I recently fasted for four days, using only fresh blended fruit/vegetable juices for nutrition. I really felt better all around during that time. Over a year ago, I ate vegan for 2 months, but than I started to feel weak. I attributed it to the diet, and decided that taking 7 classes, martial arts, and other activities required a higher energy diet. Looking back at it now, I think I could have ate the same food I was eating, but just more of it, and I would have been okay.
I will go vegan again in the future.