The current study used the nationwide NHI claims data to estimate the annual prevalence and incidence of type 2 diabetes diagnosed from 2000 to 2007. Increases in the prevalence rate were observed in both crude age- and gender-standardized annual prevalence rates from 2000 to 2007. The annual standardized incidence rates fluctuated throughout the period.
The results showed that the annual prevalence of type 2 diabetes continued to rise throughout the study period. The World Health Organization (WHO) predicted that a 39% increase in the global prevalence of diabetes would occur between 2000 and 2030. However, a 43.35% increase in prevalence has already occurred over an eight-year period in Taiwan. During the same period, the crude incidence rate did not exhibit a linear growth trend. Instead, it decreased slightly since 2005. Thus, the increase in the prevalence of type 2 diabetes may be partly explained by better diabetes care and longer survival.
The cross-sectional studies in various areas of Taiwan included the Ann-Lo district in northern Taiwan in 1988–1990 , the Kin-Hu and Kinmen offshore islands in 1991–1994 , Pu-li township in central Taiwan in 1987–1988 and 1991–1992 , Tainan city in southern Taiwan in 1996 , and the Penghu offshore islands in 1995–1997  with a prevalence of 5.6%–9.0%. All these studies were based on small samples collected in restricted areas; thus, their estimates may not be reliable and valid for national estimates. In addition, these studies were conducted before 2000. After 2000, Chang et al.  used the NHI database, which consisted of 15 million individuals from 23 million insured people registered in the NHI program of Taiwan, to estimate the prevalence of type 2 diabetes. The current study made use of 23 million insured people under the NHI program. The estimated prevalence and incidence using the same definition of type 2 diabetes and datasets were found to be similar to those of Chang et al. . The present study is more reliable and provides more precise prevalence and incidence estimates because of the larger sample size. Furthermore, this study provides prevalence and incidence rates across several socio-demographic subgroups. Compared with the prevalence rates for the same year, the prevalence based on our findings is lower than those for India (12.1% in 2000), Sri Lanka (10.3% in 2006), and Korea (7.6% in 2001); closer to those for China (6.1% in 2002), Thailand (6.7% in 2004), and the Philippines (6.5% in 2004); and higher than that for Vietnam (3.8% in 2001) [2, 6, 7].
Previous studies have shown that there is an increasing trend in the prevalence of type 2 diabetes globally; specifically, prevalence increased by 28.3% among men and 25.9% among women in Sweden from 1972 to 2001 , 48% in America from 1990 to 1998 , 47% in England from 1994 to 2001 , 69% in Canada from 1995 to 2005 , and 38% among men and 25% among women in Taiwan from 1999 to 2004 . The findings in the present study further demonstrated that the prevalence of type 2 diabetes also increased from 2005 to 2007, with a greater increase among men, which was consistent with the findings of previous studies [21, 23]. Previous studies examining the incidence trend of type 2 diabetes in Taiwan showed that the incidence rates among both men and women increased from 1992 to 1996 in Taiwan . However, the incidence rates remained stable among men and slightly decreased among women from 1999 to 2004, after considering age and gender . Thus, the trend toward a decreasing incidence among women and the stable incidence rate among men may be the reason for the higher prevalence among men than women that was observed after 2002.
The initial higher prevalence rates among females before 2002 and the subsequent decrease were supported by a previous study, which compared two nutrition and health surveys from 1993–1996 and 2005–2008 . The prevalence of diabetes in women was 5.5% in 1993–1996 and 8.0% in 2005–2008, whereas that in men was 3.2% in 1993–1996 and 12.0% in 2005–2008. This trend may be explained by the increased prevalence of obesity in men, which saw an increase from 33.4% in 1993–1996 to 51.0% in 2005–2008; however, obesity among women remained relatively stable, increasing from 33.5% in 1993–1996 to 35.9% in 2005–2008. The increased consumption of cakes, sweets, and sugary drinks, as well as the increased proliferation of a sedentary lifestyle, may have lead to this increase in obesity .
Of note, a decline in type 2 diabetes prevalence among individuals with an insurance premium greater than or equal to a median value over the period of observation was observed, which is consistent with the data reported by the International Diabetes Federation . This socio-economically advantaged subgroup also experienced the greatest decline in incidence rate, which is similar to the findings reported by Hsu et al. , who indicated that poverty is associated with higher diabetes incidence.
Increasing levels of obesity in the general population in the past two decades are believed to be one of the principal factors contributing to the rising incidence of type 2 diabetes in Asia [26, 27]. The Department of Health in Taiwan has implemented many health promotion programs in communities and workplaces in the past 10years for the prevention of chronic diseases . The major component of these promotion programs is an increase in the level of physical activity of residents in Taiwan. The protective effects of physical activities include improved body composition, glucose tolerance, and insulin sensitivity [28, 29], and this may have contributed to the decline in the incidence of type 2 diabetes since 2004, as determined in the present study.
The present study has four strengths. First, nationwide data with a large sample size was used, which emphasized the prevalence and incidence trends with a standardized method to define type 2 diabetes. Second, the NHI program in Taiwan provides continuing universal coverage for the entire population, which avoids selection bias. Third, NHI datasets were used, which eliminated the need to minimize the numbers of subjects in the cohorts lost to follow-up. In addition, a large sample of geographically dispersed patients was easily obtained. Finally, a large number of study subjects facilitated the age- and sex-stratified analyses with an ample sample size to satisfy requirements.
Nevertheless, the current study also has several limitations. First, some cases of type 1 diabetes may have been falsely classified as type 2 diabetes. Diabetic patients aged 20years and over were included, and individuals with type 1 diabetes, as defined by the ICD-9 code, were excluded to minimize the misclassification. Second, the study depended exclusively on claims data, which may have resulted in potential intentional or unintentional disease misclassification bias. Patients with at least three ambulatory claims or at least one inpatient claim with a diagnosis of type 2 diabetes were included during the specific year period to minimize potential misclassification. Thus, the estimates of prevalence and incidence may be underestimated. Finally, the body mass index, waist circumference, blood pressure, smoking history, family history of diseases, and laboratory test results were not available in the claims database. Thus, combining this information with the discussion on the prevalence and incidence of type 2 diabetes was beyond the scope of this study.
Diabetes is associated with an increased risk of co-morbidity, such as cancer , dementia , and Parkinson’s disease . Previous studies indicated that the cancer incidence density increases by 2.12, 2.80, 4.13, 2.10, and 2.58-fold for total cancer, colorectal cancer, hepatocellular cancer, pancreatic cancer, and dementia, respectively, for diabetes patients without anti-hyperglycemic medication [30, 31]. Considering the time trend of the prevalence of type 2 diabetes indicated in our study, we would estimate that the number of incident cases of total cancer, colorectal cancer, hepatocellular cancer, pancreatic cancer, and dementia would increase by 4391, 1083, 1568, 229, and 6196, respectively, based on the risk difference between individuals without diabetes and those with diabetes but no anti-hyperglycemic medication, as reported by Lee et al.  and Hsu et al. . Cancer and depression prevention may be enhanced by the primary prevention of diabetes mellitus or diabetes mellitus medication [30–32].