Pre-exercise breakfast ingestion versus extended overnight fasting increases postprandial glucose flux after exercise in healthy men

Aims: To characterize postprandial glucose flux after exercise in the fed versus overnight fasted-state and to investigate potential underlying mechanisms.

Methods: In a randomized order, twelve men underwent breakfast-rest (BR; 3 h semi-recumbent), breakfast-exercise (BE; 2 h semi-recumbent before 60-min of cycling (50% peak power output) and overnight fasted-exercise (FE; as per BE omitting breakfast) trials. An oral glucose tolerance test (OGTT) was completed post-exercise (post-rest on BR). Dual stable isotope tracers ([U-¹³C] glucose ingestion and [6,6-²H₂] glucose infusion) and muscle biopsies were combined to assess postprandial plasma glucose kinetics and intramuscular signaling, respectively. Plasma intestinal fatty acid binding (I-FABP) concentrations were determined as a marker of intestinal damage.

Results: Breakfast before exercise increased post-exercise plasma glucose disposal rates during the OGTT, from 44 g•120 min^-1 in FE [35 to 53 g•120 min^-1] (mean [normalized 95% CI]) to 73 g•120 min^-1 in BE [55 to 90 g•120 min^-1; p = 0.01]. This higher plasma glucose disposal rate was, however, offset by increased plasma glucose appearance rates (principally OGTT-derived), resulting in a glycemic response that did not differ between BE and FE (p = 0.11). Plasma I-FABP concentrations during exercise were 264 pg•mL^-1 [196 to 332 pg•mL^-1] lower in BE versus FE (p = 0.01).

Conclusion: Breakfast before exercise increases post-exercise postprandial plasma glucose disposal, which is offset (primarily) by increased appearance rates of orally-ingested glucose. Therefore, metabolic responses to fed-state exercise cannot be readily inferred from studies conducted in a fasted state.