In this study, we used demographic techniques and large national datasets to examine changes in diabetes-free life expectancy between 1980–1989 and 2000–2004 for adult men and women in the U.S. and to examine the changing role of body mass for diabetes risks. These methods offer the advantage of allowing us to examine the changing incidence of diabetes while holding the improvements in mortality during the period constant. They also allow us to estimate the contributions of changing diabetes incidence rates for different levels of body weight to overall diabetes-free life expectancy, which makes it possible to better understand the observed changes in overall diabetes risks and the differing risks in the population.
We found that the proportion of 18-year-olds who would develop diabetes in their lifetimes increased by almost 50% among women and almost doubled among men between the 1980s and the 2000s. While life expectancy for men and women in the U.S. increased, diabetes-free life expectancy at age 18 decreased by 1.7 years for women and 1.4 years for men, indicating declines in the proportion of the lifetime spent without diabetes. This pattern points to the emergence of a prolonged period of morbidity rather than longer healthy life spans, but only among obese individuals. Obese individuals experienced the greatest losses in diabetes-free life expectancy. In 2000–2004, obese 18-year-old men and women could expect to live 13.7 and 19.1 fewer years without diabetes, respectively, compared with normal/underweight 18-year-old men and women. Increases in diabetes incidence among obese people led to reductions in diabetes-free life expectancy so large that the overall diabetes-free life expectancy of the adult U.S. population decreased in spite of improvements in diabetes-free life expectancy among nonobese individuals, who represent the majority of the population.
In the main models, we standardized mortality across BMI groups to isolate the effect of changes in diabetes incidence. The approach allowed us to determine that changes in diabetes incidence rather than mortality were responsible for the decrease in diabetes-free life expectancy. It also allowed us to identify the ages at which the greatest changes in diabetes incidence occurred. Sensitivity analyses show that our results are robust to adjusting mortality rates by diabetes and weight status to the rate ratios reported in the literature.
Since our estimates did not directly model transitions between BMI groups, we have not shown the effect of gaining or losing weight on the expected life span without diabetes, as was also the case for previous studies (20
). The measures yield cumulative estimates of diabetes-free life expectancy for people starting at a given BMI level at age 18. Because the BMI composition of cohorts tends to change over time, with people gaining weight as they age, we also relaxed this assumption to account for weight increases by estimating diabetes-free life expectancy for each BMI group at age 40 and 60. Shifts in the BMI composition of the population likely have reinforced changes in diabetes risks within BMI categories; that is, as the proportion of the population who is obese has increased, higher incidence rates of diabetes seen among heavier individuals affect a larger proportion of the population, leading to increases in diabetes cases.
There are several possible explanations for our finding that the decreases in diabetes-free life expectancy have been borne almost exclusively by the obese population. One explanation is that rates of diabetes detection have changed, specifically among obese individuals. Robustness checks indicate that changes in undiagnosed diabetes in obese patients relative to others explain part but not all of the reductions in diabetes-free life experienced by this group. Another reason may be changes in weight distributions within BMI groups, because our analyses indicate that weight increases were greatest among obese men and women: the average obese American was >3.4% heavier in the 2000s than in the 1980s. Since diabetes risk increases with BMI (2
), that the average obese person was heavier in the 2000s explains some of the increases in diabetes risk. Another consideration could be changes in the racial composition within each BMI category, since previous studies showed that the prevalence of diabetes has increased more among non-Hispanic blacks and Hispanics relative to non-Hispanic whites in the normal and overweight categories but less in the obese category (21
). NHIS data show that the change in racial composition of BMI groups was lowest among obese individuals, leading to a relatively higher proportion of whites in the obese category. Thus, the racial distribution in the BMI categories is not likely to explain the observed trends. Another possible explanation is that diabetes risks are higher with younger age of obesity onset; indeed, the prevalence of obesity has tripled among children since the 1970s (22
). Our estimates are not conditional on BMI level before the age to which they pertain, so earlier onset of obesity could explain some of the increases in diabetes risk for obese individuals.
A limitation of this study is our reliance on self-reported data on diabetes diagnosis, weight, and height, which have been shown to be biased (23
) and may be systematically different by sex, ethnicity, and age (24
). Our sensitivity analyses accounted in part for misreporting of diabetes status. In addition, NHIS data are collected via rigorous in-person interviews, and the differences in reported and measured BMI are not large and do not affect health risk estimates, including those associated with diabetes (24
). Furthermore, in spite of this limitation, the NHIS is the only dataset with sufficient sample size to conduct this analysis.
Since 34% of the U.S. adult population is obese (25
), these findings entail that more than one-third of Americans can expect to develop diabetes, even if they reached adulthood without diabetes; this proportion stands at approximately two-thirds of obese men and women. These estimates suggest a large future increase in the prevalence of diabetes and its complications, especially among obese individuals. Our results suggest that in the face of budgetary or logistic constraints, new efforts to prevent diabetes can have the greatest impact among obese individuals, because those who are not obese generally have experienced decreases in risks during the past 2 decades with current prevention efforts.
This study highlights the growing prevalence of diabetes, implying greater future health care demand at younger ages and for longer life spans. This will necessitate medical and public health professionals with training in diabetes management and facilities well equipped for the management of diabetes and treatment of comorbidities among obese individuals. The many dimensions of changing population and individual health must be well understood and tracked with methods such as those demonstrated in this study to improve health care planning and health.