Bile is secreted by the liver, enters the upper small intestine by way of the bile duct, and is absorbed in the small intestine. Bile has an important role in fat digestion, emulsifying fat as the first stage in its digestion. Bypassing the major site of bile acid reabsorption in the small intestine therefore further reduces fat and fat soluble vitamin absorption. As a result, huge amounts of fatty acids which are normally absorbed in the small intestine, enter the colon where they cause irritation of the colon wall and the secretion of excessive volumes of water and electrolytes, especially sodium and potassium, leading to diarrhea. This diarrhea is the major patient complaint and has characterized jejunoileal bypass in the minds of patient and physician alike since the procedure was introduced.
Bile salts help to keep cholesterol in solution in the bile.
Following JIB, the bile salt pool is decreased as a consequence of reduced absorption in the small intestine and bile salt losses in the stool. The relative cholesterol concentration in gallbladder bile rises and cholesterol crystals precipitate in the gallbladder bile, forming a nidus for development of cholesterol gallstones in the gallbladder. Specific vitamin deficiencies also occur, Vit D and Calcium deficiencies lead to thinning of bone with bone pain and fractures as a result of osteoporosis and osteomalacia. Bypass of the terminal ileum which is the specific site of Vitamin B12 absorption, leads to Vitamin B12 deficiency with a specific peripheral neuropathy. Vitamin A deficiency can induce night blindness.