This cross-sectional study showed that pre-frailty and frailty were significantly associated with depressive symptoms in older adults after adjusting for potential covariates. Thus, our results suggest that depressive symptoms are associated with pre-frailty and frailty as defined by the Japan-Cardiovascular Health Study (J-CHS). However, other geriatric syndrome, sarcopenia, locomotive syndrome were not associated with depressive symptoms.
Previous studies have reported a prevalence of depression among older adults ranging from 17.8–27.2% [36–38]. In this study, the prevalence of depressive symptoms was 28.1%, which is relatively high. It is possible that, because the participants in this study were recruited via public hall postings, city hall notices, and newspaper leaflets, the participation rate of older adults with health anxiety was higher. In a study by Tsujiguchi et al.  among 2,470 Japanese older adults selected from the same population, the prevalence of depression was reported to be 27.2%. The results of the present study might be generalizable to other Japanese older adults, given the similar prevalence rates.
The recent interest in “overlapping comorbidities,” specifically, frailty and depression, has given rise to reviews indicating a positive association between the two [39–41]. A study with 958 adults older than 60 years found that depressive symptoms had a significant association with frailty (OR 1.8), which supports our results . Another longitudinal study reported that frailty is predictive of future depressive symptoms .
Previous studies on frailty and depression as overlapping geriatric syndromes reported that the latent factors of frailty that were significantly correlated with depression were biological syndrome (ρ = 0.68, p < 0.01), functional domains (ρ = 0.70, p < 0.01), and frailty index (ρ = 0.61, p < 0.01) . The biological syndrome model of frailty was operationalized in terms of five criteria proposed by Fried and colleagues , who analyzed data from the Cardiovascular Health Study. The frailty index is a count of 70 clinical deficits, including presence of diseases, difficulty in performing daily activities, and other physical and neurological signs and symptoms. In terms of functional domains, frailty was defined as functional impairment in at least two of four domains: physical, nutritive, cognitive, and sensory. For example, a previous study  reported that models of frailty and depression with differing numbers of classes that represent the severity of the syndromes (two in frailty; three in depression) provided a better fit to their data, although the constructs shared a substantial amount of variance (latent kappa coefficient = 0.66); moreover, these also helped identify overlapping subgroups of participants. Using confirmatory factor analysis in the Health and Retirement Study among 3,453 participants, a moderate correlation (0.68) was found between latent categorical models of depression and the “biological definition” of frailty . These findings suggest that pre-frailty and frailty are associated with depressive symptoms. Frailty comprises motor functions such as walking speed and grip strength, and these can be improved by exercise therapy, even in older adults . Therefore, it may be possible to use non-pharmacological interventions to reduce depression in older adults.
Contrary to expectations, no geriatric syndromes other than frailty were associated with depressive symptoms. Although locomotive syndrome is an independent risk factor for depression [16, 26], in this study we found no significant difference between the non-depressive and depressive groups. However, an earlier study based on the 25-item Geriatric Locomotive Function Scale (GLFS-25) reported a correlation between depressive symptoms and locomotive syndrome among 224 community-dwelling older adults . The reason for this discrepancy may be that the GLFS-25 questionnaire includes items on psychological characteristics, such as pain and anxiety, which may be better measures of depressive symptoms.
Previous studies of sarcopenia [23–25] found a correlation between sarcopenia and depression; however, the present study did not find a significant correlation between the two. In a 6-year prospective study with 1,731 community-dwelling older adults, sarcopenia alone was not associated with depression, but was associated with sarcopenia obesity . Therefore, sarcopenia, which indicates muscle wastage, is not associated with depression alone, but may be associated with depression with regard to BMI. Speed et al.  investigated the relationship between fat weight, non-fat weight, and depression by analyzing the body composition data of 800,000 people and reported that fat weight was a risk factor for depression. The aforementioned findings suggest that it is important to consider sarcopenia obesity and obesity, rather than sarcopenia alone, when assessing the risk of depression.
Our study had some limitations. First, we used screening tools rather than full diagnostic procedures to assess depressive symptoms. Second, the cross-sectional design of the study precluded the assumption of a causal relationship between depressive symptoms and frailty. Third, study data were obtained from a single city, which limits the generalizability of the results, as meaningful activities in daily life are also affected by geographic characteristics. Finally, the 363 older adults enrolled represented approximately 3% of older adults living in urban areas and were not randomly selected. Therefore, sampling bias cannot be ruled out.