Originally Posted by
Trakhak
There seems to be a consensus among researchers that aerobic exercise performed in zones 1 and 2 (using the 5-zone scale) burns fat almost exclusively, whereas exercise in zone 3 and above burns carbs at a rate that increases as the effort increases.
Yes, here's a rough example graph of carbs and fat usage (grams/minute) by intensity:
Zone 2 is already gobbling up the carbs. It's worthwhile to know that you can still deplete your glucose in zone 2, so even zone 2 efforts of 90 minutes or more benefit from carb ingestion, mainly by reducing fatigue from declining blood glucose.