This diagram is from a medical nutrition textbook. You'll see the various sections of the digestive track have specifically assigned absorption points for micro-nutrients - vitamins and minerals. I've added the notations about the standard lengths of each section of your small and large intestine so you can calculate how much of your intestines have been bypassed based on your own surgical report (from the doctor). I've shaded a section that represents 100-150cm bypass, which is the standard length for most Roux-en-Y surgeries performed today.
Any nutrient that falls within the shaded area of bypass will no longer be absorbed from the food you eat. Those absorption points are designed to grab nutrients from food as the food passes by - but if the food never enters that area again, no nutrients can be absorbed. This is why we must take supplements for the rest of our lives.