Fats, or lipids, are an essential part of your diet. According to nutritionist Elson Haas, M.D., lipids are part of every cell membrane and every organ and tissue in your body. Along with proteins and carbohydrates, lipids are a primary source of energy for your cells, and they serve as the precursors for immune molecules, sex hormones, vitamin D and cholesterol. Like other nutrients, fats must be broken down before they can be absorbed through your intestine, but fats are more difficult to digest than other nutrients.
The physical and chemical properties of fats affect the way they are processed in your stomach and intestine. Unlike proteins and carbohydrates, which mix well within the aqueous environment of your gastrointestinal tract, fats are incompatible with water and tend to rise and float at the top of your stomach's contents. Dr. Haas says this is one reason fats are acted upon last and tend to slow digestion.
The act of chewing is the first step in separating the nutrients in your food. Once exposed to the digestive environment, proteins and carbohydrates are initially attacked by enzymes produced by your salivary glands and stomach; however, effective enzymatic breakdown of fats requires emulsification by bile acids, a process that does not occur until fats leave your stomach and enter your small intestine. A June 2010 review in "Advances in Physiology Education" reports that only 15 percent of fat digestion occurs before your food leaves your stomach.
Upon entering your stomach, fats stimulate the release of cholecystokinin, or CCK, from the cells that line your duodenum, which is the first segment of your small intestine. CCK is a hormone that suppresses your appetite, triggers the release of pancreatic enzymes, stimulates contraction of your gallbladder and, according to the February 1987 issue of "The American Journal of Physiology," delays gastric emptying. This allows the fats in your stomach to enter your intestine gradually, where they can be emulsified by bile from your gallbladder and broken down by enzymes from your pancreas.
Dietary fats slow your digestion in several ways. Their incompatibility with water makes fats resistant to the digestive process, and their arrival in your stomach triggers physiologic responses that delay gastric emptying. Over the millennia, humans have evolved to extract as much energy as possible from the foods they consume. The mechanisms your body uses to retain fats in your stomach are simply adaptations, which permit the optimal use of these calorie-rich nutrients.