Moreso that the liver and muscles are said to have enough glycogen stores for up to two hours of riding. Anyone getting up in the morning and bypassing breakfast to go for a training ride won't be in glycogen debt if their ride is between 90 minutes and two hours (taking into account intensity). It also explains the drop-off in performance towards the end if they do go beyond the threshhold.

This article:

http://www.active.com/running/Articl...nk_on_purpose_

explains what so-called bonk-training is all about, and that is to do with the release of interleukin-6 which influences the body's adaptations during exercise. And it seems the jury is still out on just how to go about it.

One key point is that prolonged exercise sessions to and past the point of glycogen depletion are not the right way to go about it, but rather to undertake a second training session within hours of the first, so it starts in a state of depletion.

Personally, I think there are other dangers involved in following the advice in the article to leave even fluids out of the second training session. The most significant issue, as I see it, is the effect that depletion of glycogen will have on judgment, and the ramification of that for a cyclist on the road might be quite serious.