Cholecystitis and gallstones are common diseases. Most gallbladder infections occur due to the presence of stones in the gallbladder, which block the gallbladder duct and cause poor bile drainage, followed by bacterial infection and the formation of cholecystitis. In some patients, there are no stones in the gallbladder, but bacteria enter the gallbladder from the intestine or from the blood circulation, resulting in cholecystitis. Patients with cholecystitis are also prone to the formation of gallstones due to changes in bile composition and concentration of bile, with bacteria and inflammatory necrotic material as the core, so cholecystitis and gallstones often accompany each other. Many people know that patients with cholecystitis and gallstones should avoid fatty diets, why is this? After gallbladder surgery, is it still necessary to avoid fatty diet? By understanding the anatomy and physiology of the gallbladder and bile duct, the relationship between cholecystitis, gallstones and fatty diet can be properly understood. The gallbladder is a pear-shaped bag located below the liver, and the liver produces about 800 ml of bile every day on average, except for a small amount of bile that flows directly into the duodenum, most of which is stored in the gallbladder after being concentrated 10 times by the gallbladder. When the gallbladder contracts, bile is squeezed from the gallbladder through the cystic duct into the common bile duct and into the duodenum to help digestion and absorption of food. When the fat in the diet enters the duodenum, it stimulates the intestinal mucosa to release a substance called “cholecystokinin”, which causes the gallbladder to contract and expel bile. If there are stones in the gallbladder or stones in the common bile duct that prevent the normal discharge of bile, the gallbladder will contract strongly, causing severe pain and even triggering the onset of cholecystitis or cholangitis. Some people with bacteria in the gallbladder can also develop the disease when the body’s resistance is reduced. As the majority of patients with cholecystitis are accompanied by gallstones, patients suffering from cholecystitis and gallstones should avoid fatty diets to avoid triggering pain and attacks of cholecystitis. The gallbladder is the site of concentrated and stored bile, and some patients who have had their gallbladders removed will have diarrhea when they eat fatty foods because there is no concentrated bile to help digestion in the duodenum. In addition, in patients who have undergone cholecystectomy, the common bile duct often dilates to partially replace the function of the gallbladder. If the compensatory function of the bile ducts is dysfunctional, symptoms similar to biliary colic may occur after eating a fatty diet, so patients after cholecystectomy should also avoid a fatty diet, at least for 1-2 years after surgery.