I have always eaten something before exercising, even 5:30 am masters swimming. I have a cast iron stomach--the only time it ever bothers me is if I catch the stomach flu. When I first started training for triathlons, I ate a little carbohydrate and protein before morning training, something like an orange and a couple of pieces of cheese. Even after I went low carb, I was prone to feel out of energy and irritable if I went too long without eating (though my blood glucose was not problematically low).
I also had read that for some people with type 2 diabetes, morning high blood glucose (the dawn effect) would just go higher and higher until the person ate something to stimulate the body to produce insulin. The liver would release glucose, but the body wouldn't be able to use it until it got that insulin. I never tested that myself; it matched all my assumptions about eating before exercise. And when I was training for racing, the approach I took worked well for me, as it fit into my fueling strategy for racing.
Now that I am following the primal way of eating, I have been paying attention to the arguments for not snacking and realizing that I'm not hungry. I am less and less prone to that running out of fuel feeling. So I decided I would do a test and see if my blood glucose went high if I exercised without eating first. My test was swim practice-- 1 1/2 hours of swimming starting at 5:30 am. I tested after my warmup and an hour or so in after some hard swimming. I first tested the way I had been doing it, and then five days later I tested without eating first:
5:00 120 waking
5:25 115 before exercise, just after eating melon
6:05 93 after 1/2 hour of warmup
6:45 105 after swimming hard
7:30 125 1/2 hour after finishing workout
8:30 137 just finished slowly drinking my breakfast smoothie
9:40 119 to see if it came down
5:15 116 waking
5:50 100 after 1/2 hour warmup, no food
6:30 106 after swimming hard
7:40 120 after showering and driving home
8:20 130 after slowly drinking my breakfast smoothie
So there certainly doesn't seem to be any harm in fasting exercise for me. I felt a little less energy, and a little strange when the food hit me afterwards. But my body will adapt.