I think we really over-estimate the degree to which "fasting" for less than one day affects our ability to perform. We're blessed with such an abundance of food in the west that few of us have ever felt real hunger. Your body is so far from hitting a wall after 12 hours of no food it doesn't seem to be anything but mental. I've done tons of fasted weight training and high-intensity cardio.

I grew up doing sunrise to sunset fasts for Ramadan, with no water, and even that was mostly mental.