We found that middle aged subjects with depressive symptoms were more likely than those without depressive symptoms to develop difficulty with mobility and basic ADL over 12 years of follow-up. Difficulty in basic ADL and mobility tasks are strongly associated with the need for personal assistance[5
]. Therefore, our results suggest that middle age persons with depressive symptoms will be at higher risk for becoming disabled and losing their independence as they age.
In unadjusted analyses, subjects with depressive symptoms had over twice the risk of nondepressed subjects of becoming disabled. Subjects with depressive symptoms were very different at baseline than subjects without depressive symptoms in many risk factors that put them at higher risk for disability, including more comorbidity, lower SES, and worse health status. While adjustment for these factors, especially SES, explained much of the risk associated with depressive symptoms, even after adjustment for all these factors, subjects with depressive symptoms were at 40% higher risk of developing difficulty in mobility or ADL function.
While there is an extensive literature linking depression to disability in the elderly, most prior studies are either cross-sectional or have relatively short follow-up[7
]. Further, subjects in most studies were already of advanced age at the time of enrollment. Our study is one of the first to be informed by a conceptual framework that utilized a life course approach. Distinctive features of our study were the enrollment of a predominantly middle aged cohort free of significant disability at baseline that was then followed for an extended length of time as they advanced into later life. The life course approach to the study of potential risk factors for disability is important because most late life disability is believed to be the result of an insidious process in which risk factors for disablement accumulate over many years and slowly exert deleterious effects that impair the ability to live independently[2
]. While our study has longer follow-up than previous investigations, there is a need to further expand the lifecourse approach to depression and disability by including subjects at both younger and older ages, and following the progression of both depression and disability over time.
The recognition of disability as a life course process may have important implications for the prevention or delay of disability. For example, virtually all intervention efforts to prevent late life disability have focused on older populations. In general, the magnitude of disability prevention has been relatively modest[29
]. However, if disability actually develops insidiously over many years, it may be more effective to focus prevention efforts on earlier age groups. In some respects, the prevention of disability may be similar to the prevention of coronary artery disease where prevention interventions are initiated decades before coronary disease may become clinically apparent.
While we demonstrated an independent relationship between middle life depression and later life functional limitations, we do not know whether treatment of depression in middle life will prevent disability in later life. The IMPACT intervention demonstrated that collaborative primary care based management in late life effectively treats depression and improves physical function.[30
] It is not known whether more effective management of mid-life depression will also be effective at preventing later life disability. The recognition that subjects with depressive symptoms are at high risk for late life disability might also justify additional interventions such as physical activity interventions that lower disability risk regardless of its impact on depression.
There are several methodologic considerations that should be considered. First, we do not know who was treated for depression, and therefore do not know whether treatment modifies the risk for functional limitations. Second, we do not know whether depressive symptoms in our subjects were chronic or transient. Therefore, we can not identify whether the duration of symptoms affects disability. Third, while our ability to adjust for confounders of the relationship between depression and disability exceeded that of most other studies, our adjustment may have been incomplete. For example, we did not have data on the severity of individual comorbid conditions, and had limited data on health habits. Fourth, cognitive impairment may be an important confounder of the relationship between depression and disability but our analyses did not adjust for mid-life indicators of cognitive impairment. Fifth, while we required subjects to be disabled on two consecutive interviews to be considered persistently disabled, it is possible some of these subjects had disability free interviews between waves. Also, a small number of subjects who were disabled on two consecutive waves subsequently recovered from disability.
Another limitation of this study was the use of just a baseline measure of depression rather than a longitudinal measure. Depression is often episodic, and it is likely that the persistence and chronicity of depression is an important mediator of the relationship between disability and depression[15
]. In addition, the relationship between depression and functional limitations is probably bidirectional with multiple independent pathways and feedback loops. We were not able to examine an alternative pathway examining whether disability leads to depression. Fully describing this relationship would require examining changes in depression and functional status over multiple waves of data.
In summary, depressive symptoms significantly increase the likelihood that middle aged persons will develop difficulty with activities that are important to independent functioning as they age. It will require further study to determine whether interventions focused on depression or other risk factors for disability modify this risk.