Insulin resistance refers to reduced insulin response in the target tissues, including liver, skeletal muscle, and adipose tissues [1]. A number of studies have investigated the association of insulin resistance with atherosclerosis 2, 3, 4, 5, cardiovascular remodeling 6, 7, and CVD outcome [8], and reported inconsistent results. These studies used several insulin resistance indices, including the homeostasis model assessment of insulin resistance (HOMA-IR), Matsuda index, and parameters obtained by euglycemic-hyperinsulinemic clamp. Most of these indices were calculated based on serum insulin and glucose levels, showing that they are based on the efficiency of insulin-induced glucose uptake in target organs. Because insulin-induced glucose uptake in adipose tissues is relatively small [1], these indices mainly reflect insulin resistance in the liver and skeletal muscles [9], but not in the adipose tissues. The adipose tissue not only stores lipids but also releases free fatty acids (FFA) [10], renin–angiotensin–aldosterone system components, adiponectin, leptin, and other adipokines, all of which could affect vascular function [11], likely contributing to the development of cardiovascular disease (CVD). The adipose insulin resistance index (Adipo-IR) was a recently established valid indicator of insulin action in adipose tissue. It is calculated as the product of fasting insulin and fasting FFA levels [12]. Several studies have reported associations of Adipo-IR with glycemic status 13, 14, lipid profiles [15], obesity [16], and hypertension [17]. However, the longitudinal association between adipose tissue insulin resistance and atherosclerosis, vascular resistance, and remodeling remains poorly understood. In the clinical setting, no study was found examining the association of compensatory hyperinsulinemia and increased serum FFA levels induced by insulin resistance in the adipose tissue with vascular resistance.

The common carotid artery (CCA), part of the elastic arterial tree, can be noninvasively assessed by ultrasonography and provides useful indices for the CVD risk. The CCA diameter is an indicator of arterial remodeling [18]. Plaque formation reflects the atherosclerosis progression [19]. The “plaque score” is an atherosclerosis index obtained by summing the maximum thickness of the plaque in each of the four carotid artery segments reported by Handa et al. [20]. CCA flow parameter including resistance index (RI) shows the vascular resistance of small blood vessels [21]. Using carotid artery ultrasonography data, this retrospective cohort study aimed to investigate the longitudinal association of insulin resistance in the adipose tissue with atherosclerosis, arterial remodeling, and vascular resistance in the clinical setting. Moreover, the relationship between serum insulin and FFA levels and vascular resistance was investigated.