Another interesting study shows another benefit of fasted training:

Exercising in a glycogen depleted state could lead to more mitochondria and therefore more energy producing capabilities for future training.

As exercise with low glycogen increases PGC-1α gene expression in human skeletal muscle

We conclude that exercise with low glycogen levels amplifies the expression of the major genetic marker for mitochondrial biogenesis in highly trained cyclists.

The results suggest that low glycogen exercise may be beneficial for improving muscle oxidative capacity.

Read the full paper here