A recent, well-controlled study at the University of Ottawa examined the acute effects of exercise timing and meal glycemic index on fat burning in healthy young guys. After an overnight fast, subjects were asked to perform a moderate treadmill exercise either before consuming a 400 kcal low-glycemic or high-glycemic breakfast, or after consuming the low-glycemic or high-glycemic meal. Not suprisingly, the results revealed that the amount of fat burned during exercise was significantly greater when it was performed before breakfast compared to the same exercise performed after breakfast, as was the amount of fat burned during the entire trial. However, no significant effect of meal glycemic index on the amount of fat burned was noted, so the timing of exercise was clearly more important than the composition of the meals. The bottom line is that those who wish to maximize exercise-induced fat burning should perform pre-breakfast cardio soon after waking. A well-designed thermogenic/lipolytic formula (e.g., iSatori MX-LS7) may further increase the fat-burning effects of fasted-state cardio. Also, pre-cardio ingestion of branched-chain amino acids may be of some benefit by minimizing the burning of muscle-derived amino acids.

Reference: Bennard P, Doucet E. Acute effects of exercise timing and breakfast meal glycemic index on exercise-induced fat oxidation, Appl Physiol Nutr Metab, 2006;31:502-511.