Energy production from triacylglycerols starts with their hydrolysis into free fatty acids and glycerol. Enzymes called lipases, which catalyze the reaction, carry out this hydrolysis.
The reaction releases the three fatty acids and glycerol. An intestinal carrier absorbs the glycerol, which will eventually rejoin with fatty acids in the intestinal cells.
The body must absorb the fatty acids released by the lipases by a rather more involved mechanism. Fatty acids are poorly soluble in water, although they are more soluble than triacylglycerols. Lipids of whatever kind tend to form droplets. Protein enzymes are water‐ soluble and therefore cannot gain easy access to the lipid droplet. To be digested, lipids must be emulsified into small droplets, which have a larger surface area. In other words, the hydrophobic interactions forcing the lipids into larger droplets must be overcome. The molecules that carry out this function are called bile salts or bile acids. Metabolically, the liver creates them and secretes them into the gall bladder, from where they are pumped into the duodenum.
Bile salts are derived from cholesterol and are a major end product of cholesterol metabolism. They are powerful detergents, with a large, hydrophobic component and a carboxylic acid end‐group that is negatively charged at the pH characteristic of the small intestine. The hydrophobic component of the bile acid will associate above a specific concentration (termed the critical micelle concentration, or CMC) to form disc‐shaped micelles, that is, droplets. Common bile salts have CMCs in the 2 to 5 (millimolar) range. The micelles in the gut contain dietary lipids (triacylglycerol, cholesterol, and fatty acids) as well as bile salts, and are termed mixed micelles for that reason. Figure shows a diagram of a mixed micelle.
The bile salts form the edge of the micelle and also appear, in fewer numbers, dispersed throughout the inside of the micelle. The lipids exist in a bilayer on the inside of the disc. Bile acids are important for fatty acid absorption. Fat‐soluble vitamins (A, D, E, and K) absolutely require bile acids for absorption.
The mixed micelle provides a large surface area for the action of pancreatic lipase, which is responsible for the majority of digestive action. Pancreatic lipase uses a cofactor, a small protein called colipase, which binds both to lipase and to the micelle surface. The action of lipase leads to free fatty acids that are slightly soluble in the aqueous phase of the gut. For the most part, the cells of the small intestine absorb these free fatty acids; the bacteria in the large intestine metabolize and/or absorb those that pass through the small intestine. The bile salts are reabsorbed in the last third of the small intestine.
Bile‐acid metabolism explains the ability of certain kinds of dietary fiber to help lower serum cholesterol. A molecule of bile acid circulates through the liver and intestine five or more times before finally being eliminated. Soluble fiber (such as that found in oat bran) binds bile acids, but itself cannot be absorbed. Therefore, fiber‐bound bile acids are eliminated in the stool. Because bile acids derive from cholesterol, synthesizing more bile acid drains the body's stores of cholesterol, which leads to a reduction in serum cholesterol, and therefore, to a lower risk of coronary artery disease. Eating oat fiber cannot overcome an excessive dietary cholesterol consumption, of course. In other words, consuming excessive amounts of well‐marbled steak and expecting to overcome the effects by eating a bran muffin would be foolish.
Lipids in the bloodstream
Free fatty acids are transported as complexes with serum albumin. Cholesterol, triacylglycerols, and phospholipids are transported as protein‐lipid complexes called lipoproteins. Lipoproteins are spherical, with varying amounts and kinds of proteins at their surfaces. The protein components, of which at least ten exist, are called apolipoproteins. Lipoproteins are classified in terms of their density.
The lightest and largest of the apolipoproteins are the chylomicrons, which are less dense than water by virtue of their being composed of more than 95 percent lipid by weight (remember that oils float on water because they are less dense than water). Triacylglycerols make up most of the lipid component of chylomicrons, with small amounts of phospholipid and cholesterol. Chylomicrons contain several kinds of apolipoproteins.
Very‐Low‐Density Lipoproteins (VLDL) are less dense than chylomicrons. They contain more protein, although lipids (fatty acids, cholesterol and phospholipid, in that order) still make up 90 to 95 percent of their weight. Low‐density lipoproteins (LDLs) are about 85 percent lipid by weight and contain more cholesterol than any other kind of lipid. VLDL and LDL contain large amounts of Apolipoprotein B. The VLDL and LDL are sometimes referred to as “bad cholesterol” because elevated serum concentrations of these lipoproteins correspond with a high incidence of artery disease (stroke and heart disease). The LDLs carry cholesterol and fatty acids to sites of cellular membrane synthesis.
High‐density lipoproteins (HDLs) contain a different apolipoprotein form, Apolipoprotein A. These proteins are about half lipid and half protein by weight. Phospholipids and cholesterol esters are the most important lipid components. HDL is sometimes referred to as “good cholesterol” because a higher ratio of HDL to LDL corresponds to a lower rate of coronary artery disease.
In summary, triacylglycerols from the diet are digested by lipase and associate with bile salts into mixed micelles. The free fatty acids are absorbed by the cells of the small intestine, from which they are transported via the lymph system to the liver. From the liver, they are released as apolipoproteins in the circulation, carrying fatty acids and cholesterol to the cells throughout the body.
The triacylglycerols in chylomicrons and LDLs circulate through the blood system; the former carries dietary lipids while the latter carries lipids synthesized by the liver. The triacylglycerols are substrates for cellular lipases, which hydrolyze them to fatty acids and glycerol in several steps. Carrier proteins transport the lipids into the cell. Different carriers exist for different chain‐length lipids.