These altered pathways and reduced capacity mean that the timing and location of 
nutrient absorption are significantly changed. For instance, the duodenum is a primary site for absorbing iron and calcium. In gastric bypass, bypassing this section directly impairs the absorption of these minerals. Similarly, vitamin B12 absorption requires intrinsic factor, which is produced in the stomach. With a smaller or bypassed stomach, intrinsic factor production can be reduced, hindering 
B12 absorption in the ileum (the final section of the small intestine).