A recent study published in Diabetes Care, a journal from the American Diabetes Association (ADA), sought to determine whether all-cause and cardiovascular disease (CVD) death rates declined between 1997 and 2006, a period of continued advances in treatment approaches and risk factor control, among U.S. adults with and without diabetes.
The study found that among diabetic adults, the CVD death rate declined by 40% (95% CI 23–54) and all-cause mortality declined by 23% (10–35) between the earliest and latest samples. There was no difference in the rates of decline in mortality between diabetic men and women. The excess CVD mortality rate associated with diabetes (i.e., compared with nondiabetic adults) decreased by 60% (from 5.8 to 2.3 CVD deaths per 1,000) while the excess all-cause mortality rate declined by 44% (from 10.8 to 6.1 deaths per 1,000).
The authors concluded that “Death rates among both U.S. men and women with diabetes declined substantially between 1997 and 2006, reducing the absolute difference between adults with and without diabetes.” Nevertheless, the authors warned that diabetes prevalence is likely to rise in the future if diabetes incidence is not curtailed.
Background
Diabetes has been associated with an average 10 years of life lost for individuals diagnosed during middle age. Fortunately, numerous evidence-based interventions exist, ranging from glycemic and cardiovascular disease (CVD) risk factor control to early screening for diabetes complications. These have been paralleled by population-wide improvements in glycemic control, CVD risk factors, and rates of several diabetes complications. Despite these improvements, it remains unclear whether longevity has increased uniformly among diabetic populations.
Analyses of consecutive cohorts of the U.S. population from the 1970s through the 1990s, however, found that all-cause and CVD death rates declined among diabetic men but not diabetic women. However, no national studies have examined mortality trends among the U.S. diabetic population since the 1990s, and the intervening years have been a period of continued advances in treatment approaches and risk factor levels. Newly available mortality follow-up data linked to the National Health Interview Survey (NHIS) provide a unique opportunity to determine whether CVD and all-cause mortality has improved among the U.S. population during recent decades as well as whether the excess mortality associated with diabetes has declined.
Research and Design Methods
The authors compared 3-year death rates of four consecutive nationally representative samples (1997–1998, 1999–2000, 2001–2002, and 2003–2004) of U.S. adults aged 18 years and older using data from the National Health Interview Surveys (NHIS) linked to National Death Index. The NHIS is an ongoing survey of the health status, health care access, and behaviors of the U.S. civilian noninstitutionalized population conducted by the National Center for Health Statistics (NCHS).
The authors used data from 242,383 (approximately 30,000 per year) adults aged 18 years and older (one randomly selected from each family to receive additional NHIS questions) from the survey years 1997–2004 whose data were linked to the National Death Index, a computer database of all deaths in the U.S. compiled by the NCHS. Approximately 89% of all participants’ data (range of 86–93% across survey years) were considered adequate for accurate linkage. All NHIS surveys undergo human subjects oversight and participants give informed consent. More detailed descriptions of the NHIS design have been published elsewhere.
Measurements
Interviewers assessed diabetes status by asking participants if a doctor or other health professional had ever told them that they had diabetes or sugar diabetes and the number of years since diagnosis. In addition, respondents were queried for age, race/ethnicity, sex, education, family income, history of CVD, and self-reported height and weight, which were used to compute BMI (kg/m2). Income was used to compute the poverty index ratio, an index of income assessed in relation to need, with a score of 1 representing the official federal poverty threshold, a score of <1 indicating a relative level of poverty, and a score of >1 representing income as a multiple of the poverty level.
The study’s primary objective was to examine whether death rates in later samples of persons with diabetes were different from those of earlier samples. Secondary objectives examined whether changes in mortality over time differed between the diabetic and nondiabetic cohorts and between various age, sex, race, and socioeconomic subgroups of the diabetic population.
Results
Among the population with diabetes, there are consistent increases over time in the levels of education, income, and obesity and a decrease in the proportion of smokers, sedentary behavior, and difficulty walking. There were no significant changes in age, race/ethnicity, history of CVD, or diabetes duration. Demographic trends over time were similar for the nondiabetic population, except that there was also a slight increase in age and the number of Hispanics.
Among the population with diagnosed diabetes, 3-year CVD death rates declined by 4.0 deaths per 1,000 person-years from the 1997–1998 sample (9.5 per 1,000 person-years) to the 2003–2004 sample (5.6 per 1,000 person-years). In multivariate analyses adjusting for age, sex, race/ethnicity, and diabetes duration, diabetic adults in the most recent sample (2003–2004) had 40% lower CVD mortality and 23% lower all-cause mortality than people in the earliest sample (1997–1998). There were no significant changes in the rates of cancer mortality among persons either with or without diabetes. In sensitivity analyses excluding those with less than 2 years of diabetes duration from the most recent sample, there was essentially no difference in the rate ratios associated with later cohorts.
CVD death rates declined among the nondiabetic population as well, but the magnitude of decline was weaker (from 3.7 to 3.3 deaths per 1,000) than that observed for diabetic adults and there was not a significant decline in all-cause mortality. As a result, the excess CVD death rate associated with diabetes (i.e., compared with those without diabetes) declined from 5.8 to 2.3 deaths per 1,000, and the all-cause death rate difference between people with and without diabetes declined from 10.8 to 6.1 deaths per 1,000.
All-cause and CVD death rates in the diabetic population declined among both men and women. However, there was a slightly greater magnitude of decline among men (5.2 deaths per 1,000 decline for men vs. 3.5 per 1,000 for women). These trends for both men and women again paralleled less dramatic reductions in the nondiabetic population, resulting in reductions in the excess CVD mortality from 7.5 to 2.5 deaths per 1,000 for diabetic men and from an excess 4.8 to 1.8 deaths per 1,000 women.
Conclusions
This analysis of nationally representative samples of adults with and without diabetes reveals impressive reductions in CVD and all-cause mortality between 1997 and 2006. The rates of improvement among those with diabetes have exceeded those of the nondiabetic population, resulting in more than a 50% reduction of the excess death rates that have been repeatedly attributed to diabetes. Although excess mortality risk remains high—about 2 deaths per 1,000 due to CVD and about 6 all-cause deaths—this excess risk is now considerably lower than previous reports and consistent with improvements in several risk factors, complications, and indicators of medical care and representative of gradual, ongoing improvement in health for people with diagnosed diabetes.
Improvements were observed approximately equally in women and men, which contrasts with earlier analyses. The findings of improved life expectancy support recent regional studies in the U.S., including North Dakota, Framingham, and Minnesota, as well as population-based studies in Ontario, Denmark, Scotland, Norway, and Finland. These trends parallel other improvements in levels of risk factors and rates of complications among the overall U.S. diabetic population.
In addition, steady improvements in quality and organization of care, self-management behaviors, and medical treatments, including pharmacological treatment of hyperlipidemia and hypertension, could each have contributed to reductions in death rates. Incidence of lower extremity amputation, end-stage renal disease, and CVD hospitalization have each declined steadily. Reductions in mortality are likely to be influenced by multiple factors, however, and thus may lag behind declines in specific risk factors.
Although the NHIS provides the largest nationally representative cohort data with diagnosed diabetes, the findings were limited by reliance on self-report to define diabetes; at least 20% of cases with diabetes are undiagnosed. Because the fraction of diabetes cases that remain undiagnosed may be decreasing, later cohorts of diabetes could be enriched with people who had their diabetes detected earlier, possibly contributing to lower mortality.
Ultimately, the authors call for vigilant efforts to prevent vascular and neuropathic complication and early mortality associated with diabetes along with efforts to reduce diabetes incidence will continue to be major demands into the future.”