We present data from 87 subjects (40 with type 1 diabetes and 47 age- and sex-matched nondiabetic controls) recruited for the clamp substudy between 2005 and 2008. Initial recruitment included 91 subjects (44 with diabetes and 47 nondiabetic controls). Four diabetic subject clamps were excluded from the final analysis due to errors in clamp and/or overnight insulin preparation detected by inappropriate insulin responses in the subjects and postclamp measurement of infusate insulin concentration.
Subjects in the clamp study were representative of the full CACTI cohort seen at the 6-year follow-up with no differences found for parameters known to impact insulin sensitivity, including age (years: 44.8 ± 7.9 vs. 45.0 ± 9.1, P
= 0.87), BMI (kg/m2
: 26.2 ± 4.2 vs. 26.8 ± 4.9, P
= 0.19), visceral fat (log visceral fat area: 10.67 ± 0.52 vs. 10.74 ± 0.59, P
= 0.32), and habitual daily physical activity (log Kcal: 7.28 ± 1.00 vs. 7.15 ± 1.45, P
= 0.24). Similarity between the clamp cohort and the full 6-year visit cohort was also found within sex and diabetes strata (not shown). In addition, type 1 diabetic subjects in the clamp study were similar to the full CACTI cohort in diabetes duration (28.6 ± 8.0 vs. 29.4 ± 8.8 years, P
= 0.57). Clinical characteristics of the study population by diabetes status are shown in . Type 1 diabetic and nondiabetic groups were well matched for age and BMI. In addition, the two groups were similar for other measures of body composition, including total percentage of body fat, waist circumference, waist to hip ratio, and habitual physical activity. As expected, type 1 diabetic and nondiabetic groups differed significantly in diabetes-related parameters of GHb and fasting glucose. Total and LDL cholesterol and triglycerides also differed significantly by diabetes status, with the nondiabetic subjects exhibiting the higher cardiovascular risk phenotype of higher LDL and triglycerides. Statin use was significantly more frequent in type 1 diabetic subjects, but the difference in lipid profile remained after adjustment for statin use (not shown). Type 1 diabetic subjects were more likely to be taking antihypertensive medication, but there was no difference in blood pressure between type 1 diabetic and nondiabetic subjects who were not taking medications (not shown). Adiponectin was significantly higher in type 1 diabetic than nondiabetic subjects as has been reported previously (42
Baseline characteristics for clamp study cohort
Type 1 diabetic subjects in the clamp cohort exhibited stable glycemic control over the full duration of the study from baseline through the clamp visit (mean GHb = 7.71 ± 1.0, 7.57 ± 1.1, 7.9 ± 1.0, and 7.51 ± 0.87 at baseline, 3-year, 6-year, and clamp visits, respectively).
Impaired peripheral glucose utilization.
Three-stage hyperinsulinemic-euglycemic clamps were performed in 87 subjects (46% with type 1 diabetes, 55% women). Final clamp glucose and insulin levels were not different between the groups (). Whole-body insulin sensitivity represented by GIR during the last 30 min of the clamp was significantly lower in type 1 diabetic than nondiabetic subjects (5.8 ± 3.5 vs. 13.2 ± 5.7 mg/kg FFM/min, P < 0.0001). This difference was only modestly attenuated after multivariate adjustment for age, fasting glucose, final clamp glucose and insulin, and BMI or visceral fat area ().
Insulin sensitivity by glucose infusion rate from hyperinsulinemic-euglycemic clamp
Correlates of insulin resistance in type 1 diabetes.
Insulin resistance in type 1 diabetes correlated with the expected parameters known to predict insulin resistance in nondiabetic and type 2 diabetic subjects, but relationships were left-shifted in type 1 diabetes. For instance, triglyceride levels and BMI correlated with insulin resistance similarly in both groups (). In an analysis of the whole group, there was no interaction by diabetes for the relationship of GIR to triglycerides or BMI (P = 0.966 and 0.734, respectively), but the y intercepts were significantly different for triglycerides and trended toward significance for BMI (P = 0.002 and 0.156, respectively). Similar relationships were found for waist circumference, visceral fat area, and total body fat (not shown).
FIG. 1. Correlation of insulin sensitivity to triglycerides and BMI is retained, but left-shifted, in type 1 diabetes. For the relationship of GIR to triglycerides (top panel) the regression equation in type 1 diabetes is GIR = 8.478 – 0.038 (triglycerides), (more ...)
Impaired insulin-mediated NEFA suppression.
Insulin-mediated serum NEFA suppression was also impaired in type 1 diabetic subjects (, top panel). The increase in serum NEFA levels during the first clamp stage in diabetic subjects reflected an initial insulin infusion rate (4mU/m2/min) that was lower than their basal requirement. The second stage insulin infusion rate (8mU/m2/min) was sufficient to lower glucose and NEFA levels in all type 1 diabetic subjects. Despite this, NEFA levels remained significantly higher in type 1 diabetic than nondiabetic subjects at the end of the second clamp stage (least squares mean ± SE: 370 ± 27 vs. 185 ± 25 μmol/l, P < 0.0001) after adjustment for age, sex, BMI, fasting glucose, and time point insulin. The highest insulin infusion rate (40mU/m2/min), was sufficient to similarly suppress NEFA levels in type 1 and nondiabetic subjects. NEFA levels at all clamp stages were strongly inversely correlated with peripheral glucose uptake (r = −0.56, P < 0.0001, r = −0.63, P < 0.0001, r = −0.40, P = 0.0002 for stage 1, 2, and 3 NEFA levels, respectively).
FIG. 2. Insulin-mediated NEFA and glycerol suppression are impaired in type 1 diabetic subjects. NEFA and glycerol values are μM. Data are least squares mean ± SE (adjusted for age, sex, BMI, fasting glucose, and time point insulin). ■, (more ...)
Insulin-mediated suppression of glycerol followed a similar pattern to NEFA levels (, bottom panel). The defect in glycerol suppression in type 1 diabetic subjects remained significant in the unadjusted data during the third clamp stage, though this was attenuated at the final time point by adjustment for age, sex, BMI, fasting insulin, and time point insulin level.
Impaired peripheral glucose utilization and insulin-mediated NEFA suppression correlate with coronary artery calcification.
A relationship between insulin resistance and CAC is suggested by a plot of the raw data of peripheral glucose utilization (GIR) versus CAC volume at the 6-year visit (). In fact, after adjustment for age, GIR correlated inversely with CAC volume at the 6-year CACTI follow-up visit and with an increase in CAC volume from baseline to the 6-year visit and from 3-year to 6-year follow-up visits (ρ, P = −0.42, <0.0001; −0.41, <0.0001; −0.24, <0.028, respectively) (). In a logistic regression analysis adjusted for age, sex, BMI, and diabetes status, lower GIR was independently associated with the presence of CAC: odds ratio (OR) 0.45, 95% CI (0.22–0.93) for a one standard deviation (SD) increase in GIR. Thus, for every 6.1 mg/kg FFM/min increase in GIR (signifying greater insulin sensitivity), the odds of having CAC decreased by 55%.
Plot of raw data for CAC volume at 6-year follow-up visit as a function of GIR (n = 87). Open (white) squares = nondiabetic subjects, black triangles = type 1 diabetic subjects.
Correlation of insulin resistance to CAC volume; OR of any CAC at 6-year visit associated with insulin resistance by GIR and by failure of NEFA suppression in clamp stage 2
The NEFA levels during stage 2 of the clamp were similarly but positively correlated with CAC volume at the 6-year visit, and CAC increase from baseline to 6-year and from 3-year to 6-year follow-up visit (). Partial adjustment for sex and diabetes status attenuated all correlations somewhat (), but there was no significant interaction with diabetes status. In a logistic regression analysis adjusted for age, sex, BMI, and diabetes status, higher stage 2 NEFA levels (insulin resistance), were independently associated with the presence of CAC, OR 2.40, 95% CI (1.08–5.32) for one SD difference in NEFA level. Thus, for every 186 μmol/l increase in stage 2 NEFA (signifying insulin resistance), the odds of having CAC increased by 140%. These one SD differences in measures of insulin action are comparable to the differences seen between type 1 diabetic and nondiabetic subjects in this study. In other logistic regression models LDL and HDL levels did not have a significant effect on the odds ratio for the presence of CAC in the whole cohort, nor did insulin dose in logistic regression analysis of the diabetic group (not shown).
The above analyses were also performed on the diabetic and nondiabetic groups separately. Similar correlation coefficients resulted from these individual analyses. However, statistical significance was lost for the 3-year to 6-year visit change and otherwise attenuated to near statistical significance in the diabetic group only (, models 3 and 4). Similarly, logistic regression analysis of the odds ratio for the presence of CAC at the 6-year follow-up visit associated with a change in GIR was analyzed separately for diabetic and nondiabetic subjects. Odds ratios for the two groups were very similar, but statistical significance was lost for the diabetic group (e.g., for GIR: OR = 0.38 [95% CI: 0.11–1.37] and 0.40 [95% CI: 0.18–0.90], respectively).
Insulin resistance does not correlate with poor glycemic control.
Neither peripheral glucose uptake nor insulin-mediated NEFA suppression correlated with measures of current or recent glycemic control in this study (). Pearson coefficients revealed no significant correlation between GIR or stage 2 NEFA levels and GHb or CGM measures of mean glucose, percentage of values >180 mg/dl, percentage of values <70 mg/dl, and overall SD (glycemic variability).
Insulin resistance does not correlate with recent glycemic control. Pearson correlation coefficients are shown for correlation of GIR and stage 2 NEFA levels to GHb and CGM measures of glycemic control for all type 1 diabetic subjects
In addition, GIR and stage 2 NEFA did not differ by GHb quartile (ANOVA, P = 0.89; and data not shown) over the range of GHb values represented by this cohort (<9.5%). BMI also did not differ by GHb quartiles (26.4, 28.1, 27.4, and 26.0 kg/m2, ANOVA, P = 0.27), excluding the possibility that glycemia-related changes in insulin sensitivity were countered by glycemia-related changes in BMI.
FIG. 4. Insulin resistance does not correlate with poor glycemic control. Insulin sensitivity is expressed as glucose infusion rate per fat-free mass (GIR, mg/kg FFM/min) and shown by quartile of GHb measured 3 days before the clamp study day. GHb range for each (more ...)