Why running on fat does't work for moderate distance events - technical details

Everyone knows that burning a gram of fat produces more energy than burning a gram of glucose - roughly twice the amount. However, that fact is of little relevance to most athletes, as the supply of fuel is not the rate limiting parameter when it comes to performance. (ultra long races excepted)

The rate limiting step is the supply of oxygen to the working muscle. As much as Tim Noakes would like to, you can't magically increase the amount of oxygen that your heart pumps to your muscles during a race where you are running at close to VO2 max.

Rather than looking at the energy supplied by one gram of fat, we need to look at the amount of energy produced per unit of oxygen when burning fat vs that produced when burning glucose.
Check the biochem textbooks:

Glucose   467.1 kJ/mol O2

Fat           436.5 kJ/mol O2

Glucose is the clear winner. If you are running fast, and want your muscles to generate the maximum amount of energy in a given time, you want to be burning glucose, not fat.