Lifestyle factors and cognitive ageing in the Lothian Birth Cohort 1936: exploring the role of confounding by prior cognitive ability
Corley, Janie Elizabeth
With an increase in life expectancy, the number of older people affected by cognitive decline and dementia is rising, causing major, global public health concerns. However, there is substantial variation in the rate and magnitude of cognitive decline experienced among ageing individuals. Evidence suggests that many age-associated changes in cognitive functioning can be explained by modifiable lifestyle factors such as smoking, physical activity and diet choices. The weight of the evidence supports the promotion of a healthy lifestyle as an effective strategy for healthy cognitive ageing. Many epidemiological studies have drawn causal conclusions with regard to the positive and direct benefits of lifestyle, yet few have considered the possible confounding role of prior cognitive ability in explaining the lifestyle and cognition relationship in older age. Given the potential for reverse causation, whereby better prior cognitive functioning leads to a greater uptake of healthy behaviours rather than vice versa, it is a mechanism which should be studied, but rarely is. The present thesis focuses on the possible confounding effect of prior cognitive ability on the cross-sectional relationships between lifestyle factors and cognitive ability domains in later-life. The core of the thesis is a series of independent, peer-reviewed (six first-author and one co-author) journal articles in the public domain. Data were derived from the Lothian Birth Cohort 1936 study (n = 1091), a sample of relatively healthy, community-dwelling men and women aged 70 years from Edinburgh, Scotland, for whom childhood (age 11) mental test scores are available. The lifestyle factors investigated were caffeine consumption, alcohol consumption, dietary patterns, body mass index, smoking, serum cholesterol, and physical activity. Cognitive function was assessed across five major ageing-related domains: age 70 IQ (based on the same test that was taken in childhood), general cognitive ability (g), processing speed, memory, and verbal ability. General linear models (ANCOVA) were adjusted for the following covariates: age; sex; childhood cognitive ability; and socioeconomic status (SES). Other potential covariates were additionally adjusted for as necessary. Overall, the positive and significant associations observed between ‘healthy’ lifestyle factors and better cognitive functions at age 70 were consistent with previous research; their effect size was around 1% of the variance in cognitive tests scores. However, these relationships were markedly attenuated (by on average 80%) by a higher childhood cognitive ability and adult SES; for the most part, associations were reduced to non-significance. None of the lifestyle factors were consistent predictors of performance across cognitive domains, though smoking avoidance, a physically active lifestyle, and moderate intake of alcohol, appeared to have the most potential. The key novel finding of this thesis is that, in addition to having predictive value for lifestyle choices over 60 years later, cognitive ability at age 11 accounted for the majority of the cross-sectional associations between lifestyle factors and cognitive abilities in later-life. This finding is consistent with the theory of confounding or even reverse causation. That is, individuals with higher lifetime ‘trait’ cognitive ability may be more likely to adopt a lifestyle which protects against cognitive decline. Rather than a unidirectional or indirect effect of health behaviours on cognitive function, the present findings suggest there may be a dynamic cycle involving cognition, self-management of health and ultimate cognitive outcomes.