Pathways from physical frailty to activity limitation in older people: Identifying moderators and mediators in the English Longitudinal Study of Ageing☆
Graphical abstract
Introduction
Frailty is widely regarded as the multidimensional loss of an individual's body system reserves which results in vulnerability to developing adverse health-related outcomes (Espinoza and Walston, 2005, Lally and Crome, 2007, Pel-Littel et al., 2009), such as death, disability, falls, hospitalization, and institutionalization (Daniels et al., 2012, Ensrud et al., 2009, Ensrud et al., 2008, Jones et al., 2005, Kiely et al., 2009, Pilotto et al., 2012, Woo et al., 2012). Across a spectrum of definitions applied, the prevalence of frailty is estimated to be about 10% among people aged 65 years or older (Collard et al., 2012). The potential adverse outcomes of frailty and its size of problem combine to create significant health and social impact for ageing populations. Consequently, frailty plays a central role in influencing the well-being of older people and holds major public health importance (Woo et al., 2006).
As an adverse outcome of frailty, functional disability reduces the quality of life in older people (Murphy et al., 2007, Walker and Lowenstein, 2009). The latest WHO classification of disability defined three levels of functioning. They are impairment, activity limitation, and participation restriction (ICF, 2002). Typically, activity limitation is measured in terms of needing assistance in basic and instrumental activities of daily living (BADL and IADL). BADL items include bathing, dressing, toileting, transferring, feeding and walking (Katz et al., 1963). Activity limitation exerts a negative impact on older people. Those with increasing levels of activity limitation have lower levels of well-being, which manifests as higher prevalence of depression, less life satisfaction, poorer quality of life, and more loneliness, even after stratifying for age (Demakakos et al., 2010). Moreover, activity limitation compromises healthspan, which is measured by length of healthy life (Crimmins, 2015), and is equally if not more important than lifespan for many older people.
Frailty and functional disability, represented by activity limitation, are considered distinct entities with some degree of overlap (Fried et al., 2004). More importantly, frailty indicators predict future activity limitation in terms of BADL and IADL dependence among community-dwelling older people (Avila-Funes et al., 2008, Gobbens et al., 2012b, Romero-Ortuno et al., 2011, Vermeulen et al., 2011). However, the precise mechanisms by which frailty exerts this effect are less clear. There is sparse knowledge on pathways from frailty to eventual activity limitation. Better understanding of these pathways including the identification of moderators and mediators on them can inform public health and social policy with respect to organizing effective population-level interventions that could potentially minimize the impact of frailty where it already occurs. This may in turn slow down or even delay the onset of activity limitation in older people.
To conceptualize pathways from frailty to activity limitation, a good starting point is the working framework proposed by the Canadian Initiative on Frailty and Aging (Bergman et al., 2004) which is simplified and has its relevant portion shown in Fig. 1. Biological, psychological, social, and societal assets and deficits are represented as moderators on pathways to adverse outcomes which include disability. These assets and deficits represent potential target conditions for intervention to reduce the negative impact of frailty. More recently, the integral concept of frailty (Gobbens et al., 2010b) incorporated a similar set of frailty pathways adapted from those of the Canadian working framework. Other frailty pathways have also been proposed, but are largely restricted to the biological sphere, and are therefore less suitable for a broader investigation of the effects of frailty. Thus, the Canadian working framework offers a useful foundation on which to build a conceptual model for pathways from frailty to activity limitation.
With the basis for a conceptual model of frailty pathways available, the challenge is then to identify a frailty specification which is suitable for investigation of these pathways. In his seminal work, Strawbridge recognized the multidimensional nature of frailty and conceptualized frailty as involving problems in at least two from among physical, nutritive, cognitive, and sensory domains (Strawbridge et al., 1998), More recently, the view of frailty being multidimensional has been expressed in part through the development of frailty identifiers that measure deficits across more than a single domain (Bielderman et al., 2013, Gobbens et al., 2010b, Rockwood, 2005). However, some of these multidimensional elements in these frailty specifications, including those components in the Canadian working framework in Fig. 1, are also hypothesized to be key conditions on pathways from frailty to its adverse outcomes. Having these elements as part and parcel of the frailty specification complicates the task of teasing out the relationship between frailty and these key deficits. As an alternative, the integral concept of frailty explicitly specifies frailty as having three distinct domains namely physical, psychological, and social (Gobbens et al., 2010a). Being able to specify frailty on the basis of a single domain facilitates its disentanglement from conditions related to the other two domains. This in turn facilitates less constrained exploration of the relationship of frailty with multidimensional conditions which may turn out to be mediators or moderators on its effect.
Among these three frailty domains, physical frailty offers the most promising choice as a frailty specification for the investigation of related pathways. There are a number of reasons for this. Firstly, physical frailty is far better understood than psychological or social frailty. Secondly, physical frailty contributes most to prediction of disability among the three frailty domains (Gobbens et al., 2012a). Finally, there exists an excellent prototype for physical frailty in the CHS frailty phenotype (Fried et al., 2001). It conceptualizes physical frailty has having five indicators, which are slow walking speed, weak grip strength, self-reported exhaustion, unintentional weight loss, and low physical activity level. However, exercise as a counter of low physical activity, is a modifier of frailty's effect (Daniels et al., 2008). Thus, given that low physical activity is a lifestyle condition on pathways from frailty to its adverse outcomes, it may be argued that it should be excluded from the set of indicators for a physical frailty specification implemented for examining its relationship with activity limitation. On the other hand, the other four indicators are either symptoms or physical measurements that are not considered to be conditions on frailty pathways which need to be excluded from being a physical frailty indicator. Indeed, our previous work argues that specifying physical frailty with three of the five indicators, namely slow walking speed, weak grip strength, and exhaustion retains face and content validity. In addition, we demonstrate construct and concurrent validity for this physical frailty specification. Weight loss did not enhance these aspects of validity, and can therefore be omitted from the final set of indicators (Ding, 2016). In the light of these points, physical frailty specified by these three indicators holds promise for the investigation of pathways from frailty to activity limitation.
Our conceptual model for investigating the relationship of physical frailty with activity limitation is shown in Fig. 2. In this model, indirect or mediated effects through physical and psychological conditions are included in addition to the direct effect. Furthermore, moderation of these effects by social conditions is also included (dotted lines). We base these hypothesized pathways in part on the Canadian working framework, while advancing beyond to also include indirect effects. These pathways are also consistent with current thinking that posits psychosocial resources as possible moderators and mediators of the effects of frailty (Dent and Hoogendijk, 2015).
Thus, the overarching aim of our study is to identify and estimate the effects of multidimensional conditions which have roles as moderators and mediators of the relationship between physical frailty and future activity limitation in older people. Within this broad aim, we seek to answer three research questions, namely: 1) whether the effect of physical frailty on activity limitation varies across various levels of key social conditions; 2) whether physical frailty has an indirect effect on activity limitation through key lifestyle and psychological conditions; and 3) whether the effects of physical frailty on activity limitation vary across gender and age. To answer these questions, we use longitudinal data from the English Longitudinal Study of Ageing (ELSA).
Section snippets
Data
Our study population comprises a cohort of 4638 older respondents who are aged 65 to 89 years at wave 2 (2004) of ELSA (Marmot et al., 2015). Those aged 90 years and older are excluded given that their age is uniformly coded as “90”, and that their number is small. ELSA is a longitudinal survey of a representative sample of the English population aged 50 years and older living in their homes at baseline (Steptoe et al., 2013). It offers a broad range of reliable and multidimensional data across
Results
Table 1 describes the characteristics of the study population including physical frailty indicators, factor scores, frailty status, and activity limitation across waves 2, 4, and 6. In addition, predictors at wave 2 are also shown. Additional information on mediators at waves 2, 4, and 6 is also provided in the Supplementary Materials (Table A.1). The mean age is 74 years and women comprise approximately 55%. Activity limitation increases on over time with 27% needing assistance in one or more
Discussion
For community-dwelling older people in England, increasing levels of physical frailty independently predict more activity limitation change two years later. This means that the significantly worse trajectory of activity limitation conferred by physical frailty remains even after taking into account the effects of a broad set of concurrent physical, psychological, and social predictors. This confirms previous work by others (Lang et al., 2007, Vermeulen et al., 2011). In terms of magnitude, one
Acknowledgements
The authors would like to thank the UK Data Service for providing access to the data for the English Longitudinal Study of Ageing, as well as University College London (UCL) Research Department of Epidemiology and Public Health, Institute for Fiscal Studies (IFS), National Centre for Social Research (NATCEN), and The University of Manchester, School of Social Sciences, for collecting and processing this data. Without them, this research would not be possible.
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Institution where the work was conducted: Department of Methodology, London School of Economics, Columbia House, Houghton Street, London WC2A 2AE, United Kingdom.