Health literacy, cognitive ability and health

Abstract

Poorer health literacy—the ability to acquire, understand and use health information to make better health decisions—has been associated with worse health outcomes. Poorer cognitive ability has also been found to predict increased risk of morbidity and mortality. Health literacy is often assessed using brief tests of health-related reading comprehension and numeracy. Scores on tests of health literacy have moderate-to-strong correlations with cognitive ability test scores. Despite this, few studies have investigated the associations of both health literacy and cognitive ability with respect to health outcomes. This thesis examined whether health literacy and cognitive ability, when studied together, have unique associations with health. The first study in this thesis investigated the unique contributions of health literacy and cognitive ability to smoking status in a sample of 8,734 middle-aged and older adults from the English Longitudinal Study of Ageing (ELSA). Limited health literacy (OR=1.13, 95% CI 1.03-1.25) and poorer cognitive ability (OR per SD=0.94, 95% CI 0.89-0.99) were associated with increased odds of reporting ever smoking. These associations were attenuated and non-significant after adjusting for education and social class. In participants who reported ever smoking, limited health literacy (OR=1.34, 95% CI 1.17-1.54) and poorer cognitive ability (OR=0.88, 95% CI 0.81- 0.95) were associated with being a current smoker, and this remained significant even after adjusting for socioeconomic variables. The second study investigated whether health literacy and cognitive ability were independently associated with diabetes, using a sample of ELSA participants (n=8,669). When examined concurrently, adequate health literacy (OR=0.82, 95% CI 0.69-0.98) and higher cognitive ability (OR per SD=0.78, 95% CI 0.70-0.86) were independently associated with lower odds of self-reported diabetes. Adjusting for health behaviours attenuated these associations and they were no longer significant. Individuals who did not have diabetes were then followed up for up to 10 years. Adequate health literacy (HR=0.72, 95% CI 0.59-0.87) and higher cognitive ability (HR=0.79, 95% CI 0.71-0.88) were associated with a lower risk of developing diabetes. These associations were attenuated by health behaviours and education. The third study sought to determine the role of cognitive ability, measured in childhood and in older age, in the association between health literacy and mortality. Using data from 795 elderly participants from the Lothian Birth Cohort 1936, this study found that lower scores on two tests of health literacy—the Newest Vital Sign (OR per 1 point increase=0.89, 95% CI 0.80-0.99) and the shortened Test of Functional Health Literacy in Adults (OR per 1 point increase=0.95, 95% CI 0.91- 0.98)—were significantly associated with increased risk of mortality. These associations were almost unchanged when childhood cognitive ability was added to the model. When additionally adjusting for cognitive ability in older age, the health literacy-mortality associations were attenuated and no longer significant. Cognitive ability in older adulthood, but not childhood cognitive ability, accounted for most of the association between health literacy and mortality. The genetic architecture of health literacy, cognitive ability, and health was examined in the fourth study. This study investigated whether polygenic profile scores for cognitive, education, and health-related traits were associated with performance on a test of health literacy using 5,783 ELSA participants. Greater odds of having adequate health literacy were associated with higher polygenic scores for better cognitive ability (OR per SD increase=1.34, 95% CI 1.26-1.42) and more years of schooling (OR=1.29, 95% CI 1.21-1.36). Reduced odds of having adequate health literacy were associated with higher polygenic scores for poorer self-rated health (OR=0.92, 95% CI 0.87-0.99) and schizophrenia (OR=0.91, 95% CI 0.85- 0.96). The association between health literacy, cognitive ability and health may, in part, be due to shared genetic influences. This thesis provided an examination of the role of health literacy and cognitive ability in various aspects of health, including health behaviours, morbidity, and mortality. The findings suggest that that at least some of the associations between health literacy and health may be accounted for by cognitive ability, and that the association between health literacy and cognitive ability may be partly due to shared genetic aetiology. The degree of attenuation may depend on the health outcome used and the health literacy and cognitive ability measures used.