Easton et al. BMC Public Health 2010, 10:459 http://www.biomedcentral.com/1471-2458/10/459 R E S E A R C H A R T I C L E Open Access Health in the ‘hidden population’ of people with low literacy. A systematic review of the literature Phyllis Easton1*, Vikki A Entwistle2, Brian Williams2 Abstract Background: Much of the evidence of an association between low functional or health literacy and poor health comes from studies that include people who have various cognitive difficulties or who do not speak the dominant language of their society. Low functional or health literacy among these people is likely to be evident in spoken conversation. However, many other people can talk readily about health and other issues but have problems using written information. Consequently, their difficulties may be far less evident to healthcare professionals, creating a ‘hidden population’ whose functional or health literacy problems have different implications because they are less likely to be recognised and addressed. We aimed to review published research to investigate relationships between low functional or health literacy and health in working age adults who can converse in the dominant language but have difficulty with written language. Methods: We searched reviews and electronic databases for studies that examined health-related outcomes among the population of interest. We systematically extracted data relating to relationships between low functional or health literacy and both health status and various possible mediators or moderators of the implications of literacy for health. We developed a narrative review. Results: Twenty-four studies met our inclusion criteria. Lower functional or health literacy in this population was found to be associated with worse health status. This may be mediated by difficulties accessing healthcare, and poorer self-management of health problems. It is currently unclear whether, how or to what extent these difficulties are mediated by poorer knowledge stemming from low functional or health literacy. The variation in functional or health literacy measures and comparisons make it difficult to compare study findings and to establish the implications of different literacy issues for health outcomes. Conclusions: There is evidence in the literature that low functional or health literacy is associated with poor health in the ‘hidden population’ of adults whose literacy difficulties may not be evident to health care providers. Further research is needed to help understand the particular disadvantages faced by this population and to establish appropriate responses. Background Despite economic and social growth, low literacy con- tinues to be a significant issue across the developed world. The 1996 OECD International Adult Literacy Survey found 22% of US adults and 23% of UK adults to be performing at the lowest level of a 5-point scale of functional literacy [1]. Functional literacy was defined for this survey as “the ability to read, write and speak in * Correspondence: phyllis.easton@nhs.net 1NHS Tayside, King’s Cross, Clepington Road, Dundee, UK Full list of author information is available at the end of the article English, and to use mathematics at a level necessary to function at work and in society in general” [1]. In healthcare contexts, the concept of health literacy is often preferred. Health literacy has been defined as “the cognitive and social skills which determine the motiva- tion and ability of individuals to gain access to, under- stand and use information in ways which promote and maintain good health” [2]. Low levels of health literacy have also been reported in a number of contexts [3-6]. Both low functional literacy and low health literacy may contribute to poor health status through a variety of mediators and moderators [7]. They may contribute © 2010 Easton et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.