Digestion and absorption of dietary lipids, OR More than you ever wanted to know about what happens to food after you swallow it

The fat in our diet consists mainly of triglycerides (95%), with the other 5% being phospholipids, cholesterol and cholesterol esters, and fat soluble vitamins. Digestion of fat begins minimally in the mouth with the enzyme lingual lipase, and in the stomach with gastric lipase. These lipases mainly break down milk fats, which have short and medium chain fatty acids. Most of the fat in our diet is long chain.
This fat enters the small intestine intact, where the gut hormone CCK is released and stimulates the release of bile, pancreatic lipase, and colipase. The hormone secretin stimulates the release of bicarbonate, which helps keep a less acidic atmosphere so that enzymes can become active.
Bile is made from cholesterol in the liver and stored in the gallbladder. It is released in response to CCK and acts in the small intestine. It acts as an emulsifier and breaks the fat into smaller particles, which increases the surface area so that pancreatic lipase can attack it more easily. Once bile has finished its job it continues on to the ileum of the small intestine where 90% of it is reabsorbed, returning to the liver (enterhepatic cycle). The rest goes to the colon and is excreted. The entire pool of bile is recycled twice per fat containing meal. Soluble fibers from our diet and some cholesterol lowering drugs bind to bile so that more of it is excreted. This can help lower blood cholesterol.
Pancreatic lipase breaks the bonds connecting the fatty acids on carbon #1 and #3 of the triglycerides. This results in free fatty acids, 2-monoacylglycerol, and some glycerol.
Cholesterol esters are broken down to cholesterol by the enzyme cholesterol enterase. Phospholipids are broken down to lysophospholipids by the enzyme phospholipase.
Short and medium chain fatty acids are water miscible, so they can be absorbed directly from the intestinal cell to the portal vein, which brings them to the liver. In order to absorb long chain fatty acids, they must be packaged into a delivery device called a micelle. Micelles are tiny droplets emulsified by bile salts which contain all of the remaining dietary components (long chain fatty acids, 2-monoglycerides, glycerol, fat soluble vitamins, cholesterol, and lysophospholipids) along with bile. Micelles bring these lumps of particles to the microville of the small intestine, where everything except the bile gets absorbed into the cell.
But the fun doesn't stop here! Inside the intestinal cell, the dietary particles are rebuilt, once again, into triglycerides. The fatty acids are activated to fatty acyl coenzyme A (FACoA). FACoA reacts with 2-monoacylglycerol to form a diacylglycerol, and then with another to reform a triglyceride.
The triglycerides are very hydrophobic and cannot be transported alone, so they get packed with apoproteins (mainly B-48), phospholipids, cholesterol and vitamins to form what is known as a chylomicron. Chylomicrons are a type of lipoprotein. They are mainly made of triglyceride because they are representative of the fat in our diet. They have a single layer of phospholipid on the outside with the hydrophillic end facing out, which keeps water from entering. The hydrophobic components are hidden on the inside. The apoproteins sit on the outside of the structure. Chylomicrons are too large to go directly into the blood, so they enter the lymphatic system. They circulate to the thoracic duct and enter the bloodstream from there. As a chylomicron matures it receives more apoproteins upon its surface from HDL. ApoE and ApoCII are only found on these mature chylomicrons, and both serve important purposes. The enzyme lipoprotein lipase is activated based upon the presence of ApoCII. Lipoprotein lipase is found in the capillaries of the adipose and muscle cells, and interacts with the chylomicron and digests some of the triglycerides into fatty acids and glycerol. The fatty acids are taken up for storage into adipose and/or muscle cells, and the glycerol goes to the liver. What is left over is called a chylomicron remnant, and is returned to the liver. The presence of ApoE on the chylomicron remnant allows it to be recognized by the liver cells so that it can enter them by endocytosis and get digested by lysosomes and its contents reused.
The triglyceride components get reused, along with newly synthesized triglycerides, by creating VLDL in the liver. VLDL gets released into the bloodstream, where some of the triglycerides get taken up by adipose cells. The VLDL then becomes IDL (intermediate density lipoprotein) or LDL.