1 Division of Diabetes Translation, National Center for Chronic Disease Prevention and Health Promotion, Centers for Disease Control and Prevention, Atlanta, GA.
2 Division of Adult and Community Health, National Center for Chronic Disease Prevention and Health Promotion, Centers for Disease Control and Prevention, Atlanta, GA.
Received for publication December 9, 2003; accepted for publication March 18, 2004.
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ABSTRACT |
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blood pressure; cardiovascular diseases; cholesterol; diabetes mellitus; risk factors; smoking
Abbreviations: Abbreviation: NHANES, National Health and Nutrition Examination Survey.
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INTRODUCTION |
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In the general population, reductions in serum cholesterol, blood pressure, and cigarette smoking may partly explain the decline in heart disease mortality (1, 6, 7). Among persons with diabetes, however, data on the national prevalence of these major risk factors for cardiovascular disease are scarce. Thus, we assembled data from the National Health and Nutrition Examination Survey (NHANES) I (19711974), NHANES II (19761980), NHANES III (19881994), and the first 2 years of continuous NHANES (19992000), allowing us to examine whether cardiovascular disease risk factors have improved in people with diabetes.
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MATERIALS AND METHODS |
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Measures
Each NHANES consists of a detailed standardized medical examination in a mobile examination center and an interview to obtain information on sociodemographic variables. Weight and height are measured with standardized techniques and equipment in the mobile examination center and used to compute body mass index (kg/m2). For this analysis, participants were categorized as White or Black for NHANES I and NHANES II and as non-Hispanic White, non-Hispanic Black, or Mexican American for NHANES III and NHANES 19992000; other racial or ethnic groups, including "Hispanic other" in NHANES 19992000, were included in the White category. We excluded persons aged more than 74 years because persons in that age group were not sampled in the first two surveys. The response rates among adults for the four surveys were 70, 69, 77, and 76 percent, respectively.
Diagnosed diabetes was defined as a positive response to the question, "Have you ever been told by a doctor that you have diabetes or sugar diabetes?" In the NHANES III and NHANES 19992000, women who reported being diagnosed only during pregnancy were not considered to have diabetes; this information was not available for the earlier two surveys. Three cardiovascular disease risk factors were analyzed: serum total cholesterol concentration, systolic and diastolic blood pressure, and smoking status.
Cholesterol measurement
For the first three surveys, a detailed description of the procedures used for blood sample collection and measurement of total cholesterol has been published (14). For NHANES 19992000, a description of blood collection and measurement of total cholesterol is provided in the NHANES Laboratory/Medical Technologists Procedures Manual (12). The samples were frozen at 20°C and shipped weekly on dry ice to the laboratory conducting the lipid analyses. The NHANES I measurements were made in the Centers for Disease Control and Prevention lipid standardization laboratory (15). The NHANES II, NHANES III, and NHANES 19992000 cholesterol measurements were made in laboratories of lipid research clinics that were standardized for cholesterol measurements using the criteria of the Centers for Disease Control and Prevention-National Heart, Lung, and Blood Institute Lipid Standardization Program (1620). In all surveys, serum total cholesterol was measured regardless of fasting status. Missing data for total cholesterol ranged from 11 percent to 1 percent. We defined high total cholesterol concentrations as serum total cholesterol of 5.17 mmol/liter or more (200 mg/dl), regardless of use of lipid-lowering medications.
Blood pressure measurement
As described previously (21), differences in protocol across the four surveys included the number of blood pressure measurements, the temporal sequence of blood pressure measurements within the entire examination, types of staff who performed the measurements, whether blood pressure was measured in both the seated and supine positions, and whether blood pressure measurements were performed in the home in addition to the mobile examination center. In our analysis, we used only blood pressure measurements conducted in the mobile examination center with subjects in a seated position. When more than one blood pressure measurement was available, we calculated a mean blood pressure. Our estimates of mean blood pressure are based on up to two blood pressure measurements for NHANES I and NHANES II, up to three blood pressure measurements in NHANES III, and up to four blood pressure measurements in NHANES 19992000. Because the accepted definition of high blood pressure changed across the years of the surveys (i.e., 19712000) (2224), we defined high blood pressure levels as a mean systolic blood pressure of 140 mmHg or more and/or diastolic blood pressure of 90 mmHg or more regardless of self-reported history of hypertension and use of antihypertensive drugs.
Smoking behaviors
To determine current cigarette smoking, we asked respondents the questions, "Have you smoked at least 100 cigarettes in your entire life?" and "Do you smoke cigarettes now?". Current smoking was defined by a positive answer to both questions. In NHANES I, data on smoking behaviors were collected for a sample of adults aged 2574 years. For this reason, we restricted the analysis of smoking to the group aged 2574 years for all four surveys. Missing data ranged from 4.0 percent to 1.7 percent.
Statistical analysis
Statistical analyses used SAS for Windows software (25) for data management, SUDAAN software (26) to obtain point estimates and standard errors, and WinBUGS software (27) to fit models. Sampling weights were used to produce national estimates that accounted for the complex survey design. For comparisons across surveys, data were age adjusted by the direct method to population totals from the 2000 US Census.
Trends in the age-adjusted estimates of cardiovascular disease risk factors were assessed using a normal hierarchical Bayesian model with noninformative prior distributions on model hyperparameters (28). A Bayesian approach to modeling was preferred because inference does not depend on asymptotic assumptions. In addition to accounting for variability within survey measurements, Bayesian hierarchical models account for variability between surveys. Variability between surveys, such as methodological changes, is accounted for by incorporating random effect parameters into the model.
Our model has two levels. At the first level, we specify a probability distribution for the observed data. Letting denote the estimate for a cardiovascular disease risk factor determined from survey i and
i2 denote the variance for
estimated from SUDAAN software, we assume
The survey means i are assumed to arise from a linear model
i =
+ ß(ti t1) + Zi,
where ti represents the midpoint year of survey i, represents the mean value of the risk factor from the first survey (intercept), and ß represents the annual change in the risk factor over time. The random effect Zi represents the variability in
i remaining after accounting for the linear trend, and it is assumed to be distributed N(0,
2).
At the second level of our model, we assign probability distributions to each hyperparameter (, ß,
). Normal priors with variances of 10,000 are assigned to
and ß, while
1 is approximated with a
(0.001, 0.001). Combining the prior probabilities specified at the second level with the probability distribution for the observed data specified at the first level, we express knowledge about model parameters through the posterior distribution derived using Bayes theorem. Random draws from this posterior distribution are generated through an iterative Markov chain Monte Carlo simulation known as the Gibbs sampler.
We simultaneously ran three Gibbs samplers and saved 50,000 of the 60,000 iterations from each. Model convergence was assessed visually by inspecting trace plots of sampled values versus iteration number and inspecting the Gelman-Rubin statistic for each iteration. We used the mean of the posterior distribution of ß as our point estimate. Significant decreases in cardiovascular disease risk factors were defined by 95 percent Bayesian credible intervals for ß that did not contain zero. In addition, we estimated the probability that ß is negative. A significant decrease was defined as a probability greater than 97.5 percent. Because there is particular interest in recent declines in the prevalence of cardiovascular disease risk factors, we also obtained the Bayesian estimate for the probability of a decrease between the two most recent surveys (NHANES III and NHANES 19992000) for each factor. Separate Bayesian models were fit for the overall population, men, women, and persons aged 2059 and 6074 years.
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RESULTS |
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From 1971 to 2000, the percentage of persons with high blood pressure decreased by 1 (95 percent credible interval: 2.7, 1.1) percentage point per year (a probability of decrease of 93.4 percent) (figure 1). The proportion of men with high blood pressure decreased from 60 percent to 31 percent (a probability of decrease of 95.0 percent), and the proportion of women with high blood pressure decreased from 67 percent to 43 percent (a probability of decrease of 89.4 percent). Between 1988 and 2000, little change was seen in the proportion of persons with high blood pressure for either sex or any age group.
Trends in smoking
The age-adjusted prevalence of cigarette smoking among persons with diabetes dropped 15 percent (figure 1), with a median annual change of 0.5 (95 percent credible interval: 1.4, 0.6) percentage point per year (a probability of decline of 89.5 percent). The magnitude of decrease was greatest for men, but it was limited to NHANES I through NHANES III (table 3). Among women, the decline occurred over all four surveys (table 4).
Trends in multiple risk factors
The age-adjusted percentages of persons with diabetes and at least one, at least two, or all three cardiovascular disease risk factors (high total cholesterol, high blood pressure, or current smoking) were also estimated. Over the study period, the proportion of persons with diabetes who had at least one risk factor decreased from 92 percent to 74 percent, with the greatest decline between NHANES III and NHANES 19992000 (from 81 percent to 74 percent; a probability of decrease of 96.6 percent) (data not shown). The percentage of those with two risk factors also decreased steadily from 19711974 to 19992000, with a median change of 1.0 (95 percent credible interval: 2.1, 0.1) percentage point (data not shown). The percentage of persons with all three risk factors was 15 percent in NHANES I but fell to 3 percent in NHANES III and NHANES 19992000 (data not shown).
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DISCUSSION |
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Recent data for the US general population have indicated a reduction in the temporal declines in mean cholesterol concentrations for both men and women (32, 33). In contrast, we found that, among people with diabetes, total cholesterol continued to decline from 19881994 to 19992000, especially in men and in younger as well as older adults. In women, cholesterol values were stable up to 19881994 and then declined. The overall improvement in recent years may reflect the aggressive promotion of lipid control in persons with diabetes, following the results of clinical trials of lipid-lowering drugs published in the 1990s and the effort of the National Cholesterol Education Program (29, 3438).
In 19992000, half of the adults with diabetes had total cholesterol concentrations above the level recommended by the National Cholesterol Education Program (5.17 mmol/liter or 200 mg/dl) (38). Our results underscore the need to enforce current recommendations to consider diabetes a risk factor for heart disease and to aggressively treat lipid abnormalities in people who have diabetes. We note that, even if the decline in the proportion of adults with diabetes having high total cholesterol levels would continue at 0.6 percentage point per year (the annual decrease we found for the entire study period), 49 percent of people with diabetes will still have high cholesterol levels in 2010.
Findings from randomized trials have demonstrated the benefits of tight blood pressure control to protect against renal disease and cardiovascular events in patients with diabetes (30, 39, 40). For this reason, more stringent criteria for blood pressure control have been suggested for this group (24, 41). Even so, we found that the proportion of persons with blood pressure above 140/90 mmHg did not decrease between 19881994 and 19992000 and, at the last survey, a substantial proportion of US adults with diabetes still had a blood pressure measurement above 140/90 mmHg. More recently, the recommended blood pressure for people with diabetes has been lowered to 130/80 mmHg (41); on the basis of this new criterion, an even greater proportion of persons with diabetes will be candidates for intervention.
The favorable changes in the major cardiovascular disease risk factors among adults with diabetes have occurred alongside a dramatic increase in mean body mass index. Like diabetes, obesity is associated with insulin resistance, hypertension, lipid abnormalities (42), and cardiovascular disease mortality (43). Thus, one would have expected results the opposite of what we obtained. It is possible that the shift in body mass index reflects an increase in detecting undiagnosed diabetes, because people are at an earlier stage and have less severe disease. Similarly, changes in the diagnostic criteria for diabetes in the most recent years and increased screening may help to identify individuals with a less severe form of the disease, who would likely have a better cardiovascular disease risk profile. If true, this could in part explain the downward trend in risk factors.
Despite the favorable trends in the three major risk factors for cardiovascular disease, a national study found that heart disease mortality had declined to a lesser degree among adults with diabetes than in the US general population (5). There are a few potential explanations for this. First, many risk factors other than the three we studied are associated with cardiovascular disease mortality in people with diabetes. For example, low high-density lipoprotein cholesterol, high triglycerides, abnormal low density lipoprotein particle size, hyperinsulinemia and insulin resistance, coagulation abnormalities, inflammation, and physical inactivity all can increase susceptibility to atherosclerosis in diabetes (44). Second, it may require additional time before a decline in cardiovascular disease risk factors is reflected in changes in cardiovascular disease mortality. Finally, people with diabetes may derive less benefit from the medical treatment of cardiovascular disease. For example, they experience worse outcomes of coronary revascularization than do persons without diabetes (45) and have an adverse long-term prognosis after myocardial infarction (46). Moreover, among patients with coronary artery disease, those with diabetes are less often tested or treated for dislipidemia (47).
Our study had some limitations. First, the sample size of people with diabetes, especially in the most recent survey, limited the number of stratified subgroup analyses that we could conduct. Second, differences in participation rates by race/ethnicity may have varied over time, which may have influenced the findings. We were unable to examine this in our analyses, because of the fact that the number of people with diabetes and non-White race was too small to provide reliable or meaningful inferences and because information on race/ethnicity was not collected uniformly across the four surveys. However, we conducted follow-up sensitivity analyses in which we estimated cardiovascular disease risk factor levels across the four surveys only in non-White participants. We found that trends among non-Whites were of similar magnitude to that observed in the overall sample, but the standard errors were very large because of the small sample size. Third, given the lack of data from the earliest survey, we could not assess temporal trends in high density lipoprotein cholesterol, low density lipoprotein cholesterol, and triglycerides. Fourth, some of the methodological changes that occurred across the four surveys may have influenced our estimates, especially for blood pressure levels. Our choice of analytical approach reduces the effect of these changes on our estimates; thus, we believe that most of the observed decline in cardiovascular disease risk factors is real.
In conclusion, between 1971 and 2000, cardiovascular disease risk factors among people with diabetes declined. The biggest declines were among men and older people. Although we have strong evidence that control of blood pressure, lipids, and smoking can reduce cardiovascular disease mortality among people with diabetes, one of two people with diabetes still has a total cholesterol concentration above the recommended targets, one of three has high blood pressure, and one of six is a smoker. Considerable opportunity to tackle cardiovascular disease risk in people with diabetes still exists.
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NOTES |
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REFERENCES |
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