Our systematic review showed a discrepancy among studies regarding the relationship between health literacy or numeracy and several health outcomes in people with diabetes. Consistent evidence suggested a positive association between health literacy and diabetes knowledge, but even this evidence was only rated sufficient. Likewise, there is likely sufficient evidence to support a positive relationship between health literacy and self-care activities. On the other hand, the evidence for an association between health literacy and clinical indicators was weak. We found little evidence to support (or refute) an association between health literacy and important clinical events (such as mortality, cardiovascular disease), other than self-reported hypoglycemia and presence of diabetic complications. The majority of this evidence comes from cross-sectional studies, however, limiting causal inference.
It is important to note that substantial discrepancies exist in the literature, which could be due to methodological issues and challenges in the identified studies. One potential source of discrepancy could be the different tools used to measure health literacy41
and differences in thresholds used to distinguish between health literacy levels.42
This variation in estimates and thresholds, in addition to the fact that these instruments measure different aspects of health literacy and thus reflect different skills,43
could have influenced the magnitude and the precision of the observed estimates in these studies.
Another potential reason for discrepancy could be adjustment for confounders. Most studies adjusted for age, sex, race, and educational level in the analyses; however, few studies also adjusted for other factors such as diabetes duration, diabetes knowledge, self-care, self-efficacy, health status, treatment regimen, and many others, where some of these were included as mediators in the pathway between health literacy and outcomes.27
Adjusting for these confounders that are possibly intermediate variables could have induced over-adjustment bias in estimating direct effects of health literacy on outcomes.44
Another equally interesting observation in this review was from recent studies that explored factors that mediate the relationship between health literacy and diabetes-related health outcomes. Osborn and colleagues39
found that health literacy was not directly associated with self-care and A1c; however, it was indirectly associated with these outcomes through social support. The same investigators in subsequent analysis of the same study found that health literacy was indirectly associated with A1c through self-efficacy.27
Future research should further investigate these mediators and others to better understand the relationship between health literacy and health outcomes and what factors should be the target of intervention, health literacy or the mediators.
Other methodological issues that might have introduced the inconsistent results include the lack of power in some studies, or heterogeneity of participants across studies. This could indicate that health literacy might be related to certain outcomes in particular diabetes populations, but not in others. These speculations are difficult to examine, however, with the limited data and available studies.
A recent review by Berkman and colleagues explored the relationship between health literacy and health outcomes in patients of all ages and was not limited to any patient groups.4
Their findings were similar to ours, particularly on the relationship between health literacy and disease knowledge, and they reported inconsistent results regarding other outcomes that were not addressed in our review, such as healthcare utilization and costs. Other reviews focused on specific populations such as emergency room patients,45
and ambulatory care patients.49
These reviews also had similar results to ours with respect to disease knowledge; most were not able to provide firm conclusions on other outcomes due to insufficient evidence.
This review, as any other systematic review, reflects the quality of the published literature. Although the quality of the included studies ranged from fair to good, these ratings did not reflect the limitations imposed by the cross-sectional design of the majority of the studies, using different measures of health literacy across studies, choosing different cut-points for analysis, the inconsistent and potentially inappropriate control for confounders, and poor reporting, which made comparisons between studies difficult. In addition to methodological limitations, the majority of the studies were conducted in primary care clinics in the US; only a few were population-based and very few were conducted outside of the US (Japan, China, and Ireland).