Originally Posted by gregf83

The amount of oxygen used doesn't change for a given power level regardless of the ratio of carbs and fats being burned. The products of carbohydrate and fat oxidation are CO2 and H2O. If you burn fat you will produce less CO2 which but more H2O.

How are you measuring your power output? If you have a properly calibrated power meter you shouldn't need to rely on HR to determine your effort as HR can be influenced by many factors including hydration levels. After a long ride my HR is always higher for a given level of effort than at the beginning due to dehydration. [/LEFT]