Purely speculating, but this may be why some people who get headaches while fasting (low glucose levels) can make them go away with a bit of iron pumping. Also; it suggests, once again, that most exercise experts — those who counsel you to feed before, after, and often — just have no serious clue about complex energy pathways naturally selected for over millions of years.
The evolutionary life way is so much smarter.
From an evolutionary perspective, the result of this study is a no-brainer. Imagine what could have or did happen to all of the organisms that lost their wits along with their glucose when running from predators. They were obviously a light snack for the animals able to use lactate.
As another example of this sort of modern ignorance, anyone recall the fasting and blood glucose experiment performed by myself and others a while back, where our glucose levels increased markedly during exercise and remained elevated for some time, even though fasted for many hours? It's along the same lines as those unnecessarily obsessed (i.e., otherwise healthy people) with hypoglycemia during any sort of strenuous exercise. I've been doing most of my workouts fasted for many months, sometimes as much as 30 hours or more. It's my preferred way to work out and some of the really fabulous results I've obtained in huge strength increases (100-200% in many cases) with only an hour of training per week suggests to me that — go figure — our genes are best expressed the more we seek to model and simulate our evolutionary heritage.
Short version: predatory animals don't often hunt on full bellies.