Diabetes Care 30:1241-1247, 2007 DOI: 10.2337/dc06-2558 © 2007 by the American Diabetes Association
Incidence of Coronary Heart Disease in Type 2 Diabetic Men and WomenImpact of microvascular complications, treatment, and geographic location
1 Division of Metabolic Diseases, University of Padua, Padua, Italy Address correspondence and reprint requests to E. Ferrannini, MD, Department of Internal Medicine, via Roma, 67, I-56100 Pisa, Italy. E-mail: ferranni{at}ifc.cnr.it
OBJECTIVECardiovascular disease (CVD) is the main cause of morbidity/mortality in diabetes. We set forth to determine incidence and identify predictors (including microvascular complications and treatment) of first coronary heart disease (CHD) event in CVD-free type 2 diabetic patients. RESEARCH DESIGN AND METHODSA cohort of 6,032 women and 5,612 men, sampled from a nationwide network of hospital-based diabetes clinics, was followed up for 4 years. Baseline assessment included retinopathy, nephropathy, and foot ulcers. First CHD events (myocardial infarction, coronary artery bypass grafting, percutaneous transluminal coronary angioplasty, and electrocardiogram-proven angina) were analyzed for 29,069 person-years. RESULTSThe age-standardized incidence rate (per 1,000 person-years) of first CHD event (n = 881) was 28.8 (95% CI 5.432.2) in men and 23.3 (20.226.4) in women. Major CHD (myocardial infarction, coronary artery bypass grafting, and percutaneous transluminal coronary angioplasty) was less frequent in women (5.8 [4.37.2]) than in men (13.1 [10.915.4]; a sex ratio of 0.5 [0.40.6]). Incidence rates of all outcomes were higher in patients with microvascular complications (for major CHD, age-adjusted rate ratios were 1.6 [1.22.21] in men and 1.5 [1.02.2] in women). By multivariate Cox analysis, age and diabetes duration were risk predictors common in both sexes. In men, glycemic control and treated hypertension were additional independent risk factors, but residing in the south was associated with a significant 29% risk reduction. In women, higher triglycerides/lower HDL cholesterol and microvascular complications were independent risk factors. CONCLUSIONSIn CVD-free patients with type 2 diabetes, risk of first CHD event depends on sex, geographic location, and presence of microvascular disease. Hyperglycemia and hypertension, particularly in men, and diabetic dyslipidemia, especially in women, are risk factors amenable to more aggressive treatment.
Abbreviations: AMI, acute myocardial infarction CHD, coronary heart disease CVD, cardiovascular disease OHA, oral hypoglycemic agent WHO, World Health Organization
Diabetes is estimated to be responsible for 5.2% of all deaths (1). Since the Framingham Study (2), epidemiology has consistently shown that diabetes confers an increased risk for coronary heart disease (CHD) and cardiac mortality (36). Salient features of this association are the following: 1) relative risk of CHD (7) and fatal CHD (8) is higher in diabetic women than in diabetic men, 2) classical and diabetes-related risk factors both contribute to total CHD risk (6), and 3) insulin treatment may be associated with a worse cardiovascular prognosis (9,10). The reasons for the excessive relative CHD risk in diabetic women compared with diabetic men are not completely understood. In the Strong Heart Study (11), the greater risk for cardiovascular disease (CVD) in women was explained in part by an apparent larger negative impact of diabetes on CVD risk factors. With regard to diabetes-related risk, the World Health Organization (WHO) multinational study found that in type 2 diabetic patients, proteinuria and retinopathy were independent predictors of CVD mortality, fatal and nonfatal acute myocardial infarction (AMI), and stroke (6). Finally, the adverse prognostic value of insulin treatment has traditionally been ascribed to the presence of more advanced disease (12). However, as plasma insulin levels per se have been reported to be significant, if weak independent CVD predictors (12), the possibility that exogenous hyperinsulinemia may counter the beneficial effects of insulin-induced metabolic control on CVD risk has not been ruled out conclusively. Estimates of CHD incidence in diabetic patients vary across studies and countries. Source data are remarkably heterogeneous with regard to selection criteria and risk assessment, and few observational studies provide information on the natural course of CHD in patients who periodically refer to hospital-based outpatient clinics. Furthermore, the natural history of CVD in diabetes is changing. In a recent analysis of the Framingham original and offspring cohorts (13), incident CVD among adults with diabetes was reported to have halved between the examinations in 19591966 and 19771995. In the present study, we set forth to determine the incidence of first CHD events in a large, recent cohort of type 2 diabetic patients who are regularly followed at outpatient clinics and to identify risk factors that are associated with CHD burden, including presence of microvascular complications and pharmacological treatment.
The DAI (Diabetes and Informatics Study Group, Association of Clinical Diabetologists, Istituto Superiore di Sanità) is an observational study of type 2 diabetic patients attending hospital-based diabetes clinics of the National Health Service (in Italy, 80% of known diabetic patients are seen at these clinics at least once a year). A detailed description of the study methodology has been reported (14). In brief, the reference population consisted of all patients with type 2 diabetes (according to WHO criteria) diagnosed after 39 years of age who were seen between September and December 1998 or between March and June 1999. At each clinic, patients were chosen (on a 1:4 basis) so as to create a sample representative of the diabetic population seen at that center. A systematic sampling technique was applied by including every fourth patient. A total of 201 clinics throughout the country volunteered for the prevalence study (14), and 157 of them were involved in the incidence study. For this analysis, patients with prevalent CHD (see below for definition), cerebral thromboembolism, or peripheral amputations were excluded; the present analysis is thus restricted to 11,644 patients (6,032 women [4.4% premenopausal] and 5,612 men) without evidence of macrovascular disease at baseline. Follow-up data were collected yearly between 2000 and 2003. A total of 1,665 patients (788 men and 877 women) were lost to follow-up; their baseline clinical characteristics were essentially similar to those of the patients with follow-up data (data not shown).
Data collection and definitions
Measurements
Statistical analysis The relationship between insulin treatment and CHD risk was explored by propensity score methodology, which is an alternative technique to control for confounding in observational studies (15). The propensity score is defined as a subject's probability of treatment assignment, conditional on measured covariates. It is a one-dimensional variable that summarizes the multidimensional pretreatment covariates computed by logistic regression. The propensity score was used as a covariate in additional Cox models. All analyses were performed using the Stata 8.0 statistical package.
During 4 years of follow-up, 881 incident CHD events were observed, yielding an age-standardized incidence rate (per 1,000 person-years) of 28.8 (95% CI 25.432.2) in men and 23.3 (20.226.4) in women. AMI, major CHD events (AMI, coronary artery bypass grafting, and percutaneous transluminal coronary angioplasty), and fatal CHD were all significantly more frequent in men than women, whereas rates of CHD other than AMI were similar (Table 1). In the comparison of patients with or without incident CHD (Table 2), age was slightly higher, diabetes duration was longer, and prevalence of hypertension and insulin treatment (alone or in combination with OHAs) was higher in both men and women with a CHD event. In addition, fewer men with incident CHD resided in the southern and island regions of Italy than in north and central Italy. In women with incident CHD, serum triglycerides were more often >150 mg/dl (46 vs. 41%, P < 0.04). In both men and women, the prevalence of microvascular complications (any or none) at baseline was significantly higher among patients who developed CHD than among those who did not. Correspondingly, incidence rates of all outcomes were higher in patients with rather than without complications (Table 1).
To explore whether insulin treatment was associated with CHD risk, a propensity score was constructed that included age, diabetes duration, BMI and waist girth, fasting plasma glucose, A1C, presence of microvascular complications, and geographical area, i.e., variables presumed to inform treatment choice. In women, but not in men, the relative CHD risk adjusted for the propensity score was higher in insulin-treated than noninsulin-treated patients (relative risk 1.40 [95% CI 1.091.79]).
In preliminary analyses on the entire cohort, sex interacted with several risk factors; subsequent analyses were therefore performed separately by sex. In univariate models, age, disease duration, serum triglycerides, microangiopathy, antihypertensive therapy, and insulin treatment were shared risk factors, whereas waist girth, glycemic control, total cholesterol, blood pressure, and geographic area were additional risk predictors in men and HDL cholesterol and lipid-lowering treatment were so in women. The output of a multivariate Cox model of incident CHD, including all risk factors significant at the P < 0.1 level in univariate analysis, is shown in Fig. 1. Age was a strong predictor in both men and women, a risk increase of 14 and 23% per decade, respectively, as was diabetes duration. A1C was an additional risk factor in men (with an estimated 14% risk increase for each 20% increment above the upper limit of normal), whereas a serum triglyceride level
In this nationwide survey of type 2 diabetic patients regularly attending diabetes clinics, we found that 1) incident CHD was lower than in several previous surveys, 2) major CHD was twice more frequent in men than women, 3) microvascular complications (renal, ocular, or both) carried an independent risk of incident CHD (especially in women), 4) the pattern of CHD risk factors was partly different in men and women, and 5) insulin treatment was not associated with an increased CHD risk. These findings require specification.
First, in observational studies of CHD in diabetes, incidence rates have widely varied as a function of ethnic background, inclusion criteria, definition of end points, and duration of follow-up. In the PROactive (PROspective pioglitAzone Clinical Trial in macroVascular Events) Study, for example, 16% of type 2 diabetic patients had a first major coronary event within
Second, our findings on sex differences in incident CHD follow the trend emerging from recent meta-analyses of prospective studies showing that the impact of diabetes on both CHD (7) and fatal CHD (8) is greater in women than men. In fact, in the only population-based, nationwide dataset available for Italy (the CUORE Study) (23), the age-standardized incidence of major CHD events in the 3574 year age-group was 6.6 (95% CI 5.87.3) per 1,000 person-years in men and 2.0 (1.62.3) in women (an age-adjusted women-to-men ratio of 0.3 [95% CI 0.20.3]) (S. Giampaoli, personal communication). The corresponding rates in our cohort in the same age range were approximately twice those of the general population (women-to-men ratio of 0.5 [0.40.6]). Thus, CHD, in all its severe manifestations, was less frequent (in absolute terms) in our diabetic women than in diabetic men but with a women-to-men gradient Third, the excess CVD risk associated with microangiopathy has been repeatedly reported in cross-sectional studies or small incident cohorts (2529). The WHO multinational study of vascular disease in diabetes (6) found that heavy proteinuria and retinopathy carried a significant relative risk for CVD mortality; the type 2 diabetic cohorts in that study were, however, relatively small, and confounding by A1C was not analyzed. No large prospective study in type 2 diabetic patients free of CVD has assessed the risk of a first CHD event associated with microangiopathy in the context of the full set of classical risk factors. The current data offer robust evidence that microvascular complications markedly enhance the risk of a subsequent first coronary event in men as well as women. Interestingly, the excess risk ranged 6090% for AMI and 2040% for non-AMI CHD but rose to 160120% for fatal CHD (Table 1). Even in the fully adjusted Cox model, microangiopathy retained an independent predictive power, especially in women (Fig. 1). This suggests that mechanisms other than exposure to hyperglycemia underlie the codevelopment of micro- and macroangiopathy. Advanced glycation end products (29), oxidative stress (30), endothelial dysfunction (31), and subclinical inflammation (32) are candidate mechanisms that can simultaneously impact on the structure and function of both small and large vessels; each has been shown to be operative in type 2 diabetes (33). Established microvascular dysfunction may directly contribute to macroangiopathy by compromising blood supply to large vessels. Alternatively in our patients, hyperglycemia over the years preceding enrollment may have been a common pathogenetic predecessor of both micro- and macrovascular damage. Fourth, in the full Cox model, the pattern of CHD risk factors was partly sex specific with glycemic control and hypertension predominating in men and diabetic dyslipidemia (i.e., high triglycerides and low HDL cholesterol) and microangiopathy predominating in women. In general, the predictivity for CHD of poor glycemic control, hypertension, and dyslipidemia in diabetic patients is well established (3436), but there is little data on sex-specific risk patterns to compare with our findings. With regard to hypertension, the stronger risk in men could be explained by the fact that our diabetic women were more often on antihypertensive treatment than men (59 vs. 44% of all patients, P < 0.001). Conversely, the stronger risk conveyed by higher serum triglyceride levels in women could be due to the fact that their baseline triglyceride levels were higher (P < 0.001) (lipid-lowering treatment was equally infrequent in men and women). In addition, in women, lower HDL cholesterol levels were a further independent predictor. Thus, diabetic dyslipidemia appears to be a confirmed risk factor, especially in women with diabetes. Interestingly, men but not women living in southern Italy had a substantially lower CHD hazard than patients elsewhere in the country. This protective effect, which resisted all adjustments, may be the first trace of a lifestyle effect to have been detected in patients with diabetes. Adherence to a Mediterranean diet (which is more prevalent in southern Italy) has been convincingly associated with a reduction in cardiovascular risk (37,38). Postmenopausal women may be inherently more resistant to a diet-related risk reduction, as suggested by a recent study (39). Interactions among risk factors may be distinct in men and women such as smoking causing a higher risk in men or menopause doing the same with dyslipidemia in women. In the UK Prospective Diabetes Study (UKPDS) (40), no influence of antidiabetes treatment on CVD end points in newly diagnosed type 2 diabetic patients emerged. In general, the evidence that insulin treatment might be a risk factor for CVD in type 2 diabetes is inconclusive (12), but an especially high mortality has been reported in older diabetic women treated with insulin (10). In our cohort, insulin treatment was a risk factor in univariate analysis, and a higher proportion of women were on insulin than men (18.9 vs.14.0%, P < 0.001; insulin alone or in combination). In women, a higher propensity score was associated with a higher CHD risk. However, in the multivariate Cox model, insulin treatment showed a borderline significant association with incident CHD in women whether or not the propensity score was included in the model (Fig. 1). Though the propensity score may work better when used for matching or stratification than for regression (15), this result would suggest that in diabetic women, insulin treatment might be an independent CHD risk factor regardless of treatment assignment. A prudent summary is that insulin treatment is not an independent CHD risk factor either in diabetic men or, most likely, in diabetic women. In conclusion, this nationwide study outlines the natural history of CHD in type 2 diabetic patients managed by current standards of care. The emerging risk profile carries important therapeutic implications. First, glycemic control should be intensified and antihypertensive treatment optimized. Second, the proportion of patients eligible for lipid-lowering therapy is certainly higher than the proportion of them actually on lipid-lowering treatment. Lowering LDL cholesterol, if quite effective, may not be sufficient, especially in diabetic women in whom diabetic dyslipidemia is a significant independent risk factor.
The study was supported in part by the Italian National Institute of Health, the Italian Association of Clinical Diabetologists (AMD), and the Diabetes and Informatics Study Group (DIAINF).
Published ahead of print at http://care.diabetesjournals.org on 8 February 2007. DOI: 10.2337/dc06-2558. A table elsewhere in this issue shows conventional and Système International (SI) units and conversion factors for many substances. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C Section 1734 solely to indicate this fact. Received for publication December 18, 2006. Accepted for publication February 1, 2007.
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