Among core participants who were interviewed at wave 2 of the English Longitudinal Study of Ageing, 8316 (94.7%) completed the assessment of health literacy. The main reasons for non-completion were sight (n=132) and health problems (n=59). Older participants were less likely to complete the health literacy assessment (83.2% of those aged more than 80; P<0.001). Outcome data were not available for a further 324 participants with no consent for follow-up (n=318) or missing survival data (n=6). After exclusion of 135 participants who died within 12 months of interview, the analytical sample comprised 7857.
Health literacy was categorised as high (maximum score, 67.2%), medium (one error, 20.3%), or low (more than one error, 12.5%). Low health literacy included those scoring 2 (9.0%), 1 (2.7%), and 0 (0.7%). Lower scores were associated with increasing age and indicators of low socioeconomic position but not with ethnicity (table 1). One quarter of adults aged less than 60 could not correctly answer all four questions (25.1%), compared with almost half of adults aged more than 80 (48.4%). Participants with no educational qualifications were four times more likely to have low health literacy than participants with degree level qualifications (21.3% v 4.9%). Lower health literacy was also associated with a higher prevalence of depressive symptoms, physical limitations, and chronic diseases; specifically heart disease, diabetes, stroke, and asthma. Smoking, physical inactivity, and alcohol consumption less than daily were positively related to low health literacy. Higher health literacy scores were associated with stronger cognitive abilities, including orientation, verbal fluency, and working memory.
Table 1 Participant characteristics at baseline by health literacy score. Values are numbers (percentages) unless stated otherwise
Length of follow-up ranged from 13 to 66 months, mean 63.2 months (5.3 years). During follow-up 621 deaths occurred: 321 (6.1%) in the high health literacy category, 143 (9.0%) in the medium category, and 157 (16.0%) in the low category.
Low and medium health literacy were associated with a 75% and 24% increased risk of mortality, respectively, compared with high health literacy, after adjusting for age and sex (table 2, model 1). Adjustment for indicators of socioeconomic position, including wealth, education, income, and ethnicity, reduced the hazard ratio for low health literacy from 1.75 to 1.57 (95% confidence interval 1.29 to 1.92, model 2). After additional adjustment for baseline health status, including major chronic diseases, disabling illness, and physical health, the relation between low health literacy and mortality was further attenuated but remained significant (1.47, 1.20 to 1.79, model 3).
Table 2 Association between health literacy score and all cause mortality, based on participants surviving more than 12 months after interview (n=7857)
The influence of health behaviours on the association between low health literacy and mortality was explored. Adjustment for smoking, alcohol consumption, and regular physical activity had a limited effect on the relation between low health literacy and mortality, with the hazard ratio decreasing to 1.41 (1.15 to 1.73, model 4).
Including cognitive measures within the regression model decreased the hazard ratio for low health literacy and mortality to 1.26 (1.03 to 1.56, model 5). In addition to low health literacy, sex, age, limiting longstanding illness, limited activities of daily living, cancer, smoking, physical activity, and the cognitive measures of fluency and time orientation were significant predictors of mortality (see table in appendix 3). Likelihood ratio tests confirmed that the addition of each block of covariates in models 2 to 5 resulted in a significantly improved fit compared with the previous model (P≤0.001).
The association between health literacy and mortality did not vary significantly by age, sex, ethnicity, education, or pre-existing illness (interaction terms, P>0.05).
The regression analyses were repeated with health literacy in four categories, by further dividing low health literacy into those scoring 2 out of 4 (n=711) and those scoring 0 or 1 (n=271). A graded effect was evident, with the lowest scoring group having a consistently higher risk of mortality. For example, in the model adjusted for health behaviours, the hazard ratio for scores of 2 was 1.34 (1.06 to 1.68) and for less than 2 was 1.59 (1.18 to 2.14). To investigate the potential influence of preterminal cognitive decline, we re-ran the regression models after exclusion of 153 participants who died within 24 months of the interview. Low health literacy remained a significant predictor, with a hazard ratio of 1.32 (1.04 to 1.67) in the fully adjusted model. Increasing the analytical sample to include all eligible participants (n=7992), regardless of survival time, did not alter the relation between low health literacy and mortality. Introducing body mass index into the model reduced the sample size (n=5685), but the hazard ratio for low health literacy remained significant. Using multiple imputation for missing values generated similar results to analyses based on complete cases; low health literacy remained a significant predictor of mortality after adjusting statistically for demographics, socioeconomic position, health status, and health behaviours. (See appendix 2 for the hazard ratios for low health literacy based on multiple imputation.)