Lipids are carbon- and hydrogen-containing compounds that are insoluble in water but soluble in organic solvents. Biologically important categories of lipids are the neutral fats (e.g., triglycerides), the conjugated lipids (e.g., phospholipids), and the sterols (e.g., cholesterol). Lipids function in the body as sources of energy for various metabolic processes. Other functions include contributing to the formation of cell membranes, bile acids, and various hormones.
Lipids are derived from both dietary sources and internal body processes. Almost the entire fat portion of the diet consists of triglycerides, which are combinations of three fatty acids and one glycerol molecule. Triglycerides are found in foods of both animal and plant origin. The usual diet also includes small quantities of phospholipids, cholesterol, and cholesterol esters. Phospholipids and cholesterol esters contain fatty acids. In contrast, cholesterol does not contain fatty acids, but its sterol nucleus is synthesized from their degradation products. Because cholesterol has many of the physical and chemical properties of other lipids, it is included as a dietary fat. Note that cholesterol occurs only in foods of animal origin, including eggs and cheese.
Nearly all dietary fats are absorbed into the lymph. Ingested triglycerides are emulsified by bile and then broken down into fatty acids and glycerol by pancreatic and enteric lipases. The fatty acids and glycerol then pass through the intestinal mucosa and are resynthesized into triglycerides that aggregate and enter the lymph as minute droplets called chylomicrons. Although chylomicrons are composed primarily of triglycerides, cholesterol and phospholipids absorbed from the gastrointestinal tract also contribute to their composition (Table 5-10).
In addition to dietary sources of lipids, the body itself is able to produce various fats. Unused glucose and amino acids, for example, may be converted into fatty acids by the liver. Similarly, nearly all body cells are capable of forming phospholipids and cholesterol, although most of the endogenous production of these lipids occurs in the liver or intestinal mucosa.
Because lipids are insoluble in water, special transport mechanisms are required for circulation in the blood. Free fatty acids travel through blood combined with albumin and in this form are called nonesterified fatty acids. Very little free fatty acid is normally present in the blood; therefore, the major lipid components found in serum are triglycerides, cholesterol, and phospholipids. These lipids exist in blood as macromolecules complexed with specialized proteins (apoproteins) to form lipoproteins.
Lipoproteins are classified according to their density, which results from the amounts of the various lipids they contain (see Table 5-10). The least dense lipoproteins are those with the highest triglyceride levels. Lipoprotein densities also are reflected in the electrophoretic mobility of the various types. As with the formation of other endogenous lipids, most lipoproteins are formed in the liver.22,23