The UCLA-HFT is a microsimulation model calibrated to represent demographics and population health, including health behaviors and disease outcomes, for county-, city-, and state-level populations in the United States. Age, race, and sex are set as demographic predictors of one's diabetes incidence, whereas cigarette smoking, physical activity, and body mass index (BMI) are set as behavioral risk factors that affect both the diabetes onset for people without diabetes and the diabetes case fatality for people with diabetes. Details of this model have been described elsewhere (10
We used the UCLA-HFT to forecast prevalence of type 2 diabetes among California's adult population under different scenarios in 2020 (). The first scenario (the baseline scenario) built on recent observations that rates of childhood overweight and obesity have been leveling off in California and nationwide (11
), assuming no further increase in obesity rates for the cohorts entering adolescence (defined as being aged 12 to 17 years) after 2003, when obesity rates first started leveling. In other words, in this baseline scenario, all cohorts that reach age 12 years after 2003 have the same BMI distribution as the cohort that turned age 12 in 2003.
Assumptions of Future Obesity and Physical Activity Trends for the 4 Simulation Scenarios, California
The second scenario (called "childhood BMI decrease") was built on the observation that a considerable decline in rates of overweight and obesity in California has occurred among young children but not yet among adolescents (11
). This scenario assumed a constant annual decrease in the BMI for children entering adolescence after 2010, until the 12-year-olds in 2028 have the same BMI distribution as 12-year-olds in 1985. In other words, we modeled a gradual return of obesity rates to the 1985 level by assuming a small annual reduction of BMI for every new cohort of 12-year-olds. Thus, the mean BMI decline for each subsequent cohort entering adolescence from 2010 to 2028 will be equivalent to the annual increase in mean BMI that was observed from 1985 to 2003. In our model, this cohort effect was modeled as a trend in the mean of the inverse of BMI, following other published research (13
), increasing by 3.1 × 10-4
annually for boys aged 12 and increasing by 4.5 × 10-4
annually for girls aged 12. This increase in the mean of the inverse of BMI translates into an annual decrease in BMI of approximately 0.19 kg/m2
for boys with an initial BMI of 25.0 kg/m2
and a decrease of 0.18 kg/m2
for girls with an initial BMI of 25.0 kg/m2
. The rate of decrease is higher for those with higher BMI initially because we modeled a mean shift in the inverse of BMI.
The third scenario (called "childhood and adult BMI decrease") made a more optimistic assumption for obesity control, assuming that annual BMI decline occurs not only among new cohorts of 12-year-olds every year (as in the second scenario), but also among adults aged 18 to 65 years who are overweight or obese. This scenario was simulated by assuming an annual BMI decrease for overweight and obese people from 2010 to 2028. The average annual BMI decrease was further assumed to be equivalent to the average annual BMI increase from 1985 to 2003. In our model, this was implemented as a stochastic increase in inverse BMI for people with a BMI more than 25.0 kg/m2 in the year. The inverse BMI increase is normally distributed with a mean of 1.4 × 10-4 and a standard error of 0.6 × 10-4 for men and a mean of 2.0 × 10-4 and a standard error of 0.9 × 10-4 for women. This translates into a mean annual BMI decrease of 0.13 kg/m2 for men with an initial BMI of 30.0 kg/m2 and 0.18 kg/m2 for women with an initial BMI of 30.0 kg/m2, with a larger decrement for those with a higher initial BMI.
The fourth scenario (called "BMI decrease with increase in PA levels") built on the third scenario by further assuming an increase of physical activity levels such that racial/ethnic disparities in physical activity levels are eliminated, an objective specified by public health professionals (15
). As estimated from the Behavioral Risk Factor Surveillance System of the Centers for Disease Control and Prevention, non-Hispanic whites (50%) are more likely to meet the federal guideline (16
) of at least 150 minutes per week of moderate-intensity physical activity or 75 minutes per week of vigorous-intensity physical activity than other racial/ethnic minority groups (African Americans, 37%; Latinos, 37%; Asians, 40%) (17
Under this fourth scenario, starting from 2011, for each sex-age stratum all racial/ethnic subpopulations achieve the same physical activity level as the most active subpopulation in 2010.
We compared 4 simulated outcomes across the scenarios: type 2 diabetes prevalence among adults, obesity prevalence among adults, obesity prevalence among people with diabetes, and life expectancy at birth.