I bonked 2 years ago biking while fasting. I had no calories at all for about 36 hours before and I was 32 miles into my ride when it happened. I had to lay down for about 2 hours and I ate something. I was back to about 90% after 2 hours but I considered a hospital visit when it first happened. Yes, it was dumb and on purpose. I was trying to drive my body into starvation mode to reduce excess skin from weight loss. Running on empty a lot so to speak worked in that regard pretty well for me but I wouldn't think it would be the healthiest way do a ride. It was intended for weight loss rather than training gains. My opinion now would be different, and I would just train and accept the slower and likely healthier weight loss resulting from a good diet and good training plan.