Literacy skills are important determinants of health, and affect one’s ability to prevent, manage, and treat disease effectively. This is the first study to examine the individual and combined effects of four literacy skills simultaneously in relation to the calculated 10-year risk of CHD. Findings suggest that when examined individually, higher reading, numeracy, and aural language scores were associated with a lower risk of CHD, although these associations were significant only for women. When examined jointly, numeracy and aural language skills were the strongest predictors. Our results also highlight the importance of listening skills. As hypothesized, higher skills in one area (e.g., aural language) may compensate for difficulties in another (e.g., numeracy) resulting in an equally low risk of CHD. Our study also suggests that reading may not be the strongest literacy measure associated with CHD risk.
The lack of association between literacy skills and 10-year CHD risk among men was surprising, but consistent with other literature illustrating stronger associations of socioeconomic position (correlated with literacy skills) with obesity33
and metabolic syndrome34
(a risk factor for CHD) among women as compared to men. This is unlikely due to lack of power alone because coefficients for men were consistently smaller than those for women. One potential explanation relates to parity. Prospective studies have demonstrated that childbirth is associated with increased long-term central adiposity and decreased HDL cholesterol levels35,36
. Given the inverse association between a woman’s educational attainment and number of children born37
, and strong correlation between literacy skills and educational attainment, one potential explanation may be the literacy-related differences in parity. We, however, did not ask women in our study about number of pregnancies, and as a result are not able to test this hypothesis empirically.
This study has several limitations. First, the sample was not nationally representative. In particular, the sample contains few Hispanics and did not include immigrants or individuals for whom English is a second language. Second, risk factors for CHD are more prevalent among individuals with lower literacy and while we were able to account for race/ethnicity and educational attainment, there may be other unmeasured factors that may account for the association between literacy skills and risk of CHD. It is not clear, however, whether such factors such as obesity or other measures of socioeconomic status such as income should be treated as potential confounding factors or whether they are better described as mediators on the causal pathway between literacy skills and CHD risk. While education was used as a proxy for socioeconomic status in these analyses, the strong link between education and literacy skills and the uncertainty about their temporal order (i.e., do higher literacy skills result in higher education or vice-versa) suggests that by controlling for education, we may be underestimating the relationship between literacy and CHD risk.
Another limitation is that this study did not include writing which has also been cited as an important component of health literacy(38). It is also not clear whether our results were affected by the use of a modified numeracy scale. Finally, most of our literacy measures were not assessed within a health context and, thus, cannot be considered measures of “health literacy.” While there are no currently available measures of the oral exchange in a health context, the measures used in this study are good proxies to capture such skills as they are readily available, normed, and validated. However, we do acknowledge that context is extremely important when studying health literacy and that the complexity of the health care system, the medical jargon used by many providers, and the exposure to novel health concepts all have the potential to exceed one’s health literacy skills, even among those with adequate literacy. As such, findings from our study may only underestimate the true association between health literacy skills and risk of CHD.
In addition to further examining the association between literacy skills and risk of CHD, more work must be done to understand the mechanisms for the association. It is not known, for example, who among the sample had access to a physician, whether food or cigarette labels or risks were clearly understood, whether antihypertensive mediations were taken as prescribed, or whether conversations with and recommendations by health care providers were effective and useful. Understanding why the association exists has important implications for interventions to lower risk of CHD, particularly among women.
While the results from this study provide important insight into the independent and interactive effects of literacy skills on risk of CHD, they also highlight the need for the development of easy-to use assessments of the oral exchange in the health care setting and the need to better understand which literacy skills specifically are most important for given outcomes.