A riding buddy of mine is a very talented athlete. At 70, he rode a bike over the highest road pass in the world, something over 18,000'. When he came back, he took 45' off the record for his age group on a local mountain TT. So he's got cred. He also has very good friends in India, with whom he stays for a few weeks every year. They have a standard Indian practice of fasting one day a week, so he did too. Coming back from those trips, he'd try to maintain that discipline, but it just didn't work with riding. Screwed up his metabolism to where he hadn't good riding energy for much of the week, so he quit doing it while in the US. He thought it didn't work because it bounced his hormones and glycogen all over the place. I've found it's best athletically to keep things nice and steady.