Metabolic flexibility and insulin resistance

Abstract

Metabolic flexibility is the capacity for the organism to adapt fuel oxidation to fuel availability. The inability to modify fuel oxidation in response to changes in nutrient availability has been implicated in the accumulation of intramyocellular lipid and insulin resistance. The metabolic flexibility assessed by the ability to switch from fat to carbohydrate oxidation is usually impaired during a hyperinsulinemic clamp in insulin-resistant subjects; however, this "metabolic inflexibility" is mostly the consequence of impaired cellular glucose uptake. Indeed, after controlling for insulin-stimulated glucose disposal rate (amount of glucose available for oxidation), metabolic flexibility is not altered in obesity regardless of the presence of type 2 diabetes. To understand how intramyocellular lipids accumulate and cause insulin resistance, the assessment of metabolic flexibility to high-fat diets is more relevant than metabolic flexibility during a hyperinsulinemic clamp. An impaired capacity to upregulate muscle lipid oxidation in the face of high lipid supply may lead to increased muscle fat accumulation and insulin resistance. Surprisingly, very few studies have investigated the response to high-fat diets. In this review, we discuss the role of glucose disposal rate, adipose tissue lipid storage, and mitochondrial function on metabolic flexibility. Additionally, we emphasize the bias of using the change in respiratory quotient to calculate metabolic flexibility and propose novel approaches to assess metabolic flexibility. On the basis of current evidence, one cannot conclude that impaired metabolic flexibility is responsible for the accumulation of intramyocellular lipid and insulin resistance. We propose to study metabolic flexibility in response to high-fat diets in individuals having contrasting degree of insulin sensitivity and/or mitochondrial characteristics.

Fig. 1.

Model for fat-induced insulin resistance describing how a failure to appropriately store lipids into subcutaneous adipose tissue (quantitatively predominant) will lead to ectopic lipid deposition into visceral fat and insulin-sensitive tissues such as liver and skeletal muscle. These tissues will progressively develop a state of lipotoxicity, altering insulin signaling and action and contributing to whole body insulin resistance and deterioration of glucose tolerance.

Fig. 2.

Different features of metabolic flexibility during overnight fasting (A), during a hyperinsulinemic clamp (B), in response to a high-carbohydrate diet (C), and in response to a high-fat diet (D). Metabolically flexible (•) and inflexible (○) subjects.

Fig. 3.

Correlation between metabolic flexibility [steady-state respiratory quotient (RQ) − fasting RQ = ΔRQ] and fasting RQ (A) and insulin-stimulated glucose disposal rate (B).