- Understand the special barriers to absorption of lipids supplied in the diet
- Describe the phases of lipid digestion
- Understand how lipid digestion is facilitated by gastric events
- Define the mechanisms of lipid digestion in the intestinal lumen
- Identify how bile acids and micelles participate in the process of lipid assimilation
- Describe events at the level of the intestinal epithelium that govern uptake of different classes of lipids
- Understand how the products of lipolysis cross the brush border
- Delineate pathways for lipid processing in the enterocyte
- Describe how chylomicrons are formed and their eventual disposition
- Define how lipid digestion and/or absorption can be altered in the setting of disease
Lipids are defined as organic substances that are hydrophobic, and thus are more soluble in organic solvents (or cell membranes) than in aqueous solutions. Lipids form an important part of most human diets. First, they are denser in calories than either proteins or carbohydrates, increasing the nutritional content of a given meal. Second, several vitamins are lipids (the so-called fat-soluble vitamins
). Third, many of the compounds that account for the flavor and aroma of foods are volatile hydrophobic molecules, meaning that lipids serve as an important vehicle to render food palatable. In short, dietary lipids are tasty!
Barriers to Assimilation of Hydrophobic Molecules
In the last chapter, we considered the barriers to assimilation of the water-soluble nutrients, carbohydrate, and protein. These molecules are readily soluble in the aqueous environment of the gut lumen, but, following digestion, they require special mechanisms to facilitate their transport across the hydrophobic domain of the enterocyte apical membrane. Conceptually, when considering the assimilation of dietary lipids, the opposite problems pertain. The products of lipid digestion—lipolysis
—are, in a large part, readily able to cross cell membranes to allow for absorption into the body. However, lipids are not “at home” in the aqueous milieu of the intestinal contents. Likewise, they must interact with lipolytic enzymes that are themselves soluble proteins. Finally, the products of lipolysis must arrive at the brush border at a sufficient rate to allow for uptake before being propelled along and out of the gut. Systems therefore exist to maintain lipids in suspension in the gut contents with a sufficiently dispersed surface area to allow for lipolysis at the oil–water interface. Additional phase transitions allow for efficient trafficking of lipids to the enterocyte surface, where they can be absorbed.
Dietary and Endogenous Sources of Lipids in Intestinal Content
Lipids represent a major source of calories in most Western diets, with an average of 120–150 g consumed on a daily basis by a typical adult. Despite their hydrophobicity, the process of lipid assimilation has evolved to be highly efficient, with significant reserve capacity also present in the system. The ready availability of lipid-rich foods in developed countries may therefore be an important contributor to the burgeoning problem ...