Originally Posted by
jim p
Maybe I do have it backwards about which energy source is used first by the body but it would seem that glucose would be used first because it is already in the blood stream.
Glucose is in your blood stream, and so is a little bit of fat ("free fatty acids"), and they are both continuously being used. Glucose accounts for most of the resting metabolism, and fat is the preferred fuel for your muscles. Only at high levels of exercise intensity, when muscles can't cope with the load by burning fat, they start tapping into glucose stores.