Proteins and fats have to undergo a series of chemical reactions to form simple compound and then only they can enter Krebs Cycle. After forming simple compounds only then they can enter the cycle and help to produce energy which takes a lot of time. Glucose on the other hand can be easily oxidised without any complex or time consuming reactions and also it present in a quite large quantity. Due to above reasons fats and proteins are not used as respiratory substrate and glucose is used.
