I asked this question to various doctors while my symptoms were being investigated and I never got a clear reply. So if you are experiencing pain, sickness, diarrhoea or even just foul breath and eggy burps after eating fatty food then hopefully I can help you to understand why.
Firstly how is fat digestion supposed to work? The majority of fat digestion occurs in the small intestine. When the fat reaches the start of the small intestine (duodenum) it triggers the release of hormones which in turn initiate the release of enzymes from the pancreas and bile from the liver (via the gall bladder and bile duct, see here for an excellent diagram of the digestive system). The healthy stomach is very acidic mainly to assist in protein digestion and to break down or damage any pathogens (germs) that may have found their way into the stomach. However our digestive enzymes are proteins too so they do not work as well in an acidic environment. One function of bile, along with sodium bicarbonate released from the pancreas, is to restore an almost neutral pH to the bowel so the digestive enzymes can function properly. Bile is also essential to fat digestion by making the fat molecules water soluble (emulsification) which makes the digestive enzymes job much easier. The most important enzyme for fat digestion is pancreatic lipase, it completes the digestion process and allows the small intestine wall to absorb the fat molecules.
So this is how the fully functional fat digestion process works but why and how does this process end up causing pain and sickness in some people? If you have had all the tests and scans showing no ulcers, inflammation, gallstones, pancreatitis or other disease then the fault will possibly lie with irritable bowel syndrome (IBS).
The main symptoms of IBS are caused by erratic bowel spasms. If one or more of these spasms occurs in the duodenum it can result in the stomach not being able to drain properly (sometimes known as gastroparesis) leaving an uncomfortable full sensation, bloating, bad breath, nausea, stomach cramps and feelings of ‘indigestion’ (dyspepsia). I have found eating smaller meals more frequently helps with all of these symptoms and low dose (10mg) amitriptyline helps with the discomfort and pain.
Why are these symptoms worse when fat is present? This could be the result of ‘bile reflux’. Bile is only released in the presence of fat in the duodenum and bowel spasms may push this bile into stomach. Bile is not normally present in the stomach so this upsets the normal digestive process. There are some reports that bile can damage the stomach lining resulting in symptoms of gastroenteritis (gastritis) or worse lasting damage but I do not believe these have been scientifically verified and should not be something to worry about. Bile acid sequestrant medication can help with these symptoms. Bile acid sequestrants include: Cholestyramine (Questran) , Colesevelam (Cholestagel in Europe, Welchol in the USA) and Colestipol (Colestid, Colestipid).
The worst symptoms I get after eating fatty foods are extreme stomach pain (usually occurring in the night or very early morning), vomiting and diarrhoea match closely those of biliary colic. Biliary colic is the pain resulting from gallstones blocking one or more of the ducts leading from or to the gallbladder causing it to distend. So it is possible that the bowel spasms somehow prevent draining of bile from the gallbladder into duodenum causing the gallbladder to painfully distend. In a similar way bowel spasms could mimic pancreatitis as bowel spasms could prevent draining of the pancreas via the pancreatic duct into the duodenum. Both amitriptyline and bile acid sequestrants help to control the diarrhoea but the best treatment I have found to prevent these extreme episodes is to eat a very low fat diet. I have not found any medication that helps, even powerful painkillers do not control the pain it is simply a matter of waiting it out. Unfortunately there is no ‘cheat day’ on this diet even one sneaky chocolate bar would result in a really bad night. For more information on my diet see here.