Diabetes Care 30:1851-1856, 2007 DOI: 10.2337/dc06-2152 © 2007 by the American Diabetes Association
Prognostic Impact of Metabolic Syndrome by Different Definitions in a Population With High Prevalence of Obesity and DiabetesThe Strong Heart Study
1 Division of Cardiology, Weill Medical College of Cornell University, New York, New York Address correspondence and reprint requests to Giovanni de Simone, MD, Department of Clinical and Experimental Medicine, Federico II University Hospital, v.S.Pansini 5, 80131 Naples, Italy. E-mail: simogi{at}unina.it
OBJECTIVE This study analyzed which definition of the metabolic syndrome is more predictive of cardiovascular events in both diabetic and nondiabetic members of a population-based sample. RESEARCH DESIGN AND METHODS A 10-year, longitudinal follow-up of the Strong Heart Study cohort has been evaluated. The analysis included 3,945 participants (2,384 female) with complete data (1,700 with diabetes and 1,468 with arterial hypertension) for evaluation of metabolic syndrome. Those with prevalent cardiovascular disease were excluded (n = 287, of whom 127 were female). Prevalence of metabolic syndrome was assessed based on the World Health Organization (WHO), the National Cholesterol Education Program Adult Treatment Panel (NCEP ATP) III, and International Diabetes Federation (IDF) definitions. The main outcome was 10-year incidence of combined fatal and nonfatal cardiovascular events, including stroke, coronary heart disease, and congestive heart failure. RESULTS Fatal and nonfatal cardiovascular events occurred in 1,120 participants. After adjusting for age, sex, and diabetes, metabolic syndrome by all definitions was significantly associated with higher incidence of cardiovascular events (all P < 0.0001). In nondiabetic individuals, incident cardiovascular event rates were about 3040% higher in those with metabolic syndrome, without a significant difference among definitions (0.03 < P < 0.001), and remained significant in WHO and NCEP ATP III definitions even after further adjustment for obesity, hypertension, and low HDL cholesterol. In the diabetic group, metabolic syndrome risk for cardiovascular events was greatest using the WHO definition (P < 0.002 vs. other models). CONCLUSIONS In individuals without diabetes, metabolic syndrome is associated with incident cardiovascular disease, especially with WHO and NCEP ATP III definitions. Metabolic syndrome also predicts higher cardiovascular event rates in diabetic participants, a prediction that is greatest using the WHO definition.
Abbreviations: HOMA, homeostasis model assessment IDF, International Diabetes Federation NCEP ATP, National Cholesterol Education Program Adult Treatment Panel SHS, Strong Heart Study WHO, World Health Organization
The metabolic syndrome represents clusters of cardiovascular risk factors, assuming that cardiovascular risk is amplified more than is expected from the effect of single risk factors (1). Many studies support the utility of this definition (2,3), but others have questioned the incremental utility of this approach (4). An element of potential confusion concerning the real prognostic utility of defining metabolic syndrome is the availability of several definitions (59), reflecting different strategies, either identifying a main characteristic as a necessary factor (5,7,9) associated with other variable risk factors or accepting varied combinations of characteristics (6,8). Other differences may also be important, including partition values and methods to define abnormalities. At present, few data exist comparing the correlates and prognostic significance of the definitions of metabolic syndrome in the same population. Accordingly, we compared the ability of the most used definitions of metabolic syndrome to predict cardiovascular events in the Strong Heart Study (SHS) cohort.
The SHS is a population-based cohort study of cardiovascular risk factors and disease in 4,549 American Indians from three communities in Arizona, seven in Southwestern Oklahoma, and three in South and North Dakota, as extensively described (1014). Participants seen during the baseline exam, in 19891992, were representative of the source population (15). For the present analysis, individuals with prevalent cardiovascular disease (n = 287, 127 of whom were female) were excluded. Prevalent and incident cardiovascular events (cardiovascular death, stroke, congestive heart failure, myocardial infarction, and coronary heart disease [coronary angiography, combination of typical symptoms with positive treadmill tests or abnormal imaging stress test, or revascularization procedures]) were confirmed by the SHS mortality and morbidity committees, using specified criteria for causes of fatal and nonfatal cardiovascular events (16). Diabetic participants were included; participants with fasting triglyceride levels >750 mg/dl were excluded. Thus, 3,945 participants (2,384 women) without prevalent cardiovascular disease and available data (1,700 with diabetes and 1,468 with hypertension) were included in the analysis.
Laboratory tests and definitions of metabolic syndrome
Table 1 shows the criteria used to define metabolic syndrome by three guidelines: those by the World Health Organization (WHO) (5,9), the National Cholesterol Education Program Adult Treatment Panel (NCEP ATP) III (6), and the IDF (9). Insulin-glucose homeostasis for the WHO definition was estimated by the homeostasis model assessment (HOMA) equation (19). Based on the WHO recommendation, a partition value for HOMA index was arbitrarily determined in the nondiabetic SHS participants as the lower boundary of the highest tertile (4.3). Thus, insulin resistance status was defined as the presence of type 2 diabetes or fasting glucose 110 mg/l or HOMA index >4.3. Hypertension was defined by Joint National Committee VII criteria (blood pressure 140/90 mmHg or use of antihypertensive treatment).
Statistical analysis The 10-year relative risk of combined fatal and nonfatal cardiovascular events was estimated for each definition. Log-cumulative hazard functions were computed by Cox regression, adjusting for age (years), field center, sex, and diabetes. Additional models were also adjusted for the other components of metabolic syndrome. Cox regression was also run separately for diabetic and nondiabetic participants.
To compare the independent prognostic effect of metabolic syndrome by the three definitions, likelihood functions were compared (20). The difference between two 2 log likelihoods has a
Table 2 shows that the IDF definition yielded the highest prevalence of metabolic syndrome in both men and women. The WHO definition resulted in a similar proportion of metabolic syndrome in men and women, whereas NCEP ATP III and IDF showed a substantially higher prevalence in women, with the greatest sex difference by the NCEP ATP III definition (both P < 0.0001). The coefficient of concordance (k) among the different definitions in the recognition of metabolic syndrome status was not excellent: 0.56 and 0.59 for men and women between WHO and NCEP ATP III, 0.58 and 0.77 between NCEP ATP III and IDF, and only 0.49 and 0.50 between WHO and IDF definitions, respectively.
Table 2 also shows that, due to differing high blood pressure criteria, the WHO definition was associated with slightly higher blood pressure levels than was NCEP ATP III or IDF. Urinary albumin-to-creatinine ratio was highest in participants with metabolic syndrome by the WHO definition (including this parameter among criteria for definition), but also higher in the NCEP ATP III and IDF definitions (all P < 0.0001), than in participants without metabolic syndrome. The criteria for adiposity proposed by the WHO identified 83% of participants with central fat distribution in the absence of metabolic syndrome (compared with 96% in those with metabolic syndrome). The difference between groups without (66% with central fat) vs. with metabolic syndrome (100% with central fat) was more accentuated in the IDF and maximal with the NCEP ATP III definition (50 vs. 91%, respectively). In all definitions, one-half or more of individuals free of the syndrome exhibited central fat distribution.
Cardiovascular risk in the metabolic syndrome
Alternative Cox regression models adjusted for obesity, low HDL cholesterol, and hypertension, in addition to the covariates used in the previous model. Although reduced, the hazard ratios (HRs) of metabolic syndrome remained statistically significant for the definitions by the NCEP ATP III (HR 1.28 [95% CI 1.041.56], P < 0.02) and the WHO (1.35 [1.111.64], P < 0.002) but not by IDF (1.12 [0.921.36], P > 0.2).
Cardiovascular risk in diabetic and nondiabetic subjects
This study demonstrates that in the SHS cohort, metabolic syndrome by all three examined definitions is independently associated with a significantly greater rate of incident composite fatal and nonfatal cardiovascular events and is a marker of preclinical cardiovascular disease, represented by the increased urinary albumin-to-creatinine ratio. The independent association of metabolic syndrome with cardiovascular events was seen in population strata with or without diabetes. In participants without diabetes, increased cardiovascular risk is independently associated with metabolic syndrome, suggesting that this diagnosis can help identify high-risk individuals. Similarly, among diabetic individuals, recognition of the metabolic syndrome impressively enhanced the prediction of cardiovascular events, consistent with National Health and Nutrition Examination Survey III results (21), where the age-adjusted prevalence of coronary heart disease in the diabetic population without metabolic syndrome (8%) was similar to that in subjects without diabetes or metabolic syndrome (9%), while increasing to 14% in nondiabetic subjects with metabolic syndrome and to 19% in those with both conditions. However, independent of the presence of full-fledged metabolic syndrome, attention to all cardiovascular risk factors is paramount in the presence of diabetes. Compared with nondiabetic populations, detecting the full presentation of metabolic syndrome might be less important for decision making when diabetes coexists with even one additional risk factor. This study identifies similarities among the three definitions of metabolic syndrome but also reveals differences. In the whole population, combining participants with or without diabetes, the model using the WHO definition was a better predictor than models using NCEP ATP III or IDF definitions, with IDF exhibiting the lowest fit. When forcing single cardiovascular risk factors into the proportional hazard model, only the WHO and ATP III definitions maintained an independent prognostic impact, whereas the IDF definition did not, possibly due to lower specificity associated with the generally lower partition values for single factors. In nondiabetic participants, the risks predicted by the three definitions were not statistically different. In contrast, a substantial difference was evident among diabetic participants, with the WHO definition significantly superior to those of both the NCEP ATP III and IDF (the HR of which was not statistically significant). Although the NCEP ATP III definition was not intended for diabetic individuals, it should be noted that it performed well in participants with or without diabetes, though slightly less well than WHO in those with diabetes. To evaluate correctly the implications of these findings, both the characteristics of the SHS population and the differences among the different definitions of metabolic syndrome have to be taken into account. The American Indian population of the SHS is characterized by higher prevalence of obesity than the overall population in the U.S. and Europe (12), though this difference is rapidly attenuating (2225). As a consequence, in the SHS cohort, the distribution of measures of adiposity is skewed toward higher values, similar to findings in hypertensive populations (26). Due to the high prevalence of obesity, the adopted measures of fatness in the WHO definition do not discriminate well between participants with or without the metabolic syndrome. Also with the other definitions, substantial proportions of participants without metabolic syndrome exhibited central fat distribution, maximal with the IDF definition, which uses very low partition values for waist girth. Thus, in the context of a population with very high prevalence of obesity, measures of adiposity or abdominal obesity do not strongly separate individuals with or without metabolic syndrome, since obesity is also highly prevalent among individuals without metabolic syndrome. The present analysis extends a previous study of metabolic syndrome in nondiabetic SHS participants. That analysis did not report significant independent associations between metabolic syndrome (by the NCEP ATP III definition) and cardiovascular risk (27), but it included only 4.2 years of follow-up and was limited to some cardiac events. In contrast, the present analysis also included diabetic participants, the follow-up was substantially longer, and all clinical manifestations of cardiovascular disease, including cerebrovascular events and congestive heart failure, were considered as end points. In particular, incident congestive heart failure (394 adjudicated events) could be very important, because heart failure is a relevant end point for obesity (2830). Overall, our findings suggest that the metabolic syndrome may take several years to manifest its effects on clinical events (as Fig. 1 also suggests). In conclusion, the NCEP ATP III and WHO (but not IDF) definitions of metabolic syndrome predict cardiovascular disease independently of single components of the syndrome; their value may be similar in individuals without diabetes, but in diabetes the WHO definition seems to be more useful. Results of these analyses, obtained in a population with high prevalences of obesity and diabetes, are likely applicable to other populations of different ethnicities in which there is an epidemic rise of prevalences of overweight and obesity (31), triggering diabetes and other metabolic abnormalities (32). The identification of the metabolic syndrome can be valuable to focus aggressive intervention strategies, especially when diabetes has not yet occurred.
This work has been supported by grants HL41642, HL41652, HL41654, HL65521, and M10RR0047-34 (General Clinical Research Center) from the National Institutes of Health, Bethesda, Maryland. The authors wish to thank the Indian Health Service, the SHS participants, the participating tribal communities, and the SHS Center coordinators for their help in the realization of this project.
Published ahead of print at http://care.diabetesjournals.org on 17 April 2007. DOI: 10.2337/dc06-2152. The views expressed in this paper are those of the authors and do not necessarily reflect those of the Indian Health Service. 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 October 18, 2006. Accepted for publication March 30, 2007.
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