Lipids, or fats, are essential nutrients in the diet, along with proteins and carbohydrates. Ingested lipids serve as a calorie-rich source of energy for the body. Because lipids prove characteristically insoluble in water, a multistep process occurs to enable absorption of ingested fats. The small intestine, pancreas, liver and gallbladder interact to accomplish the digestion and absorption of dietary lipids.
Dietary lipids pass from the stomach to the small intestine in a mud-like mixture of food particles and stomach acid known as chyme. The presence of chyme triggers the release of bile from the gallbladder into the small intestine. Bile reacts with fat globules in chyme, breaking them into tiny bile-containing fat droplets, explains the National Institute of Diabetes and Digestive and Kidney Diseases. This process, known as emulsification, prepares the ingested lipids for the next step in the digestive and absorptive process.
The presence of chyme in the small intestine stimulates the release of pancreatic digestive fluids and enzymes, including the fat-digesting enzyme lipase. Lipase reacts with the emulsified fat droplets, breaking down the complex dietary fats into smaller lipid particles known as fatty acids and monoglycerides, notes biologist David Sadava, Ph.D., coauthor of "Life: The Science of Biology." Thus, lipase converts dietary lipids into smaller, absorbable fat particles.
Fatty acids and monoglycerides pass through the outer membrane into the intestinal lining cells. The smallest fatty acids pass through the intestinal cells into the bloodstream and travel with a carrier protein to the liver for further processing.
Monoglycerides, large fatty acids and cholesterol undergo processing within the intestinal lining cells. The cells reconstruct the absorbed monoglycerides and large fatty acids into lipid molecules called triglycerides, explains biochemist Stephen Chaney, Ph.D., of the University of North Carolina School of Medicine. The cholesterol and triglycerides attach to protein carriers, forming chemicals known as chylomicrons. The protein carriers transform the water-insoluble lipids into a water-soluble form, enabling export from the intestinal cells.
The intestinal cells extrude the chylomicrons formed from absorbed dietary fat into a liquid transport network known as the lymphatic system. The lymphatic system transports the absorbed chylomicrons to the bloodstream, notes "Life: The Science of Biology." The liver and other body tissues process chylomicrons for storage or immediate utilization.
Impaired Lipid Absorption
Diseases that adversely affect the production and release of bile and pancreatic digestive fluids can lead to impaired absorption of dietary lipids. A variety of inherited, autoimmune, infectious and toxic conditions of the liver can cause decreased bile production. Similarly, disease processes that adversely affect the pancreas may lead to diminished production of digestive enzymes and fluids, notes "Harrison’s Principles of Internal Medicine."
Patients with impaired lipid absorption pass large amounts of undigested fat in the stool, a condition known as steatorrhea. Weight loss and malnutrition prove common among patients with impaired lipid absorption.