The way I understand it is that the ratios shift as inputs/outputs change, and also as certain body mechanisms are enacted by these changes.
Comparatively Increased Carb Intake = Increased Carb Utilization
Comparatively Increased Fat Intake = Increased Fat Utilization
Muscle Glycogen decrease, increased utilization of FAT and PRO catabolism.
Blood Sugar decrease, glucagon increase and increased FAT utilization.
Blood Sugar increase, insulin increase, increased glycogen, increased CHO usage.
Protein however is not used as readily, and when inputs of FAT and CHO drop then the ratio of PRO being broken down for E is increased.
FAT and CHO are both being used all the time, it is just a matter of conditions that alter the ratios of their utilization. And their are many others that may contribute, ie. Exercise intensity, and I would guess stress also.
The body systems are highly-malleable. Their plasticity fights for homeostasis. I'm quickly becoming convinced that BOBO is correct in "balanced" E inputs as elimination of one always leads to increased utilization of another, yet the imbalance that results always leads to increased detriment(s) of some kind, ie. Weight Gain or Catabolism. Manipulation of extremes doesn't get you any further ahead in the long run.
Hopefully all of the above examples are correct, someone more knowledgable needs to reply to this.