Exercising while fasted induces adaptations to mitochondria in muscle and adipose tissue including increased fatty acid metabolism that is blunted by pre-exercise feeding (meta-analysis of 46 clinical studies).
Exercising in a fasted state increased the release of fatty acids stored in adipose tissue and the use of them for energy in muscle and adipose tissue (ie. fat burning). It also increased the use of intramuscular triglycerides over glycogen in muscle tissue. Exercise while fasted also caused mitochondria to increase gene activity in genes related to fatty acid metabolism making them more efficient as using fat for energy. These adaptations were blunted by pre-exercise feeding.
Pre-exercise feeding did enhance performance in long-duration exercise (> 60 minutes) but had no effect on aerobic training shorter than 60 minutes. Pre-exercise feeding also slightly enhanced anaerobic exercise (ie. run until exhaustion) but had no effect on high-intensity interval training.