How can we measure insulin sensitivity/resistance?

Abstract

Insulin resistance represents a major public health problem, as it plays a major role in the pathophysiology of type 2 diabetes mellitus; it is also associated with increased cardiovascular risk and atherogenic dyslipidaemia, and is a central component of the cluster of metabolic abnormalities that comprise the metabolic syndrome. Thus, the development of tools to quantify insulin sensitivity/resistance has been the main objective of a number of studies. Insulin resistance can be estimated with the use of several biological measurements that evaluate different aspects of this complex situation. To that end, it requires various resources, ranging from just a single fasting blood sample for simple indices, such as the HOMA or QUICKI, to a research setting in which to perform the gold-standard hyperinsulinaemic–euglycaemic clamp test. The choice of method for evaluating insulin resistance depends on the nature of the information required (classification of individual subjects, group comparisons, precise measurement of either global, muscle or liver insulin sensitivity/resistance) and on the available resources. The aim of this review is to analyze the most frequently used assay methods in an attempt to evaluate when and why these methods may be useful.

Résumé

La résistance à l’insuline représente un problème majeur de santé publique puisqu’elle joue un rôle central dans la physiopathologie du diabète de type 2, est associée à une augmentation du risque cardiovasculaire et est un élément central d’un ensemble d’anomalies métaboliques qui définissent le syndrome métabolique. Ainsi, de nombreuses études se sont attachées à développer des outils pour apprécier et quantifier la sensibilité/résistance à l’insuline. L’insulinorésistance peut être mesurée de plusieurs façons qui permettent d’en appréhender différents aspects et qui font appel à des méthodes qui vont du simple prélèvement sanguin à jeun, qui permet le calcul des index simples HOMA ou QUICKI, à la méthode de référence du clamp hyperinsulinémique euglycémique. Le choix de la méthode pour évaluer le niveau de résistance à l’hormone dépend de la nature des informations requises (procéder à un classement de sujets, de groupes, mesurer précisément la sensibilité/résistance à l’insuline globale, musculaire ou hépatique) et des outils disponibles. Le but de cette revue est d’analyser les méthodes le plus fréquemment utilisées afin de clarifier quand et dans quelles situations, elles sont utiles.

Introduction

Insulin is a peptide hormone that exerts a variety of effects, mostly anabolic, on different cell types, but mainly in hepatocytes, myocytes and adipocytes. The hormone stimulates cell growth and differentiation, and promotes the storage of substrates in fat, liver and muscle by stimulating lipogenesis and lipid storage, and glycogen and protein synthesis, while inhibiting lipolysis, glycogenolysis and protein breakdown. Thus, resistance to the hormone leads to hyperglycaemia and hyperlipidaemia, while a lack of insulin results in protein-wasting, ketoacidosis and, ultimately, death [1]. Although insulin is central to all intermediary metabolic processes, its main action is related to glucose homoeostasis. Therefore, insulin resistance is typically defined as a decrease of insulin-mediated glucose disposal in insulin-sensitive tissues and increased hepatic glucose production (HGP). Peripheral insulin resistance—in other words, reduced insulin sensitivity—refers to a low capacity to utilize glucose in target tissues, mainly skeletal muscle, where it affects glucose transport and glycogen synthesis. At the level of adipose tissue, the main feature of insulin resistance is increased lipolysis via decreased antilipolytic insulin activity [2], [3]. In addition, hepatic insulin resistance refers to increased glucose production by the liver via impaired inhibition of glycogenolysis and stimulation of gluconeogenesis [3], [4]. Overall, insulin is implicated in numerous cellular mechanisms and in key tissues, such as muscle, in which insulin regulates over 700 genes [5].

Since the definition of the metabolic syndrome in the 1980s by Reaven [6], who first proposed that insulin resistance was clustered together with numerous cardiometabolic abnormalities (hyperglycaemia, elevated plasma triglycerides [TG], low levels of high-density lipoprotein [HDL] cholesterol and hypertension) in association with the emergence of the obesity pandemic, insulin resistance has gained in importance. Insulin resistance is a major component of several significant cardiometabolic abnormalities, including the metabolic syndrome, type 2 diabetes and cardiovascular disease (CVD) [7]. Their presence strongly suggests evidence of an insulin-resistant state, but does not either demonstrate its presence or quantify it. Some nonobese subjects may present with all of the metabolic abnormalities, whereas other obese subjects may not present with any metabolic abnormality and be classified as metabolically normal [8], [9]. Moreover, insulin resistance is not always pathological, and may be observed in physiological states such as pregnancy and puberty [10]. In addition, there is considerable overlap of the insulin sensitivity/resistance distribution between the physiological and pathological states [10]. Thus, the development of tools to quantify insulin sensitivity/resistance has been the main focus of several studies. Such studies can provide information on the underlying pathophysiological mechanisms, and may be associated with clinical and therapeutic outcomes in the field of diabetes and CVD.

The different methods available for measuring insulin sensitivity/resistance have been exhaustively reviewed elsewhere [11], [12], [13], [14], [15], [16], [17], [18]. For this reason, the aim of the present review is to discuss the most frequently used tests in clinical research and to focus on the surrogate methods currently available.

In addition to most dynamic tests to evaluate insulin resistance, all the simple surrogate indices for insulin sensitivity/resistance estimation mainly depend on insulin values. These are particularly important in the fasting state, when insulin levels are relatively low and a small difference in its measurement could considerably impact the surrogate indices. Also, it is important to consider that the insulin assay itself might be critical at both the preanalytical and analytical levels. In addition to the well-known importance of avoiding haemolyzed samples for insulin assay, it is important to note that insulin levels are higher in serum than in plasma samples, with differences that are proportional to insulin concentrations [19]. Therefore, even while using the same analytical procedures, when insulin values are derived from both serum and plasma samples in a study, they cannot be compared.

Furthermore, it is not possible to compare the results of surrogate indices where different insulin assays have been used, as may be the case in multicentre studies where insulin has not been centrally tested. Indeed, a twofold difference in insulin values has been reported in a study investigating 11 different types of human insulin assays [19]. Standardization of these insulin assays may thus improve several variations that preclude such comparisons between studies [20]. Recently, the Insulin Standardization Work Group recommended that concentrations of insulin be reported in international units (SI, pmol/L), thereby avoiding all references to traditional insulin units based on insulin biological activity per milligram of standard preparation [20], [21].

On the other hand, the Work Group also showed that most commercial insulin assays [21] can achieve consistent performance with calibration traceability based on individual serum samples, with insulin concentrations set by isotope dilution-liquid chromatography/tandem mass spectrometry measurement, a procedure that is calibrated using purified recombinant insulin. They also observed that several, but not all, insulin assays were acceptable for precision, accuracy and cross-reactivity. However, performance of these insulin assays was not similar at low concentrations, a critical point when determining cut-off values between normal and insulin-resistant states. Therefore, a serious and sustainable insulin assay standardization programme needs to be established [20], as this will likely improve the reliability of surrogate indices of insulin sensitivity.