Training after an All-Night Fast Does Not Promote Weight Loss
The body uses mainly carbohydrates as fuel at exercise intensities above 65 percent of maximum effort. However, it will use whatever fuels are available. It seems reasonable that fat use will increase during exercise when the body has minimal carbohydrate stores, such as after an all-night fast. Consequently, many bodybuilders and people trying to lose body fat will train the first thing in the morning without eating breakfast.
