Originally Posted by Hermes

Age 64. I do a lot of early morning rides starting on an empty stomach and only drink water. First, it takes energy to digest food and that means blood is diverted to the stomach. Second, absent any new source of glucose, muscles select fat metabolism and bias the set point to use glycogen higher. This conserves glycogen. If power levels increase there is a point where glycogen must be used and at some point one will run out. However, having a higher metabolic rate benefits all cyclists especially LD riders, stage racers and riders doing multiple days back to back.

Also, if one consumes too much glucose then there is an insulin reaction that inhibits fat burning. For me, eating less on the bike is better and the best result is cycling a couple of hours after eating.

We are going on a tour in France and the food is going to be amazing but it will definitely reduce my ability after breakfast and lunch. All that means is we go a lot easier for a while until the food digests.