Oily stool is stool that contains more fat than normal, giving it a greasy, slick appearance. The medical term is steatorrhea, and it’s defined as excreting more than 7 grams of fat per day in your stool. In a healthy digestive system, over 94% of the fat you eat gets absorbed. When something disrupts that process, the excess fat passes through and shows up in the toilet.
What Oily Stool Looks Like
Fatty stools are pale or light-colored, bulky, and noticeably greasy. They often leave an oily film on the surface of the toilet water or are difficult to flush. The smell is distinctly foul, stronger than a normal bowel movement. Some people notice droplets of oil floating separately from the stool itself.
This is different from mucus in stool, which looks more like a clear or whitish jelly clinging to the surface. Oil has a slick, shiny quality and spreads on water rather than holding its shape. If you’re unsure which you’re seeing, the smell is a reliable clue: fatty stools have a particularly rancid odor that mucus-coated stools do not.
How Your Body Normally Digests Fat
Fat digestion is a multi-step process that depends on your pancreas, liver, and small intestine all working together. When you eat fat, your pancreas releases enzymes (lipase and colipase) that break large fat molecules into smaller pieces called fatty acids and monoglycerides. At the same time, your liver sends bile salts into the small intestine. These bile salts act like a detergent, dissolving fat into tiny droplets called micelles so the lining of your small intestine can absorb them.
Once absorbed, those fatty acids get repackaged inside your intestinal cells, wrapped in protein, and shipped into your lymphatic system for distribution throughout your body. A failure at any point in this chain, whether it’s insufficient enzymes, too little bile, or a damaged intestinal lining, means fat passes through undigested and ends up in your stool.
Common Causes
Pancreatic Insufficiency
The most well-known cause of persistent oily stools is exocrine pancreatic insufficiency, where the pancreas can’t produce enough digestive enzymes. This doesn’t cause noticeable steatorrhea until more than 90% of pancreatic function is lost, which is why it often develops gradually. Conditions that can lead to this include chronic pancreatitis, cystic fibrosis, pancreatic cancer, gallstone pancreatitis, and long-term heavy alcohol use.
Bile-Related Problems
Without enough bile reaching your small intestine, fat can’t be properly dissolved for absorption. Liver cirrhosis, blocked bile ducts, and cholestasis (reduced bile flow) all interfere with this step. Certain types of bacterial overgrowth in the small intestine can also break down bile salts before they do their job, which both reduces fat absorption and triggers watery diarrhea as unprocessed bile irritates the colon.
Small Intestine Damage
Even when enzymes and bile are working fine, a damaged intestinal lining can’t absorb fat properly. Celiac disease is a common culprit. The immune reaction to gluten destroys the tiny finger-like projections (villi) that line the small intestine and do the actual work of absorption. Crohn’s disease can cause similar damage, especially when it affects the upper portions of the small intestine. Infections, certain medications, and other autoimmune conditions can also injure the intestinal lining enough to cause fat malabsorption.
Dietary and Temporary Causes
Not every episode of oily stool signals a medical problem. Eating an unusually large amount of fat in one sitting can overwhelm your digestive capacity temporarily. Certain fat substitutes, like olestra (once used in snack foods), pass through the body undigested by design. High-dose fish oil supplements can also produce oily stools. Weight-loss medications that block fat absorption work by deliberately preventing your body from absorbing dietary fat, making greasy stools an expected side effect. If oily stool only happens once or twice after an identifiable trigger, it’s generally not a concern.
How It’s Diagnosed
The simplest screening test involves staining a stool sample with a special dye that binds to fat globules, making them visible under a microscope. This test picks up clinically significant fat malabsorption about 90% of the time, with a sensitivity of 100% and specificity of 96% when performed correctly.
For a more precise measurement, a 72-hour fecal fat collection is considered the gold standard. You eat a controlled diet containing about 100 grams of fat per day for three days while collecting all stool produced. A result above 7 grams of fat per day (or more than 21 grams total over the 72-hour period) confirms fat malabsorption. It’s not a pleasant test, but it provides a definitive number.
Once fat malabsorption is confirmed, additional testing focuses on finding the underlying cause. This might include blood work for celiac disease, imaging of the pancreas, or tests for bile duct obstruction, depending on your other symptoms and medical history.
How Oily Stool Is Treated
Treatment depends entirely on what’s causing the fat malabsorption. If the problem is pancreatic insufficiency, the standard approach is enzyme replacement therapy: capsules containing digestive enzymes that you take with every meal and snack. These capsules supplement what your pancreas can no longer produce on its own. Dosing starts low and gets adjusted upward based on how well your symptoms improve. For adults, the typical range is 500 to 2,500 units of lipase per kilogram of body weight per meal, with smaller doses for snacks.
For bile-related causes, treatment focuses on restoring bile flow. This could mean addressing a blocked duct, managing liver disease, or treating bacterial overgrowth with antibiotics. In celiac disease, a strict gluten-free diet allows the intestinal lining to heal, and fat absorption typically improves within weeks to months as the villi regenerate.
Regardless of the cause, persistent fat malabsorption can lead to deficiencies in fat-soluble vitamins (A, D, E, and K) and unintended weight loss, since your body isn’t extracting calories from a significant portion of the fat you eat. If oily stools continue for more than a few days without an obvious dietary explanation, or if you’re also losing weight, feeling fatigued, or noticing other digestive changes, it’s worth getting checked out. These symptoms together suggest your body is consistently failing to absorb nutrients it needs.