Insulin resistance and five-year progression of carotid intimal-medial wall thickness:
The Insulin Resistance Atherosclerosis Study (IRAS)
Insulin resistance (IR) is thought to be an important risk factor for atherosclerosis. It is unclear, however, whether the effect is independent of other cardiovascular (CV) risk factors that cluster with IR, e.g., obesity, hypertension (HTN), and hyperlipidemia. IRAS is a multi-ethnic epidemiologic cohort study investigating the association between IR and CV disease. IR was measured by the frequently sampled intravenous glucose tolerance test with minimal model analysis in 773 non-diabetic persons aged 60 years (range 44 - 75) who participated in the baseline IRAS examination (1992-4). Intimal-medial thickness (IMT) of the carotid arteries was measured by B-mode ultrasonography as an index of atherosclerosis at baseline and at the 5-year follow-up exam (1998-9). Both scans were read at the follow-up time point to eliminate possible reader drift. We modeled change in IMT on IR, adjusting for baseline IMT using a general linear models approach. The models adjusted progressively for demographic variables and CV risk factors. Overall, common carotid (CCA) IMT increased 30 μm (s.e. = 8 μm). Average IR at baseline was 2.2 units (range 0 - 14.2). Adjusting for age, sex, ethnicity, clinic and baseline measurement, CCA IMT increased 20 μm (p=0.007) for a 2 unit increase in IR (change from 25th to 75th percentile). Further adjustment for baseline HDL, LDL, smoking, HTN and glycemia attenuated the effect (17 μm, p=0.05). Adjustment for BMI and waist-hip ratio explained some of the effect (13 μm, p=0.13). No significant association was observed between IR and progression in the internal carotid artery. Analyses incorporating hierarchical measurement error models for IMT were performed to confirm these results. As expected in these analyses, the IR effect was somewhat attenuated, but not eliminated. In summary, IR is associated with progression of carotid atherosclerosis, but this effect may be mediated through the associated cluster of CV risk factors.