Impact of pre‐exercise feeding status on metabolic adaptations to endurance‐type exercise training

Nutrition and exercise metabolism are vibrant physiological fields, yet at times it feels as if greater progress could be made by better integrating these disciplines. Exercise is advocated for improving metabolic health, in part by increasing peripheral insulin sensitivity and glycaemic control. However, when a modest‐to‐high carbohydrate load is consumed before and/or during each exercise bout within a training programme, increases in oral glucose insulin sensitivity can be blunted in both men of a healthy weight and those with overweight/obesity. Exercise training‐induced adaptation in the energy sensing AMP‐activated protein kinase (AMPK) and the insulin‐sensitive glucose transporter GLUT4 protein levels are sensitive to pre‐exercise feeding status in both healthy individuals and individuals classified as overweight or obese. Increased lipid oxidation may, in part, explain the enhanced adaptive responses to exercise training performed before (i.e. fasted‐state exercise) versus after nutrient ingestion. Evidence in individuals with type 2 diabetes currently shows no effect of altering nutrient–exercise timing for measured markers of metabolic health, or greater reductions in glycated haemoglobin (HbA1c) concentrations with exercise performed after versus before nutrient provision. Since the metabolic inflexibility associated with type 2 diabetes diminishes differences in lipid oxidation between the fasted and fed states, it is plausible that pre‐exercise feeding status does not alter adaptations to exercise when metabolic flexibility is already compromised. Current evidence suggests restricting carbohydrate intake before and during exercise can enhance some health benefits of exercise, but in order to establish clinical guidelines, further research is needed with hard outcomes and different populations. figure legend Candidate mechanisms linking nutrient–exercise timing to insulin sensitivity. Exercise performed in an overnight‐fasted state (before nutrient intake), increases fatty acid availability for skeletal muscle, and also increases intramuscular triglyceride utilisation. An increase in skeletal muscle lipid turnover may result in phospholipid remodelling, with a relative reduction in saturated fatty acids within skeletal muscle phospholipids. Increased fatty acid availability can also increase AMPK activity and this increase PGC‐1α levels. Exercise before versus after nutrient intake can also result in an increase in skeletal muscle GLUT4 and CHC