I found this diagram online somewhere a couple years ago (don't remember the source). It is a scan from a medical text book that shows the points of absorption for various nutrients. I have altered the picture and added the red lines. This indicates a normal bypass amount of 100-150cm --- so we are missing that section between the red lines. Any nutrient normally absorbed between the red lines, is no longer absorbed there because food does not pass by that part of the intestine anymore.
Thus, the need for a lifetime regimine of vitamins and supplements because our body can no longer absorb all the nutrients it needs from the food we eat.