Agree with this. I like doing my long runs empty but these are low intensity. For anything else I really struggle unless I leave at least a 2 hour gap, stomach won't settle.There is increasing evidence behind some starved training to increase fat burning ability (not for weight loss but metabolic economy) is useful but only 1 or 2 sessions a week.
This just involves lower intensity exercise in the morning with no breakfast. Also no carbs for first 1-2 hours depending on existing fitness.
The intensity has to be low as high intensity workouts should be done with good care stores to maximise training effect.
Also, it's suggested not to eat in a 2 hour window before training as you may be in the post-carb dip as you begin training. Of course, life gets in the way of the ideal!
Fuelling on long runs is a matter of preference but the body is limited to processing about 60g carbs an hour (about 3 gels).