Every study every performed looking at this question shows that a high carb/glycemic load diet destroys body composition. We see fat gain and muscle loss. Low carb diets by contrast show a potent protein sparing effect. This is particularly pronounced in the ketogenic state. Now why does a high carb diet do this? In simple terms: metabolic derangement. Nothing that we want to work is working and everything we do not want is ramped up to full throttle.
Seeing is believing however! Try a 70% carb, 20% protein and 10% fat as per the ADA recommendations and see what happens to your body comp and well-being. Make those carbs come from primarily “healthy” whole grains just to add to the fun. Try that for a month. Then try the Zone or Metabolic diet using paleo foods for a month and compare notes.