Age and Diet Modulate the Insulin-Sensitizing Effects of Exercise: A Tracer-Based Oral Glucose Tolerance Test

Diet modulates the development of insulin resistance during aging. This includes tissue-specific alterations in insulin signaling and mitochondrial function, which ultimately affect glucose homeostasis. Exercise stimulates glucose clearance and mitochondrial lipid oxidation and also enhances insulin sensitivity (IS). It is not well known how exercise interacts with age and diet in the development of insulin resistance. To investigate this, oral glucose tolerance tests with tracers were conducted in mice ranging from 4 to 21 months of age, fed a low-fat diet (LFD) or high-fat diet (HFD) with or without life-long voluntary access to a running wheel (RW). We developed a computational model to derive glucose fluxes, which were commensurate with independent values from steady-state tracer infusions. Values for an IS index derived for peripheral tissues (IS-P) and one for the liver (IS-L) were steeply decreased by aging and an HFD. This preceded the age-dependent decline in the mitochondrial capacity to oxidize lipids. In young animals fed an LFD, RW access enhanced the IS-P concomitantly with the muscle β-oxidation capacity. Surprisingly, RW access completely prevented the age-dependent IS-L decrease; however this only occurred in animals fed an LFD. Therefore, this study indicates that endurance exercise can improve the age-dependent decline in organ-specific IS if paired with a healthy diet.

Exercise is a known strategy to improve insulin sensitivity (IS), whereas aging and a lipid-rich diet decrease IS. Using a tracer-based oral glucose tolerance test, we investigated how exercise, age, and diet interact in the development of tissue-specific insulin resistance. Exercise (voluntary access to a running wheel) mainly improved IS in animals fed a low-fat diet. In these animals, exercise improved peripheral IS only at young age but fully prevented the age-dependent decline of hepatic IS. The prevention of age-dependent decline in IS by exercise is tissue-specific and blunted by a lipid-rich diet.