If you have chronic kidney disease you would have abnormal renal function on basic labs. Sounds like you’ve had a lot of blood work so this likely would have been detected. Iron is absorbed in the first part of the small intestine so anything that damages the mucosa in this region can affect it (Celiac, Crohn’s, ulcers, etc). I’m not sure why those with a j pouch would have more iron deficiency anemia as the pouch is a good 15 feet away. Possibly from chronic low level bleeding from the cuff, pouchitis, erosions at anastomotic sites, etc. Also, perhaps patients with pouches have more restrictive diets. I can’t think of a reason why having a pouch would cause malabsorption. More likely j pouchers have the potential to bleed and poor intake. Im just speculating here. I’m not familiar with the literature in this topic.