**3. Insulin resistance: a direct consequence of immunometabolic imbalance**

Insulin resistance (IR) is a condition characterized by the inability of cells to appropriately respond to insulin, which results in prolonged systemic hyperglycemia. It was considered a pathology since the 1930s, however, it was the development of insulin quantification assays and methodologies to estimate its biological action, as well as large epidemiology studies, which allowed to define the magnitude of the problem and the clinical implications.

### **3.1. Insulin resistance classification**

The gold standard for IR assessment is the 'Hyperinsulinaemic-euglycaemic clamp' described by DeFronzo et al. [69]. However, this technique is hard to perform, time consuming, invasive and expensive; therefore, it is prohibitive for large studies. Because of this, numerous indexes have been developed and validated as surrogates, one of the most used is the 'Homeostasis Model Assessment of Insulin Resistance' (HOMA-IR) described by Matthews et al. that kept a correlation of 69 and 88% with euglycaemic and hyperglycaemic clamp, respectively [70].

Cut-offs and conditions has been tested to improve IR individuals classification; Stern et al. compiled demographics, clinical, laboratory and anthropometrics data of 2321 subjects studied with the euglycaemic insulin clamp technique and determined that with a combination of two simple rules: (1) HOMA-IR > 3.60 and (2) BMI > 27.5 kg/m2 ; individuals can be classified as insulin-resistant with a sensitivity and specificity of 84.9 and 78.7%, respectively [71].

#### **3.2. Insulin resistance aetiology**

Establishment of IR arises from the interaction between environmental factors (principally obesity), and predisposition genes that confer susceptibility.

At the cellular level, there are two main mechanisms responsible of IR development: (1) cellular stress in the endoplasmic reticulum, and in the mitochondria of adipocytes, hepatocytes and myocytes; and (2) release of pro‐inflammatory cytokines, principally, TNF‐α and IL‐6 by activation of the Toll‐like receptor 4 (TLR‐4) on the surface of infiltrated macrophages of white adipose tissue and liver [72, 73]. Obtained data point towards multiple triggering paths for this processes to be started, however, it is the obesity‐associated chronic low‐grade inflamma‐ tion the most linked one [72, 73].

Moreover, in obesity, the amount and the size of adipocytes increase (hyperplasia and hypertrophy); furthermore, macrophage infiltration in white adipose tissue is higher and these processes together deregulate the secretion of adipokines. One of them, adiponectin, is negatively correlated with WAT accumulation (as mentioned before), this diminishes insulin signalling, already affected by the pro‐inflammatory milieu. To add complexity to the IR phenomenon, they have been recently described novel mechanisms of immunometabolic regulation, the miRNAs.
