Fasting is only used for aerobic workouts zone one and two and largely for those with aerobic deficiency syndrome. UA does not encourage fasting as a means of weight loss or food restrictions. The appropriate amount for calories and fueling should still be followed. It can however help your body “remember” how to burn fat for fuel while aerobic which is what it should do. However if an athlete has ADS their bodies do not.
There area couple of links to great UA Articles below to further explain fasted training.
Talking Fasted Training With Staff Performance Dietitian Rebecca Dent