When you eat fat, any fat, even the little bit in a few nuts or a bowl of plain oatmeal, your body attempts to absorb that fat for its calories and nutrients, like fat-soluble vitamins. It uses bile acids to do that. Bile acids are produced in the liver, stored in the gall bladder, and squirted into the intestine when fat is detected. The bile acids emulsify the fat and make it easier to absorb.
Most bile acids are reabsorbed in the small intestine before they reach the large intestine or colon. Sometimes they aren’t (lots of reasons, especially if there is damage to the small intestine or ileum). Bile acids in the colon cause irritation and pain in the colon, less-solid stools, frequent bowel movements, and diarrhea. (They also increase the risk for colon cancer, which I’ve discussed.)
So, eating fat leads to bile secretion; eating lots of fat leads to lots of bile secretion, some of which can end up in colon, irritating it.
The less fat you eat, the less bile you secrete, which calms the intestines. We’ve known this for decades:
Fat-Reduced Diet In The Symptomatic Treatment Of Small Bowel Disease, Metabolic studies in patients with Crohn’s disease and in other patients subjected to ileal resection, Gut 1974
A fat-reduced diet is recommended in the symptomatic therapy of chronic diarrhoea in patients suffering from diseases of the ileum.
Effect Of A Low-Fat Diet On Bile Salt Excretion And Diarrhoea In The Gastrointestinal Radiation Syndrome, Acta Radiologica: Oncology, Radiation, Physics, Biology, 1979
Nine patients with diarrhoea and bile salt malabsorption after pelvic irradiation for malignant gynaecologic tumours were treated with a low-fat diet (40 g of fat per day) for 3 to 6 months. Faecal excretion of bile salts decreased in all patients concomitant with relief of symptoms.
If you are experiencing any irritable bowel or intestinal discomfort, try it. Just try it. Stop eating fat for 1 week. No oil. No butter or cheese or eggs or meat or even peanut butter or nuts. Just 1 week. You can go back to eating anything you like after that. What do you have to lose?