Introduction

Diabetes and its complications are major causes of early death worldwide [1, 2]. Four hundred and fifteen million people worldwide (8.8% of adults aged 20–79 years) are estimated to have diabetes [3, 4]. This figure is expected to increase by at least 25% by 2030 to 552 million. This upward projection can be explained by population growth, an ageing population and a rise in the prevalence of risk factors for diabetes, including obesity [5]. In Australia alone, 1.5 million adults have diabetes and the number is predicted to increase to 2.9 million by the year 2025 [6, 7]. The prevalence of diabetes is much higher in Indigenous Australians and the onset of diabetes occurs at a significantly younger age than in non-Indigenous Australians [8–10].

The National Health Surveys (NHS) reported an increase in the prevalence of diabetes in Australia from 3% in those aged over 25 years and 7.5% in those aged over 65 years in 1989–90, to 5.1% and up to 13.2% in 2014–2015 in the same age groups [11, 12]. The landmark cohort study, the Australian Diabetes, Obesity and Lifestyle (AusDiab), reported an increase in the prevalence of clinically diagnosed diabetes in Australians aged 25 years or older from 8.5% at baseline (year 1999–2000), to 9.3% in 2004–2005 and to 12% in 2011–2012 [13–15]. The AusDiab remains the primary reference study for historical trends and projected forecasts for the prevalence of diabetes in Australian adults [16]. However, since the final wave of the AusDiab study in 2011–12 [15], there has been a rise in the prevalence of diabetes-related risk factors [17]. Furthermore, Indigenous Australians were underrepresented in these studies. Accordingly, more current population-based prevalence data that stratifies study participants by Indigenous status will prove valuable for monitoring the nationwide burden of diabetes.

Several studies have estimated the prevalence of diabetes in Indigenous Australians in a limited number of geographic areas [8, 9, 18–21] and only a few studies have reported the national prevalence of diabetes in this group [10, 22, 23]. The National Indigenous Eye Health Survey (NIEHS) was conducted in 2008 and found the prevalence of self-reported diabetes in Indigenous Australians age 40 years and over to be 37.3%, approximately 8 times higher than that of the general population of Australia [24]. The most recent Australian Aboriginal and Torres Strait Islander Health Survey (2012–13) found that approximately one in six (18%) Indigenous persons aged 25 years and over had self-reported diabetes, with the prevalence increasing to 39% in those aged 55 years or more [23]. Considering the burden of diabetes in Indigenous Australians, up-to-date prevalence data in this high-risk group through a population-based survey is required.

This paper presents the prevalence of self-reported diabetes in Indigenous and non-Indigenous Australians recruited from 30 different regions in Australia as part of the National Eye Health Survey (NEHS).

Materials and Methods

Results

Differences between those who self-reported diabetes versus those who did not

Non-Indigenous participants.

Non-Indigenous participants who self-reported diabetes were significantly older (mean [SD] = 68.62 [8.90] years vs. 66.24 [9.77] years, p = <0.001) and had fewer years of educational attainment (mean [SD] = 12.06 [3.78] years vs. 12.61 [3.72], p = 0.004) than those who did not self-report diabetes (Table 1). The proportion of participants who were male differed significantly between those who had diabetes and those who did not, with 56.15% of diabetics being male, compared with 44.81% of non-diabetics being male (p<0.001). A smaller proportion of non-Indigenous participants with self-reported diabetes were found to speak English at home compared with their non-diabetic counterparts (88.86% vs. 95.24%, p = <0.001). The prevalence of self-reported diabetes increased with age until it plateaued after the age of 70 years (age 50–59 years, 8.85%; 60–69 years, 14.82%; 70–79 years, 16.14%; >80 years, 17.74%; p = <0.0001).

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Table 1. Comparison of key demographics between participants with self-reported diabetes and those without, stratified by Indigenous status.

https://doi.org/10.1371/journal.pone.0169211.t001

Indigenous participants.

As was found in non-Indigenous participants, Indigenous participants who self-reported diabetes were significantly older (mean [SD] = 58.31 [9.58] years vs. 53.09 [9.58] years, p<0.001), had fewer years of educational attainment (mean [SD] = 10.51 [3.31] years vs. 11.26 [3.28], p<0.001) and had a lower proportion of English speakers (self-report diabetes = 93.95% vs. no diabetes = 97.44%, p<0.001) compared to those who did not self-report diabetes. The proportion of Indigenous Australians with diabetes who were male (37.52%) was significantly lower than the proportion who were male in the non-diabetic group (43.15%, p = 0.020). Similarly to non-Indigenous Australians, the prevalence of self-reported diabetes increases markedly with age, plateauing after the age of 70 years (age 40–49 years, 21.78%; 50–59 years, 37.50%; 60–69 years, 53.87%; >70 years, 53.42%; p = <0.0001).

Associations between risk factors and self-reported diabetes

Total population.

The univariate and adjusted logistic regression analysis revealed significant associations between self-reported diabetes and several socio-demographic variables in the total population (Table 2). After controlling for covariates, Indigenous status was the greatest risk factor for self-reported diabetes, with an odds ratio of 5.91 (95% CI: 4.86, 7.19). After adjustments, the prevalence of self-reported diabetes increased significantly with age (OR = 1.04 per year, 95% CI: 1.03, 1.05) and English speakers at home (OR = 0.46,95% CI: 0.33, 0.65) were at a lower risk of self-reported diabetes. Due to differences in the age inclusion criteria and sampling methodologies employed between Indigenous and non-Indigenous participants, further logistic regression analysis was stratified by Indigenous status.

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Table 2. Univariate and multivariable logistic regression analysis of associations between self-reported diabetes and sociodemographic factors.

https://doi.org/10.1371/journal.pone.0169211.t002

Stratified by Indigenous status.

In the non-Indigenous population, adjusted analysis revealed that male gender (OR = 1.61, 95% CI: 1.30, 1.98), older age (OR = 1.02 per year, 95% CI: 1.01, 1.03) and lower number of years of education (OR = 0.97 per year, 95% CI: 0.64, 0.99) were risk factors for self-reported diabetes, while English speakers at home (OR = 0.49, 95% CI: 0.32, 0.75) were at lower risk for self-reported diabetes.

For Indigenous Australians, multivariate analysis revealed that older age (OR = 1.05, 95% CI: 1.04, 1.07), female gender (OR = 0.75, 95% CI: 0.60, 0.92) and residing in very remote geographical areas (1.61, 95% CI: 1.03, 2.52) were significant risk factors for self-reported diabetes, while those who spoke English at home (OR = 0.45, p = 0.003) were at lower risk of self-reported diabetes.

Discussion and Conclusion

This paper presents the prevalence of self-reported diabetes in Indigenous and non-Indigenous Australian participants in a cross-sectional study from 30 randomly selected sites across the nation. The age-adjusted prevalence of self-reported diabetes was almost 4 times higher in Indigenous participants (43.77%) than in their non-Indigenous counterparts (11.49%). There was a clear association between increasing age and increased diabetes prevalence (OR = 1.02 per year of age, and 1.05 per year of age, for non-Indigenous and Indigenous participants, respectively, p< 0.001).

In 2008, it was estimated that the prevalence of diabetes would rise to 11.5% for those aged 45–64 years and 19.8% for those aged 65–85 years by the year 2016 [14, 27]. Our findings are closely aligned with these projections. The increasing prevalence of diabetes in Australia has been indicated by the National Health Survey (8.9–10.9%, 55–64 years) [11] and by the AusDiab study (9.4%, 40–74 years) [13]. It must be noted that estimates from the AusDiab were obtained using plasma glucose measurements and the diagnosis of diabetes for approximately half of participants was made during the course of the study. Therefore, the use of self-reported diabetes in the present study will have underestimated the true prevalence of disease. The prevalence of self-reported diabetes identified in the present study is consistent with estimates made by statistical modeling [16, 5]. Likely risk factors associated with the increase in diabetes prevalence include an ageing population, rising obesity rates and population growth [6]. Of note, our finding that non-English speaking participants were at higher risk of diabetes is consistent with previous Australian reports [28]. Genetics, immigration stressors, socio-economic and cultural factors have been implicated in the higher level of diabetes risk in this Australian sub-group [29]. Our data highlights that more work is required to address the modifiable barriers to the effective and equitable delivery of health services, including; language barriers, literacy rates, effects of stigmatisation and lack of access to culturally specific care [30].

Our findings are consistent with previous epidemiological evidence reporting a significantly higher prevalence of diabetes in Indigenous Australians compared to non-Indigenous Australians, and with an earlier age of onset of the disease [8–10]. We found that the prevalence of self-reported diabetes (43.77%) was higher than that identified in the NIEHS in 2008 (37.3%) [10]. Given the similarities in the study methods and examination rates for Indigenous participants (NEHS = 77.6% vs. NIEHS = 78.7%), robust comparisons can be made between the findings of these studies. Accordingly it appears that the prevalence of diabetes in Indigenous Australians is on an upward trajectory. These findings are supported by those of the Australian Aboriginal and Torres Strait Islander Health Survey, that reported an increase in the prevalence of diabetes in similarly aged participants (>55 years) from 32% in 2005 to 39% in 2013 [22, 23]. Previous evidence has implicated a greater genetic predisposition [31] and a rising prevalence of key diabetes risk factors, such as obesity [8, 23, 32], poor nutrition [23, 33] and low physical activity levels [23], in the higher prevalence of diabetes in Indigenous Australians. The recent Aboriginal and Torres Strait Islander Health Survey (2012–13) highlighted this disparity, reporting that Indigenous Australians displayed 1.5 times higher rates of obesity, were approximately half as likely to meet national physical activity targets and exhibited significantly poorer adherence to recommended fruit and vegetable intake guidelines when compared to non-Indigenous Australians [23]. Efforts to address these issues through the implementation of primary prevention strategies are ongoing [34].

The higher prevalence of diabetes observed in Indigenous Australians living in very remote regions concurs with findings reported by Hoy and co-workers (2007) who found that the prevalence of diabetes in Indigenous persons from three remote communities were 5.4-10-fold higher than that of the general population [9]. These findings may be related to multiple barriers that Indigenous Australians face in accessing specialist services in very remote areas, including communication, distance, and cultural factors [28].

This is the first study of its kind to provide population-based national data on the prevalence of self-reported diabetes in both non-Indigenous and Indigenous Australian adults. Key strengths of this study include the high positive response and examination rates and a representative nationwide population sample. The primary limitation of this study was the use of self-report for the ascertainment of diabetes. While we cannot deny the potential influences of self-reporting bias, self-report is a commonly utilised tool for diabetes surveillance [10, 12, 30, 31]. In addition, several studies have reported high sensitivity and specificity for diabetes self-reporting as an indicator of medically diagnosed diabetes [32–34]. The absence of glycemic testing in the study protocol is likely to have resulted in an underestimation of the true prevalence of diabetes, as it is estimated that up to 50% of cases are undiagnosed [13].

The prevalence of self-reported diabetes in Australia is high and is increasing. Notably, the age-adjusted prevalence of diabetes in Indigenous Australians in this study was almost 4 times higher than in non-Indigenous Australians. These findings portend major public health and economic challenges for Australia.

Supporting Information

Acknowledgments

S.K. conducted the literature review and contributed to all aspects of the manuscript. J.F. conducted data collection, and contributed to the introduction and discussion. J.X. conducted statistical analysis and contributed to the methods section. P.v.W. provided expert clinical advice, assisted in the study design, and made contributions to technical aspects of the manuscript. H.R.T. provided expert clinical advice, assisted in the study design, and made contributions to technical aspects of the manuscript. M.D. was the Principal Investigator of the study, and contributed to all aspects of the manuscript, from conception to final proof-reading. The guarantor of this manuscript, Dr. Mohamed Dirani, takes full responsibility for this work, including the study design, access to data, and the decision to submit and publish the manuscript. No authors of this manuscript had any conflicts of interest.

The Centre for Eye Research Australia (CERA) and Vision 2020 Australia wish to recognise the contributions of all the NEHS project steering committee members (Professor Hugh Taylor, Dr Peter van Wijngaarden, Jennifer Gersbeck, Dr Jason Agostino, Anna Morse, Sharon Bentley, Robyn Weinberg, Christine Black, Genevieve Quilty, Louis Young and Rhonda Stilling) and the core CERA research team who assisted with the survey field work (Joshua Foreman, Pei Ying Lee, Rosamond Gilden, Larissa Andersen, Benny Phanthakesone, Celestina Pham, Alison Schokman, Megan Jackson, Hiba Wehbe, John Komser and Cayley Bush). Furthermore, we would like to acknowledge the overwhelming support from all collaborating Indigenous organisations who assisted with the implementation of the survey, and the Indigenous health workers and volunteers in each survey site who contributed to the field work.