Does cognition predict mortality in midlife? Results from the Whitehall II cohort study

Abstract

The authors examined the association of ‘g’ (general intelligence) factor and five specific cognitive measures assessed in 1997–1999 with mortality till 2006 (mean follow-up of 8 years) in the middle-aged Whitehall II cohort study. In age- and sex-adjusted analysis, a decrease in 1 S.D. in memory (hazard ratio (HR), 1.19; 95% confidence interval (CI): 1.02, 1.39) and in Alice Heim 4-I (AH4-I) (HR, 1.16; 95% CI: 1.01, 1.35) was found to be associated with higher mortality. The association with ‘g’ factor, phonemic and semantic fluency did not reach significance at p < 0.05. No association was found with vocabulary. Out of education, health behaviours and health measures, it was health behaviours that explained the greater part of the association between cognition and mortality, ranging from 21% for memory to 70% for semantic fluency. All the covariates taken together explained only 26% of the association with memory and between 33 and 90% for the other cognitive measures. This study suggests that ‘g’ type composite measure of cognition might not be enough to understand the associations between cognition and health.

Introduction

There are two distinct strands of research on the association between cognition and mortality. Low cognitive scores in childhood and early adulthood, usually on intelligence tests, have been shown to be associated with shortened survival (Batty et al., 2007, Hart et al., 2005, Holsinger et al., 2007, Kuh et al., 2004, Martin and Kubzansky, 2005, Whalley and Deary, 2001). On the other hand, cognition in the elderly has also been shown to be associated with mortality (Bassuk et al., 2000, Bennett et al., 2002, Dewey and Saz, 2001, Eagles et al., 1990, Fried et al., 1998, Gale et al., 1996, Gussekloo et al., 1997, Hassing et al., 2002, Hunderfund et al., 2006, Kelman et al., 1994, Korten et al., 1999, Liu et al., 1990, Neale et al., 2001, Nguyen et al., 2003, Palmer et al., 2002, Shipley et al., 2006, Small et al., 2003, Small and Backman, 1997, Smits et al., 1999, Swan et al., 1995, Tuokko et al., 2003). Besides age, a major difference between these two strands of research is the conceptualisation of cognitive function. Among children and adolescents, cognitive function is generally measured by tests of intelligence (Batty et al., 2007, Hart et al., 2005, Holsinger et al., 2007, Kuh et al., 2004, Martin and Kubzansky, 2005, Whalley and Deary, 2001), a single composite measure is used to assess global cognitive ability and referred to as ‘g’ or the general intelligence factor (Deary and Batty, 2007). Among the elderly, multiple cognitive domains, like memory (Hassing et al., 2002, Shipley et al., 2006, Small et al., 2003, Small and Backman, 1997, Smits et al., 1999), digit symbol substitution test (Fried et al., 1998, Swan et al., 1995), others measures of processing speed (Anstey et al., 2001, Hassing et al., 2002, Korten et al., 1999, Smits et al., 1999), visuo-spatial abilities (Hassing et al., 2002, Shipley et al., 2006, Small et al., 2003), vocabulary (Anstey et al., 2001, Rabbitt et al., 2002), verbal fluency (Small et al., 2003) or global cognitive function, like the Mini-Mental State Examination (Anstey et al., 2001, Bassuk et al., 2000, Eagles et al., 1990, Gale et al., 1996, Gussekloo et al., 1997, Kelman et al., 1994, Neale et al., 2001, Nguyen et al., 2003, Palmer et al., 2002, Small et al., 2003) have been used. However, it remains unclear whether the association between cognition and mortality is specific to a particular cognitive domain or to intelligence in general. Another issue that remains debated is whether the association between cognition and mortality in adults is linear or restricted to those at the lower end of the distribution of cognitive scores (Kuh et al., 2004).

The objective of the present study is to examine, in a middle-aged population, whether the association between cognition and mortality is best captured by specific cognitive measures or by the “g” factor. We also examine whether this association applies across the continuum of the distribution of cognitive scores. A further objective is to identify the extent to which education, health behaviours and health measures explain the association between cognition and mortality.