It takes approximately 3 hours after a carbohydrate-containing meal for elevated insulin levels to return to pre-meal or waking fasting levels, which is exactly where you want them prior to a workout or race lasting longer than 60 minutes (perhaps as long as 90 minutes). To the degree that insulin levels are elevated at the start of a workout/race, the greater the body spends it muscle glycogen for energy production. The lower the insulin levels at the start of the event, the better your body spares available muscle glycogen stores. Research confirms that time to fatigue at aerobic pace is longer with pre-exercise fasting insulin levels than elevated insulin levels. Put another way, the athlete who consumes calories sooner than 3 hours prior to a 60-90+ minute event will bonk sooner than if the athlete fasted 3 hours prior.
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