Conceived and designed the experiments: KF LP. Performed the experiments: BB LP. Analyzed the data: KF BB LP. Contributed reagents/materials/analysis tools: BB LP. Wrote the paper: KF.
The authors have read the journal's policy and have the following conflicts: KF is employed by Steno Diabetes Center A/S, a research hospital working in the Danish National Health Service and which is owned by Novo Nordisk A/S. Steno Diabetes Center A/S receives part of its core funding from unrestricted grants from the Novo Nordisk Foundation and Novo Nordisk A/S. K.F. owns shares in Novo Nordisk A/S. No other potential conflicts of interest relevant to this article were reported. This does not alter the authors' adherence to all the PLoS ONE policies on sharing data and materials.
Several studies have shown associations between hyperglycemia and risk of cardiovascular disease (CVD) and mortality, yet glucose-lowering treatment does little to mitigate this risk. We examined whether associations between hyperglycemia and CVD risk were explained by underlying insulin resistance.
In 60 middle-aged individuals without diabetes we studied the associations of fasting plasma glucose, 2-hour post oral glucose tolerance test plasma glucose, insulin sensitivity as well as body fat percentage with CVD risk. Insulin sensitivity was measured as the glucose infusion rate during a euglycemic hyperinsulinemic clamp, body fat percentage was measured by dual X-ray absorptiometry, and CVD risk was estimated using the Framingham risk score. Associations of fasting plasma glucose, 2-hour plasma glucose, insulin sensitivity and body fat percentage with the Framingham risk score were assessed in linear regression models.
Both fasting and 2-hour plasma glucose levels were associated with higher Framingham risk score (fasting glucose: r2 = 0.21; 2-hour glucose: r2 = 0.24;
The association between plasma glucose levels and CVD risk is mainly explained by insulin resistance, which raises the question of whether glucose lowering
Type 2 diabetes significantly increases the risk for cardiovascular disease (CVD) and all-cause mortality. People with diabetes without prior myocardial infarction and people with a prior myocardial infarction, but without diabetes, have similar risk of survival
Fasting and post-challenge glucose levels do not confer the same risk of CVD disease and mortality. A meta-analysis of several observational studies showed that the association of CVD risk with post-challenge glucose concentration is stronger than that of fasting plasma glucose
Using the euglycemic hyperinsulinemic clamp in 60 middle-aged individuals without diabetes, we examined whether associations between hyperglycemia and CVD risk were explained by underlying insulin resistance.
Data used in this study originate from 3 separate studies that enrolled a total of 60 American men and women without diabetes. Of these, 34 had normal glucose tolerance, 10 had isolated impaired fasting glucose, 6 had isolated impaired glucose tolerance, and 10 had combined impaired fasting glucose and impaired glucose tolerance
The studies were performed in accordance with the Helsinki declaration and approved by the Colorado Multiple Institutional review Board. Informed written consent was obtained from all participants prior to the studies.
A standard 75 g oral glucose tolerance test (OGTT) was performed after an overnight fast. Blood samples for measurement of plasma glucose were drawn in the fasting state and 120 min after ingestion of glucose.
On a separate day, peripheral insulin sensitivity was measured by a euglycemic hyperinsulinemic clamp. After an overnight fast, basal blood samples were taken and a 2-hour basal period was initiated. After the basal period, a 2-hour euglycemic hyperinsulinemic clamp at 40 mU/m2/min was performed as described previously
CVD risk was calculated using the Framingham risk score, which includes information on gender, age, total and high-density lipoprotein (HDL) cholesterol, diastolic and systolic blood pressure, as well as diabetes and smoking status
Body fat percentage and fat-free mass (FFM) was determined by dual-energy X-ray absorptiometry.
Plasma glucose concentration was analyzed using the hexokinase/G6P-DH technique (Roche Diagnostics, Mannheim, Germany). Serum insulin concentration was analyzed by a radioimmunoassay (Linco Research Inc., St. Louis, MO). Plasma cholesterol and triglyceride concentrations were measured enzymatically using commercially available kits (Beckman Coulter, Inc., Brea, CA).
Linear regression models were used to study the relationships between CVD risk and fasting glucose, 2-hour glucose, insulin sensitivity and body fat percentage. Because absolute 10-year risk of CHD was not linearly related to the measures of glucose metabolism, Framingham risk score was used as outcome in the models. Measures of glucose metabolism and body fat were standardized before analysis to be able to compare the effect size of the different measures on CVD risk. The analyses were performed with and without adjustment for insulin sensitivity, glucose levels and body fat percentage. None of the analyses were adjusted for age and gender, since these were part of the Framingham risk score. All statistical analyses were performed using SAS version 9.1 and
Clinical characteristics of the study population are shown in
Men (%) | 50 |
Age (years) | 53.0 (41.5;60.5) |
BMI (kg/m2) | 29.5 (26.2;32.7) |
Body fat (%) | 33.8 (26.3;41.2) |
Fasting plasma glucose (mmol/L) | 5.2 (4.9;5.7) |
2-hour plasma glucose (mmol/L) | 5.7 (4.9;7.9) |
Glucose infusion rate (mg/min/kg FFM) | 4.6 (2.5;7.4) |
Systolic blood pressure (mmHg) | 132 (121;142) |
Diastolic blood pressure (mmHg) | 80 (73;85) |
Total cholesterol (mmol/L) | 4.9 (4.4;5.6) |
High-density lipoprotein (mmol/L) | 1.2 (0.9;1.4) |
Framingham score | 7 (2;10) |
Absolute 10-year risk of CHD (%) | 8 (4;16) |
Categories of CHD risk (%) | |
Very low risk of CHD (<10%) | 51.7 |
Low risk of CHD (≥10 and <15%) | 20.0 |
Moderate risk of CHD (≥15 and <20%) | 10.0 |
High risk of CHD (≥20%) | 18.3 |
Data are medians (IQR) or percentages. FFM: Fat-free mass.
Both fasting and 2-hour glucose levels were highly significantly related to the Framingham risk score (
FFM: fat-free mass.
Beta coefficients reflect the absolute change in Framingham risk score per SD unit increase in the explanatory variables.
Adjustment for body fat did not change the associations of fasting and 2-hour glucose with the Framingham risk score (
Insulin sensitivity was more strongly related to the Framingham risk score than were fasting and 2-hour glucose levels (r2 = 0.36,
Body fat was significantly related to the Framingham risk score in the unadjusted model, but the correlation was very weak (r2 = 0.12;
Several studies have demonstrated relationships of plasma glucose levels with CVD and mortality
Over the past decades, a considerable number of intervention studies have focused on the role of glucose-lowering strategies on CVD morbidity and mortality in patients with type 2 diabetes
Our results indicate that lowering hyperglycemia without changing the underlying processes that cause hyperglycemia may not be the optimal way of reducing CVD risk. Thus, treatment modalities for improvement of insulin resistance may be more efficient both in terms of reducing CVD risk and improving glycemic control. Indeed, there are indications that treatment strategies focused on improving insulin sensitivity have beneficial effects on CVD risk factors and events. The Actos Now for Prevention of Diabetes (ACT NOW) study showed beneficial effects of the insulin-sensitizing agent pioglitazone on carotid intima-media thickness in individuals with impaired glucose tolerance
To our knowledge, this is the first study to address whether gold-standard-measured insulin sensitivity drives the association between glycemia and CVD risk. In the San Antonio Heart Study, insulin resistance as measured by the homeostasis model assessment (HOMA-IR) was related to CVD risk factors, whereas insulin secretion was not
We found that insulin sensitivity as measured by the clamp technique explained more of the relationship between 2-hour glucose and Framingham risk score than between fasting glucose and Framingham risk score. The reason for this may be related to the underlying physiology of fasting and post-OGTT hyperglycemia. Elevated 2-hour glucose is mainly related to peripheral insulin resistance and beta cell decompensation, whereas elevated fasting glucose is predominantly caused by hepatic insulin resistance and reduced first-phase insulin secretion
There are several limitations related to the current study. First, because of the cross-sectional nature of this study we cannot determine whether insulin sensitivity in fact is related to future CVD outcomes. Second, because we did not measure beta cell function in the entire study population, we cannot exclude disturbances of insulin secretion being responsible for part of the observed findings. Third, the use of the Framingham risk score has several limitations. In general, risk engines are thought to underestimate risk in women compared with men
In summary, we demonstrated that insulin resistance is responsible for a large part of the associations between plasma glucose levels and CVD risk. If these data are confirmed in other larger prospective studies, markers of insulin resistance (beyond glucose) should be explored for their clinical utility in high-risk individuals before the onset of diabetes.
We owe the success of this work to the research participants who volunteered their time to participate and to the committed staff of the the Clinical Translational Research Center, University of Colorado Anschutz Medical Campus. K.F. is the guarantor of this work, had full access to all the data, and takes full responsibility for the integrity of data and the accuracy of data analysis.