The Case For Glycogen-Depleted Long Runs
The theory behind running your long runs on low glycogen stores is that by not having readily available muscle glycogen to burn, you body is forced to burn fat. Consequently, your body will become more efficient at using fat as a fuel source. The real question is, does this theory hold true?
A recent study conducted in New Zealand showed that cyclists who completed exercise early in the morning without eating breakfast (fasted state) improved muscle glycogen stores by as much as 50% over the group that ate breakfast before their exercise. Similar studies have made it clear that occasional fasting before exercise can improve glycogen storage and endurance performance.
However, other studies have gone further and tested the effects of training with low glycogen levels for more than one run or for extended periods of time. The research concludes that extended carbohydrate depletion impairs performance and does not enhance fat utilization.
The research makes a strong case that occasional long runs in a fasted state will improve glycogen storage and fat utilization, but extended training or multiple long runs in the fasted state will impair performance and does not provide further benefits to fat utilization.