -0.47,1.00, after controlling for covariates representing important risk and protective factors (Supplemental Table 1, PCS Model 1). In contrast, loneliness was significantly and negatively associated with PCS, B = -1.12, 95% CI: -1.76,-0.47. On average, adults who felt lonely reported a 1.12 unit lower in PCS compared to adults who did not feel lonely (Model 2). The negative impact of loneliness on PCS remained after accounting for the effect of living alone, B = -1.15, 95% CI: -1.08,-0.51 (Model 3). A similar pattern was found for MCS. As shown in Supplemental Table 1, loneliness, but not living alone, was a significant predictor of MCS, B = -7.58, 95% CI: -8.28,-6.88, after accounting for the sociodemographic and health covariates (Models 1 and 2). As predicted, the effect of loneliness on MCS held after accounting for the effect of living alone, B = -7.6, 95% CI: -8.30,-6.89 (Model 3).
Controlled for covariates and the correlations across time, the GEE model showed that living alone and loneliness were both significant longitudinal predictors of PCS, B = 0.59, 95% CI: 0.03,1.14 and B = -0.71, 95% CI: -1.32,-0.11, respectively (Table 2, PCS Models 1 & 2). Their effects remained significant after accounting for the effect of the other, B = 0.67, 95% CI: 0.12,1.22 for living alone, and B = -0.77, 95% CI: -1.38,-0.17 for loneliness (Model 3). Regarding MCS, living alone was no longer a significant longitudinal protective predictor after accounting for the effect of loneliness, B = -0.23, 95% CI: -0.89,0.43. In contrast, the effect of loneliness held after controlling for the effect of living alone, B = -7.28, 95% CI: -8.13,-6.43 (Table 2, MCS Model 3).
Evaluating the Moderating Role of Living Arrangement in the Association of Loneliness with Health-Related Quality of Life
To further investigate the moderation, an interaction between living alone and loneliness was included in the final model (Model 4) of the regression models. With cross-sectional data, the effect of loneliness on PCS was not contingent on adults' living arrangements, B = 0.57, 95% CI: -1.05,2.19 (Supplemental Table 1, PCS Model 4). On the other hand, in predicting MCS, the interaction term involving living alone and loneliness was significant. Furthermore, the coefficient was positive, B = 1.99, 95% CI: 0.23,3.76 (Supplemental Table 1, MCS Model 4), indicating that the effect of living alone amplified the negative impact loneliness had on MCS. Specifically, among adults who reported being lonely, there was a 1.99-unit difference in the average MCS scores between adults who lived alone and those who lived with others. Results from longitudinal data, on the other hand, showed the negative impact of baseline loneliness was not amplified or attenuated by living arrangement in predicting PCS and MCS at Time 2 (Table 2, Model 4).
Evaluating The Mediation Pathways With Longitudinal Structural Equation Modeling
The structural model of the concurrent and longitudinal relations among living alone, loneliness and HRQoL is shown in Fig. 2. The relations were tested separately for PCS and MCS. Regarding the model predicting PCS, the model fit for the full structural model met criteria for a reasonable fit, χ2 (3) = 64.24, p < .0001, CFI = 0.96, RMSEA = 0.071 (90% CI: 0.057,0.087), SRMR = 0.028. The path involving cross-sectional data showed that the indirect effect of loneliness was significant for the associations between living alone and PCS, B = -0.54, SE_B = 0.09, p < .001. When it came to predicting PCS at Time 2, the results suggested that loneliness at Time 1 was not a significant mediator for the effect of living alone, B = -0.10, SE_B = 0.07, p = 0.17. Instead, it was the lingering sense of loneliness observed over time (i.e., baseline and Time 2) that significantly mediated the associations of baseline living alone and poorer PCS three years later, B = -0.09, SE_B = 0.02, p < .001 (Table 3). The overall amount of variance accounted for in this model was R2 = 0.19, which is a medium effect size according to Cohen (1988) [39].
Regarding the model predicting MCS, the model fit for the full structural model also met criteria for a reasonable fit, χ2 (3) = 79.73, p < .001, CFI = 0.95, RMSEA = 0.08 (90% CI: 0.065,0.096), SRMR = 0.03. A similar pattern emerged, such that loneliness significantly mediated the associations of living alone with MCS with all baseline data, B = -1.4, SE_B = 0.15, p < .001. Similarly, loneliness observed across two measurement occasions significantly mediated the associations of baseline living alone with MCS three years later, B = -0.28, SE_B = 0.04, p < .001 (Table 3). The overall amount of variance accounted for in this model was R2 = 0.12 and considered a medium effect size.
The mediations models were replicated after controlling for the effects of covariates (Supplemental Table 2). In general, the patterns remained, and the replication emphasized the robust role of loneliness in explaining the impacts of living on lowered HRQoL.