Cognitive activity, education and socioeconomic status as preventive factors for mild cognitive impairment and Alzheimer's disease

Abstract

Growing epidemiological evidence suggests that premorbid participation in cognitive leisure activities (CLA) reduces the risk of dementia by increasing cognitive reserve. We investigated the differential effect of CLA, education, and socioeconomic status (SES) on the development of mild cognitive impairment (MCI) and Alzheimer's disease (AD). Participants in the prospective population-based ILSE study (*1930–1932; 12-year follow-up) were examined in three examination waves (t1:1993/94; t2:1997/98; t3:2005/07). In total, 381 subjects of the original cohort (n = 500) were re-examined at t3. Of these subjects 29% received the diagnosis of MCI and 7% of AD. Subjects participated in a thorough psychogeriatric examination and neuropsychological testing. Moreover, they took part in a detailed autobiographical interview and completed questionnaires including socio-demographic data and current frequency of participation in CLA. Subjects who were highly cognitively active at t1 had a significantly reduced risk of developing MCI/AD at t3 (scores adjusted for education, SES, gender, and depressive symptoms). Additionally, high education and high SES separately reduced the risk of MCI and AD. Our results confirm the hypothesis that a high level of CLA acts as a protective factor against the development of MCI and AD by increasing cognitive reserve. This effect is not accounted for by important potential confounders.

Introduction

Recent studies investigating protective factors against neurodegenerative disorders have frequently referred to the concept of cognitive reserve (Stern, 2002). This concept is based on the repeated observation that brain pathology and subsequent clinical dementia do not necessarily highly correlate if investigated on an individual level (Katzman et al., 1988, Stern, 2006). This finding suggests the existence of interindividual differences concerning the brain's ability to compensate for pathological changes.

A wealth of epidemiological studies has focused on identifying protective factors against Alzheimer's disease (AD) which represent potential constituents of cognitive reserve at the same time. One of the most frequently studied determinants in this context is education. Epidemiological studies consistently report that a high level of education is associated with a reduced risk of AD (Stern et al., 1994, Ganguli et al., 2000, Qiu et al., 2001, Anttila et al., 2002, Lindsay et al., 2002, Fratiglioni and Wang, 2007, Ngandu et al., 2007). The results indicate that early determined variables have the potential to moderate the occurrence of neurodegenerative processes decades later. Education might reflect the extent of early cognitive stimulation of the brain which might influence cognitive abilities and their cerebral correlates in turn (Le Carret et al., 2003). Accordingly, education is considered to be an essential aspect of cognitive reserve.

However, cognitive reserve is not only determined by education. According to epidemiological studies, numerous lifestyle habits practiced throughout people's lifespan have the potential to contribute to cognitive reserve. One aspect that recent studies have focused on is the relationship between late-life leisure activities and dementia risk. Studies consistently agree on a significant protective effect of cognitive leisure activities (CLA) (Scarmeas et al., 2001, Wang et al., 2002, Wilson et al., 2002a, Wilson et al., 2002b, Verghese et al., 2003, Karp et al., 2006, Wilson et al., 2010). As all of the cited studies controlled for education, the results suggest that early as well as late life variables have a high potential to significantly moderate the risk of AD by increasing cognitive reserve. Thus, cognitive reserve should not be seen as a fixed trait which is determined early in life but may be actively enhanced by individual lifestyle.

An important methodological issue in many studies has been the consideration of socioeconomic status (SES) due to its high association with education. Results concerning the association between SES and AD risk are rather inconsistent, with some studies suggesting a significant protective effect of high SES (Stern et al., 1994, Anttila et al., 2002) and others that do not report this relation (Wilson et al., 2005). One possible explanation for these inconsistent results might be the application of heterogeneous definitions of SES (childhood vs. late life SES). Some studies were only able to confirm a protective effect of SES if education was not hypothesized to function as a confounder (Evans et al., 1997, Karp et al., 2004), suggesting that education represents the actual relevant factor in this context.

Even though studies investigating the relationship between CLA and dementia controlled for potential confounders – such as age, education and sex – SES and its potential impact on the ability and motivation to participate in certain leisure activities have not been considered sufficiently. Moreover, mild cognitive impairment (MCI), which is associated with a higher risk of developing subsequent AD, has not been considered adequately in previous studies. Another issue that has not been considered by previous studies is that cohort effects might have influenced study outcomes, as earlier studies did not investigate birth cohorts. A birth cohort design provides the advantage of investigating subjects who grew up under very similar life conditions, which allows for a more solid interpretation of the findings — especially with respect to the impact of education and lifestyle factors.

In the present study, we thus investigated the potential impact of CLA on the development of both MCI and AD. In addition, we analyzed the relationships and interdependencies between CLA, education, and SES referring to the concept of cognitive reserve. The German Interdisciplinary Longitudinal Study on Adult Development and Aging (ILSE), which includes a representative birth cohort of subjects born between 1930 and 1932, served as the basis of our investigations.