I was listening to a podcast which dealt with this issue. Seemingly it takes the body 3-4 weeks to adapt to the ketogenic (fat-burning) mode. After that, clinical studies have shown that there is no difference in athletic performance and endurance, and actually in longer-term studies, there are certain benefits from ketogenic diets. Having said that, these studies were done on healthy people. Anyone with a blood sugar imbalance should find a great difference in their energy levels once they acclimatise to a low-carb diet, because the energy production will be regularised, without all the peaks and troughs of blood sugar, leading to higher and better sustained energy levels.

Here's the podcast:

Dr. Stephen Phinney on energy metabolism and athletic performance on a ketogenic diet.

http://www.thelivinlowcarbshow.com/...-1-episode-170/

I found it really interesting, and hope that you do too.