Metabolic Flexibility and Insulin Resistance: From Molecular Mechanisms to Therapeutic Opportunities

Metabolic flexibility describes the capacity to adjust fuel utilization to changes in nutrient supply and energy demand. Impaired switching between lipid and carbohydrate metabolism accompanies insulin resistance, but the relationship depends on the physiological challenge and measurement method. Evidence was assessed across human substrate-oxidation studies, skeletal-muscle mitochondrial function, lipid intermediates, adipose substrate delivery, exercise and circadian regulation. Experimental and clinical findings show that flexibility is a dynamic property of an integrated system rather than a synonym for high mitochondrial capacity or low tissue lipid content. Exercise can improve insulin action despite increased intramyocellular lipid storage, while pharmacological improvement in insulin sensitivity does not always require a parallel increase in maximal mitochondrial ATP synthesis. Studies adjusting for glucose disposal further caution against interpreting every reduced respiratory-quotient response as an independent mitochondrial defect. Therapeutic studies should specify the metabolic challenge, quantify substrate availability and distinguish improved switching from forced dependence on a single fuel.