As I was preparing to teach on digestion today, I came across a great paragraph in the student's textbook that is a reminder of how amazing the body is. Also, it gives some insight to why your body gets cranky when you make drastic changes to your diet.
Pancreatic secretions contain a mixture of enzymes to digest carbohydrate, fat, and protein. How does the pancreas know how much of each type of enzyme to provide?
The pancreas knows what its owner has been eating, and it secretes enzyme mixtures tailored to handle the food mixtures that have been arriving recently (over the last several days). Enzyme activity changes proportionately in response to the amounts of carbohydrate, fat, and protein in the diet. If a person has been eating mostly carbohydrates, the pancreas makes and secretes mostly carbohydrases [enzyme to digest carbs]; if the person's diet has been high in fat, the pancreas produces more lipases [enzyme to digest fats]; and so forth. Hormones from the GI tract, secreted in response to meals, keep the pancreas informed as to its digestive tasks. The day or two lag between the time a person's diet changes dramatically and the time digestion of the new diet becomes efficient explains why dietary changes can "upset digestion" and should be made gradually.
I thought this explanation was written so well, it was worth just quoting in its entirety. Thanks to Whitney and Rolfes for writing it in Understanding Nutrition (13th Ed).