Dietary fats, like those in butter, meat or cooking oils, are basically organic compounds composed of carbon, hydrogen, and oxygen. They consist of complex molecules and are the most highly concentrated source of energy in our daily diet. They belong to a class of substances called lipids. Unfortunately, dietary fats do not dissolve in water, as a result they are not easily broken down by fat-digesting enzymes (lipase) in the watery content of the gastrointestinal tract. Thus fats tend to take longer to digest than carbohydrates or proteins.
How Fat Is Digested
Although a small amount of lipase is secreted by Ebner's glands on the tongue, and by the stomach, these digestive actions are not significant, as almost no real breakdown of fat occurs until the fats reach the duodenum in the form of gastric chyme.
Fat Breakdown In The Small Intestine
Fat digestion and absorption requires that the complex fat molecules be broken down into smaller more manageable molecules. This is done by mixing the fat with the digestive enzyme lipase, which enters the duodenum from the pancreas - the main source of enzymes for digesting fats and proteins. Lipase chops up lipid molecules into fatty acid molecules and glycerol molecules. However, because fat does not dissolve in water, the fat molecules enter the duodenum in a congealed mass, which makes it impossible for the pancreatic lipase enzymes to attack them, since lipase is a water soluble enzyme and can only attack the surface of the fat molecules. To overcome this problem the digestive system uses a substance called bile, produced in the liver but stored in the gallbladder, which enters the duodenum via the bile duct. Bile emulsifies fats - meaning, it disperses them into small droplets which then become suspended in the watery contents of the digestive tract. Emulsification allows lipase to gain easier access to the fat molecules and thus accelerates their breakdown and digestion.
How Fat Is Digested And Absorbed Into The Bloodstream
Lipase and other digestive juices break down the fat molecules into fatty acids and types of glycerol. Absorption of fat into the body, which takes 10-15 minutes, occurs in the villi - the millions of finger-like projections which cover the walls of the small intestine. Inside each villus is a series of lymph vessels (lacteals) and blood vessels (capillaries). The lacteals absorb the fatty acids and glycerol into the lymphatic system which eventually drains into the bloodstream. The fatty acids are transported via the bloodstream to the membranes of adipose cells or muscle cells, where they are either stored or oxidized for energy. Since glucose rather than fat is the body's preferred source of energy, and since only about 5 percent of absorbed fat (the glycerols) can be converted into glucose, a significant proportion of digested fat is typically stored as body fat in the adipose cells. The glycerol part is absorbed by the liver and is either converted into glucose (gluconeogenesis), and/or used to help breakdown glucose into energy (glycolysis).
