I used to think why does the body maintain the level of glucose in the blood. What is the use for it? I can understand that glycogen is required, but why keep it forever in the blood when its so bad. Why not generate it only when required?
The answer seems to be the insolubility of triglycerides and amino-acids. The two need to be transported through the blood, which is water based and can dissolve only polar molecules. Here glucose comes in handy. It attaches to the amino-acids and triglycerides and makes them polar and hence water soluble.
So the blood needs to maintain the required level of glucose. It may need to have more glucose after digestion, and further more when glycogen replenishes. So during these times glucose level will rise anyway, even if we don't have much carbohydrate in the diet.
I have also read that dietary fats do not travel through the blood but rather through the lymph node system. So fats should not cause a spike in glucose, but proteins should, as they will need to be transported.