© 2004 by the American Diabetes Association, Inc.
Insulin Sensitivity, Insulinemia, and Coronary Artery DiseaseThe Insulin Resistance Atherosclerosis Study
1 Barbara Davis Center, University of Colorado HSC, Denver, Colorado Address correspondence and reprint requests to Marian Rewers, MD, PhD, Barbara Davis Center, University of Colorado Health Sciences Center, B-140, 4200 E 9th Ave., Denver, CO 80262. E-mail: marian.rewers{at}uchsc.edu
OBJECTIVEThe aim of this study was to evaluate whether low insulin sensitivity (Si) measured using a modified frequently sampled intravenous glucose tolerance test with minimal model analysis is associated with coronary artery disease (CAD) independent of other cardiovascular risk factors. RESEARCH DESIGN AND METHODSWe studied 1,482 women and men, age 4069 years old, African American (28%), Hispanic (34%), or non-Hispanic white (38%), with normal (45%), impaired (23%), or diabetic (32%) glucose tolerance. CAD defined as confirmed past myocardial infarction, coronary artery bypass graft, coronary angioplasty, or presence of a major Q-wave was found in 91 participants. RESULTSThe odds ratio (OR) for CAD was greatest among individuals in the two lowest quintiles of Si (2.4, 95% CI 1.05.6 and 4.7, 2.110.7) compared with the highest Si quintile. After adjusting for demographic and cardiovascular risk factors, a decrement from the 75th to 25th percentile in Si was associated with a 56% increase in CAD (P = 0.028). Similar increments in fasting or 2-h insulin levels were associated with, respectively, only 15 (NS) and 3% (NS) increases in CAD. The association between Si and CAD was partially mediated by insulin, HDL cholesterol and triglyceride levels, hypertension, diabetes, and obesity, but not LDL cholesterol or cigarette smoking. CONCLUSIONSLow Si is associated with CAD independently of and stronger than plasma insulin levels. Part of the association is accounted for by dyslipidemia, hypertension, diabetes, and obesity.
Abbreviations: CAD, coronary artery disease ECG, electrocardiogram FSIGT, frequently sampled intravenous glucose tolerance test HOMA, homeostasis model assessment IGT, impaired glucose tolerance IRAS, Insulin Resistance Atherosclerosis Study
Low insulin sensitivity underlies the metabolic syndrome that includes central obesity, dyslipidemia, hyperglycemia, hypertension, impaired fibrinolysis, and atherosclerosis (1,2). However, measurement of insulin sensitivity is technically difficult, and only a few relatively small studies (35) have demonstrated a strong association between insulin sensitivity measured directly and coronary artery disease (CAD). On the other hand, hyperinsulinemia (a marker of low insulin sensitivity) has been related to CAD in numerous prospective (612) and cross-sectional studies (13). Insulinemia is generally inversely related to insulin sensitivity, but the relationship is not linear (14), and it is usually absent in diabetic individuals (15,16) who account for a significant proportion of people with low insulin sensitivity. The Insulin Resistance Atherosclerosis Study (IRAS) (17) and others (18,19) have previously shown that low insulin sensitivity is associated with atherosclerosis, defined by the intima-media thickening of the carotid arteries. In this study, we test the hypothesis that low insulin sensitivity is also cross-sectionally associated with clinical CAD, independent of insulin levels and other cardiovascular risk factors.
The design of IRAS, a four-center epidemiological study exploring relationships among insulin sensitivity, insulin levels, cardiovascular risk factors, and cardiovascular disease across a broad range of glucose tolerance, has been previously published (20). Briefly, IRAS evaluated 1,624 women and men aged 4069 years, representing normal (44%), impaired (23%), and diabetic glucose tolerance (33%). Of the 479 diabetic patients included in this report, 294 were previously diagnosed (average duration of diabetes 6.9 ± 6.4 years). Of those, 73% were taking oral hypoglycemic agents, whereas the remaining were treated with diet alone. Individuals with impaired glucose tolerance (IGT) and type 2 diabetes were over-sampled to achieve sufficient statistical power in these subgroups. Nondiabetic IRAS participants had, however, fasting blood glucose levels similar to those in nondiabetic individuals of the same ethnic group in the general population (20). IRAS clinics in Oakland and Los Angeles, California, studied non-Hispanic whites and African Americans recruited from Kaiser Permanente health maintenance organizations. The centers in San Antonio, Texas and San Luis Valley, Colorado recruited non-Hispanic whites and Hispanics from ongoing population-based studies (21,22). Race and ethnicity were assessed by self-report using the U.S. census definitions; African-Americans comprised 29%, Hispanics 34%, and non-Hispanic whites 37% of the study participants. Exclusion criteria included insulin treatment in the past 5 years, fasting glucose 16.7 mmol/l [300 mg/dl], unstable angina, decompensated congestive heart failure, or serious illness within the past month. All study protocols were approved by institutional review boards, and informed consent was obtained from all participants.
Measurement of glucose tolerance, insulin, and insulin sensitivity Insulin sensitivity was assessed by the FSIGT with minimal model analysis (25). Glucose (0.3 g/kg in 50% solution) was injected through an intravenous catheter at 0 min, and regular human insulin (0.03 U/kg) was injected at 20 min. Blood was collected at -5, 2, 4, 8, 19, 22, 30, 40, 50, 70, 100, and 180 min for insulin and glucose determination. Si was calculated by mathematical modeling (MINMOD, version 3.0, 1994). The injection of insulin was necessary to ensure adequate plasma insulin levels for accurate computation of insulin sensitivity in a diabetic person (26). This version of the FSIGT was validated by comparison with the hyperinsulinemic-euglycemic clamp (27).
Definition of CAD
Other measurements
Statistical analysis The Si was estimated to be 0 for 231 of the 298 participants in the lowest Si quintile. In all logistic regression models shown, an indicator variable was included for individuals with Si = 0, as previously described (17), but it was statistically not significant (P > 0.05).
This report includes 91% (1,482 of 1,624) of the study participants who completed the FSIGT. Univariate comparison of the characteristics of the 91 case subjects and 1,391 control subjects studied (Table 1) confirmed known associations between CAD and type 2 diabetes, male sex, older age, central obesity (higher waist-to-hip ratio), dyslipidemia (low HDL cholesterol and high triglycerides), hypertension, and cigarette smoking. Case subjects had significantly lower Si levels than control subjects. Fasting insulin levels were only on the borderline of being higher among case subjects than control subjects. There was no difference in the levels of 2-h insulin between case subjects and control subjects.
To explore the linearity of the relationship between Si and the CAD, the ORs of CAD were estimated by quintiles of the Si distribution, adjusting for age, sex, ethnicity, and clinic (Fig. 1). Adjusted CAD ORs for quintiles of fasting and 2-h insulin levels were included for comparison. The quintile of highest Si or lowest fasting insulin or 2-h insulin levels served as the reference. The ORs for CAD were greatest among individuals in the second lowest Si quintile (OR = 4.7, 95% CI 2.110.7), followed by those with the lowest Si (2.4, 1.05.6). The ORs were nearly identical when the analysis was stratified by diabetic status. For instance, the ORs for CAD in nondiabetic participants were greatest among individuals in second lowest Si quintile (OR = 4.7), followed by those with the lowest Si (OR = 2.5).
After adjustment for demographic factors CAD (Table 2, model 1a), an interquartile decrement from the 75th to 25th percentile in Si (2.21 to 0.41 x 10-4 min · µU-1 · ml-1) was associated with a 91% increase in CAD (P = 0.001). Similar interquartile differences in fasting insulin (from 60 to 132 pmol/l) (model 1b) or 2-h insulin levels (from 216 to 816 pmol/l) (model 1c) were associated with, respectively, only 34% (P = 0.034) and 16% (NS) increases in CAD. A simultaneous estimation of the effects of Si, fasting, and 2-h insulin (model 1d) indicated that only Si was significantly and independently associated with CAD (OR = 1.84, P < 0.006).
Because nearly a one-half of the case subjects had type 2 diabetes, which is known to increase the risk of CAD, we carried the analyses also stratified by diabetic status. The results were virtually identical for diabetic and nondiabetic participants, and the interaction between the effects of Si and diabetes was nonsignificant (P > 0.9) in all of the models. In further analyses, we combined diabetic and nondiabetic participants. Adjustment for HDL and LDL cholesterol levels, triglycerides, smoking, and hypertension attenuated the independent association between CAD and Si (model 2a) and removed any association between CAD and fasting (model 2b) or 2-h insulin levels (model 2c). Models 2d, 2e, and 2f further suggested that Si, rather than fasting or 2-h insulin levels, was the independent determinant of CAD. The decrease in the CAD ORs with adjustment for cardiovascular disease risk factors, from 1.91 for (model 1a) to 1.56 (model 2a), was consistent with the likely scenario that some of these factors could mediate the association between Si and CAD. A stepwise addition of these risk factors to model 1a (data not shown) indicated that HDL cholesterol levels or closely correlated triglyceride levels and hypertension, but not LDL cholesterol and cigarette smoking, might mediate the effect of Si. Further adjustment for diabetes status (model 3) and obesity (model 4) removed most of the remaining association between Si and CAD. One may, however, argue that this could be a case of over adjustment, because the vast majority of participants with low Si were diabetic and/or obese. After adjustment for all covariates (model 5), insulin levels did not have any effect on CAD, but the interquartile decrement in Si was still associated with a 29% increase in the odds of CAD. Although not statistically significant, this finding may suggest that additional factors not included in these analyses may also play a role in the association between low Si and CAD.
This is the largest epidemiological study to date that has assessed directly insulin sensitivity and related it to fasting and postload insulin levels, traditional cardiovascular risk factors, and CAD. Our findings of an association between low insulin sensitivity and CAD, largely independent of the effects of major cardiovascular risk factors, are consistent with previous studies that used fasting insulin levels as a marker of insulin sensitivity (613). In contrast to some of these previous studies (8,9,30), the association between Si and CAD was highly significant and independent of the effects of lipids, hypertension, and cigarette smoking. These results are also consistent with the previously reported (1719) association between low Si and carotid artery wall thickness, which is an index of atherosclerosis. A comparison of the intima-media thickness of the internal carotid arteries in the IRAS CAD case subjects and control subjects (Fig. 2), confirmed that the most insulin-resistant CAD case subjects had the most extensive carotid atherosclerosis. Thus, low insulin sensitivity is associated with both subclinical carotid atherosclerosis and clinical CAD.
The association between Si and carotid wall thickness (17) or CAD (current report) was independent of and much stronger than the associations with fasting or 2-h insulin levels. The exact contribution of the proposed atherogenic effect of insulin (13) to the association between insulin resistance and CAD is difficult to quantify in this cross-sectional analysis but appears to be relatively small (Table 2, model 1a versus 1d). This is consistent with the variable and generally weak associations between insulin levels and CAD reported previously (30). On the other hand, our data confirm that hypertension (31), dyslipidemia (32), and diabetes (acting through hyperglycemia or other risk factors [33,34]) mediate a significant part of the association between low Si and CAD. This study is the first to measure insulin sensitivity directly in a large population of people with normal, impaired, or diabetic glucose tolerance. Whereas it is more difficult to measure Si than fasting insulinemia, the interpretation of Si (effectiveness of insulin on glucose kinetics) is easier than that of fasting insulin levels. Fasting insulin levels increase with insulin resistance but to a variable extent limited by the pancreas ability to secrete insulin and modified by ambient glycemia and insulin clearance. Therefore, fasting insulinemia is a less useful marker of insulin sensitivity in individuals with diabetes, impaired insulin secretion (e.g., a large proportion of individuals with IGT [15]), some forms of hyperinsulinemia (e.g., insulinoma), and disorders of insulin clearance (e.g., cirrhosis). The major advantage of the IRAS protocol was the ability to measure insulin sensitivity in individuals with diabetes who are at a two- to fourfold increased risk of CAD (12,35). They have typically been excluded from previous studies (611), yet diabetes affects, in the U.S., 614% of people aged 3064 years and 1832% of those over 64 years (36).
Despite the advantages of the minimal model analysis in assessment of insulin sensitivity, the method resulted in a "zero Si" estimate in The present study has several limitations. First, the relation between Si, insulin levels, and CAD were assessed cross-sectionally, and the proposed role of low insulin sensitivity as one of the causes of CAD needs to be confirmed in longitudinal studies. The IRAS cohort is being followed prospectively with major cardiovascular disease end points ascertained through annual participant interviews and committee review of medical records of reported fatal and nonfatal events. A 10-year follow-up of the study cohort will be completed in 2005. Second, the IRAS cohort is not strictly population based. The study participants were drawn from two existing population-based epidemiological studies and from two health maintenance organization populations; however, individuals with IGT and diabetes were over-sampled by design. On the other hand, demanding protocol and specific exclusion criteria removed from the study population individuals with the most severe diabetes or CAD. Less than expected carotid artery atherosclerosis among the most insulin-resistant IRAS participants reported previously (17) and lower than expected CAD prevalence found in the current study in that group could be due to a "survivor bias." This could occur if individuals with the most severe CAD have died, elected not to participate, or were excluded. This potential selection bias would tend to underestimate the true association between Si and CAD. Third, the study population included Hispanic and non-Hispanic whites as well as African Americans, but relatively few end points in each of these subgroups limited our ability to detect any ethnic differences in the relation between low Si and CAD. There were no clear interactions between Si and ethnicity (P > 0.4, data not shown), and the present analyses were adjusted for, but not stratified by, ethnicity. Fourth, there could have been some misclassification of the CAD status using the study criteria. Only 91 participants with most severe clinical or ECG manifestations of CAD were classified as "case subjects," whereas obviously many more had some degree of CAD but were classified as "control subjects." More precise procedures to document CAD, such as coronary angiography or electron beam tomography for coronary calcification, were too invasive or expensive for this large study. Our definition of CAD most likely underestimated the true associations between CAD and risk factors, including Si. Recently, a study of just 13 case subjects with arteriographically documented CAD and 10 control subjects (3) found a significant difference in their insulin sensitivity, consistent with that reported here. Fifth, the minimal model measurement of insulin sensitivity is technically difficult in clinical practice. In search for a simpler solution, we substituted Si with the homeostasis model assessment (HOMA) measurement of insulin sensitivity that can be derived from the FSIGT (39). In none of the models, except for the simplest model 1a, was HOMA associated with CAD. Although easier to obtain than Si, the HOMA estimate of insulin sensitivity appears to be insufficiently precise for studies of IRAS size. Finally, Si and insulin levels display significant variability, partially related to precision of measurements and partially due to acute day-to-day and diurnal changes (40). The interclass correlation for Si measured twice within 1 week in 58 IRAS participants was 0.67 compared with 0.76 for fasting insulin. Thus, it is unlikely that we measured Si with more precision than fasting insulin levels and that this could account for the stronger association of CAD with Si than with fasting insulin. We confirmed that by using the average of two fasting insulin measurements (on the oral glucose tolerance test day and on the FSIGT day) instead of a single measurement in alternative models 1, 2, and 5. Although some of the ORs for fasting insulin increased slightly, the ORs for Si and the associated P values virtually did not change. We did not estimate the reproducibility of 2-h insulin levels in IRAS, but they may vary by >30% in normal subjects studied 48 h apart (41), which is comparable with the reproducibility of Si and fasting insulin. Therefore, differential measurement precision of Si and insulin levels is unlikely to explain the apparent independence and greater strength of the association between Si and CAD compared with that between insulin levels and CAD. In middle-aged women and men representative of the three major U.S. ethnic groups and including individuals with normal, impaired, and diabetic glucose tolerance, we found that CAD was cross-sectionally associated with low insulin sensitivity. This association was independent of and stronger than that between CAD and fasting or postload insulin levels. Dyslipidemia, hypertension, diabetes, obesity, and fat centrality explained part of the association between low insulin sensitivity and CAD.
This study was supported by the National Heart, Lung, and Blood Institute Awards U01-HL-47887, HL-47889, HL-47890, HL-47892, and HL-47902. In addition, the Los Angeles center was supported by grant M01-RR-43 from the National Center for Research Resources/National Institutes of Health. The authors would like to thank the women and men who participated in this study. We would also like to acknowledge the valuable contributions of additional IRAS investigators, clinical, and technical staff.
A table elsewhere in this issue shows conventional and Système International (SI) units and conversion factors for many substances. Received for publication September 1, 2003. Accepted for publication December 1, 2003.
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