Introduction 1 2 1 3 4 2 5 9 10 12 13 17 18 19 The purpose of this study was to project the prevalence of diabetes for the adult US population up to 2031, using models based on data contained in the nationally representative National Health and Nutrition Examination Survey (NHANES) II mortality survey (1976–1992), NHANES III (1988–1994) and NHANES 1999–2002. Materials and methods Diabetes prevalence model Time 2 i i i i The percentage of persons with diabetes, which is calculated thus: Time 2 Time 2 Time 2 The estimate of future diabetes is therefore based on this equation, including the number of individuals with diabetes in the previous time period, conversion to diabetes, migration, and mortality, rather than being a linear extrapolation of the change in diabetes prevalence from the known values of 1991 and 2001. Data sets The NHANES is a programme of surveys conducted by the National Center for Health Statistics and designed to assess the health and nutritional status of adults and children in the United States. The survey is unique on a national level in that it combines interviews and physical examinations. The NHANES uses a complex multistage sampling design, making it representative of the non-institutionalised US population and allowing weighted estimates to be computed. n n 20 n Variables used in models Prevalence of diabetes Diabetes burden was assessed as diagnosed diabetes plus undiagnosed diabetes. Because of the substantial proportion of people with undetected diabetes, we focused on this formula for total diabetes, rather than using diagnosed diabetes to indicate diabetes burden in the population. Moreover, by focusing on diabetes as diagnosed and undiagnosed disease, we minimised the possible impact on future diabetes prevalence of changes in screening practices for diagnosing diabetes during an ensuing time period. 21 Persons converting from a disease-free state to having diabetes 13 14 16 18 2 18 18 18 Migration of persons with diabetes 5 Persons with diabetes moving into the 20 to 29-year-old age class For 2011, 2021 and 2031 the total number of persons with diabetes in the 20 to 29-year-old age class was estimated using a linear projection of the NHANES III and NHANES 1999–2002 data. The proportion of 20 to 29-year-olds with diabetes in each race/ethnic group was held constant at the proportions found in the NHANES 1999–2002 data at the later time intervals. Mortality among individuals with diabetes 20 The NHANES II mortality cohort is based on a sample of individuals aged 30 to 75, whereas we made diabetes estimates on individuals aged 20 years and older. Consequently, we assumed no deaths due to diabetes in the 20 to 29-year-old age group over the 10-year period. Population estimates 22 Analysis In an effort to provide an estimate of future trends in diabetes and the population at high risk of developing diabetes, we employed the following procedure. We used the NHANES III data to fit a model to predict total diabetes in the NHANES 1999–2002. We used this strategy prior to making future projections, because it allowed us to develop and fit the model to an existing national estimate of diabetes prevalence. Because both the NHANES III and the NHANES 1999–2002 are based on multi-year data collection, we estimated a mid-point of 1991 and 2001 for the two surveys. The number of persons with diabetes 10 years post-baseline was calculated for 10-year age classes by first adding baseline prevalence and incidence (the number of low-risk and number of high-risk persons who developed diabetes over the 10-year interval), then adding persons with diabetes who immigrated to the United States, and persons with diabetes who moved into the 20 to 29-year-old age class, and finally subtracting the number of diabetic subjects who died. Percentage of persons with diabetes was estimated for each time period by taking the total number of persons with diabetes and dividing by the expected total population, then multiplying by 100. Varying model assumptions 10 12 In addition, to address the potential impact on mortality of healthcare interventions in management of diabetes, we examined potential reductions of 10, 20 and 30% in mortality among individuals with diabetes. Finally, we computed a model examining a combination of effects, assuming that lifestyle interventions would yield a 10% decrease of persons at high risk and healthcare interventions would yield a 10% decrease in mortality of persons with diabetes. Results 1 Table 1 Number of people (in millions of persons) with and prevalence of diabetes by year and age category Age (in years) NHANES III NHANES 1999–2002 2011 2021 2031 20–29 Number 0.2 0.4 0.5 0.7 0.9 Percentage 0.5 1.0 1.3 1.7 2.0 30–39 Number 0.6 1.6 1.8 2.1 2.3 Percentage 1.4 3.7 4.7 5.0 5.2 40–49 Number 1.5 3.5 4.3 4.3 4.9 Percentage 4.5 8.1 10.3 11.1 11.2 50–59 Number 2.5 3.9 6.6 7.2 6.9 Percentage 11.4 12.2 15.6 17.7 18.2 60–69 Number 3.1 4.1 6.5 9.8 10.2 Percentage 15.3 19.8 22.0 25.1 26.6 >70 Number 3.2 4.1 5.7 8.4 12.4 Percentage 16.2 17.4 20.5 22.6 23.9 Total Number 11.1 17.5 25.4 32.6 37.7 Percentage 6.3 8.8 11.5 13.5 14.5 NHANES 1 2 Discussion 23 5 9 3 24 25 7 7 The results have several implications for the delivery of healthcare and healthcare financing. First, we estimated our models under several assumptions for the number of individuals at high risk of diabetes in the population. Regardless of these assumptions, the US will have a substantial number of individuals at high risk of diabetes in 2011, 2021 and 2031. Interventions to modify lifestyle are critical to decrease the number of individuals at high risk, and consequently to lower the expected increase in diabetes in the future. Although some of the diabetes estimates suggest seemingly small decreases in future prevalence, based on decreases in the population at risk, the actual numbers are substantial. For example, a one-percentage point drop in the US population estimate of diabetes among individuals aged 20 and older in 2031 is quite substantial and would account for a decrease in prevalence of diabetes equivalent to 2,600,000 people. Second, the projection that a substantial proportion of the population will have diabetes indicates greater spending will be necessary to manage the disease. This will include spending on drugs, ongoing monitoring, and treating of complications including nephropathy, retinopathy, and cardiovascular disease. 26 18 18 In summary, a continued focus on effective interventions for lifestyle modifications to decrease diabetes risk, as well as vigilant ascertainment of diabetes, appears crucial if the future prevalence and burden of diabetes in the US population are to be adequately addressed. This is especially important for minority populations, particularly the Hispanic community, which is projected to have an overwhelming future diabetes burden. Considering that minorities have historically had limited access to healthcare, these findings emphasise the importance of interventions targeting these populations. Electronic supplementary material Below is the link to the electronic supplementary material.
 Table 1 a Table 2 Projections, using different assumptions, of diabetes prevalence among individuals aged >20 years in 2031 (32 kb)