Used to do it when I was more into intermitted fasting. Would go at 7am for about 2.5 hour rides, going hard uphill and relaxed riding on the flats, burning about 2000 kcal, was a bit on the limit of what I could do fasted. These days I sometimes skip breakfast when there is no work-out that day, but specially if there is a heavy work-out planned, I would eat slightly more for breakfast.