From:
http://co2factor.blogspot.com/
"Eat sugar/protein, preferably something high quality like milk and Oj, before, during and after exercise."
From my experience, this is absolutely not necesarry and eating before exercise definitely makes my exercise performances worse. Too much energy/blood goes to digestion, while all your energy/blood should go to your muscles.
Besides that, the vibration you make during running, I think kind of rubs the food throughout year gut and stomach, and this irration can cause a huge increase in serotonin. I can literally feel this irritation, and makes me feel really bad, so I try to avoid that at all cost.
Muscles contain glycogen, I think you need to exercise for quite some time before you run out of this carb source. And besides that the liver stores 120 gram of glycogen, I think you need to have fasted for hours before you run out all your stored carb sources. Therefore, I think exercising on an empty stomach is perfectly healthy and fine:
Muscle cell glycogen appears to function as an immediate reserve source of available glucose for muscle cells. Other cells that contain small amounts use it locally, as well. As muscle cells lack glucose-6-phosphatase, which is required to pass glucose into the blood, the glycogen they store is available solely for internal use and is not shared with other cells. This is in contrast to liver cells, which, on demand, readily do break down their stored glycogen into glucose and send it through the blood stream as fuel for other organs.
The same argument I think goes for coffee. I think if you are in a healthy state, you can perfectly drink coffee, or use caffeine on an empty stomach. This argument that you need coffee together with carbs just doesn't make sense to me logically, as your body stores plenty of carbs as glycogen, I think it take quites some time before you run out of all your glycogen stores. If I don't eat anything beside coffee for the first 6 hours of the day, I feel perfectly fine, and I don't have any stress reaction.
Probably, depends per person, but I don't think we should scare all people that you should eat carbs all day long to have a healthy metabolism, or that you have to eat carbs before exericse, it just doesnt make sense.