What does this "bad" cholesterol look like? LDL cholesterol is a round lipoprotein that shuttles the bulk of the cholesterol in the blood. The inner core contains oily cholesterol molecules, each with a fatty acid chain dangling from it. Also found within the core is a lesser amount of triglyceride molecules intermingling with the cholesterol esters. Seventy-five percent of the cholesterol within the LDL is in this bound-up (esterified) form, concentrated within the inner core. The greater the concentration of these "chained" cholesterol molecules in the LDL, the more susceptible the LDL molecule is to oxidation (which instigates the catastrophic events that make up the atherosclerotic process), and the higher your risk for developing heart disease.
Eating your way to a heart attack What most people don't realize is that cholesterol comes from two sources: from food and from our own cells. In fact, we actually make three times more cholesterol (about 1,000-1,200 milligrams per day) than we typically eat. So we don't need to eat cholesterol because the body is perfectly capable of manufacturing enough of this waxy substance on its own. Still, American continue to consume an appreciable amount of cholesterol from dietary sources, which can raise "bad" cholesterol levels and increase our risk of heart disease. The average American ingests approximately 300-400 milligrams of cholesterol every day from animal sources. This amount exceeds the government's cholesterol - lowering dietary recommendation of no more than 200 milligrams a day. It is simple to eat this much cholesterol: seven ounces of liver (providing 762 milligrams of cholesterol) or two egg yolks (424 milligrams) will easily put you over the top. In the intestine is a reservoir of cholesterol called the cholesterol pool. The amount of cholesterol that enters the pool from bile is up to five times greater than the amount of cholesterol derived from our diet. Where does all that cholesterol floating around in the intestine go? About half of the approximately 2,000 milligrams in the pool is excreted and half is reabsorbed into the intestinal cells, ultimately going back to the liver. If you could block reabsorption of some of the cholesterol in the intestinal pool so that it gets excreted and not returned to the liver, then you would decrease your circulating LDL level. One class of cholesterol - lowering drugs in particular capitalizes on this concept. How is cholesterol absorbed from the intestinal pool into the body? For cholesterol, the gateway into the body is a layer of cells lining the upper intestine known as enterocytes. The only way for cholesterol to get to this layer is to attach itself to a transporter called a micelle. Micelles are small, round transport packages formed by bile salts and phospholipids that carry cholesterol and fat (triglycerides) to the intestinal cell wall. The cholesterol must then take a second transport route - a protein channed called Niemann - Pick C1 Like 1, or NPC1L1 for short. The NPC1L1 traverses through the wall and into the intestinal cell where the cholesterol is packaged into chylomicrons for travel in the lymph system and ultimately the bloodstream. Zetia, the new cholesterol - lowering drug, blocks cholesterol absorption from the intestine by targeting the NPC1L1 protein. Once in the bloodstream, chylomicrons off-load lots of triglycerides to needy cells, eventually arriving at the liver as small, cholesterol-rich particles known as chylomicron remnants. These remnants signal to the liver cells that they don't have to make more cholesterol, as a new cholesterol shipment has arrived - cholesterol that may have come from your breakfast omelet. The liver extracts the cholesterol from the remnants to make whatever is required for the day. Typically, the cholesterol is converted into bile acids for its launch into the body's cholesterol recycling system. But the liver could instead package the cholesterol up as a lipoprotein (primarily VLDL) and send it back into the bloodstream, ultimately increasing the amount of "bad" LDL cholesterol. Eat a huge amount of cholesterol and you increase the delivery of cholesterol to the liver by chylomicron remnants. Therefore, you should avoid foods with high cholesterol in order to lower the amount of cholesterol in your intestinal pool. Less cholesterol overloading the liver means less VLDL exported and less LDL produced. The liver, a cholesterol factory
The liver is a glutton when it comes to cholesterol. Its cells work round the clock using huge amounts of cholesterol, mainly to manufacture bile. In fact, almost 80 percent of the cholesterol in the liver is used to produce bile, which is secreted into the small intestine for fat digestion. The liver also uses cholesterol for lipoprotein production, particularly VLDL (the parent molecule of LDL). The foremost source of "bad" LDL cholesterol is the liver, which indirectly functions as an LDL production facility. The body's cholesterol recycling system Think of the liver as not only a bile and lipoprotein manufacturing plant but also a cholesterol removal and recycling plant. How does the liver recycle cholesterol? The liver cells build bile acids from cholesterol. The bile acids are then secreted in bile (along with free cholesterol) into the upper section of the small intestine, where they aid in fat digestion. Once in the acidic environment of the small intestine, the bile acids convert into bile salts. Bile salts are continuously recycled from the intestine and returned to the liver. Each bile salt is actually reused about twenty times before it is excreted out of the body in the feces. The liver, therefore, doesn't need to churn out too much new "homemade" cholesterol because it has this neat recycling program (called enterohepatic circulation) in which the bile acids (formerly cholesterol) are continuously recycled between the intestines and the liver. Almost 95 percent of the bile acids entering the intestine are recirculated and absorbed back into the bloodstream, heading straight to the liver, where they are once again secreted into the intestine in bile. This recycling system decreases the liver's requirement for new cholesterol and actually inhibits production of new bile acids from cholesterol. How much cholesterol the liver uses up each day to make bile acids depends on the return flow of bile salts from the intestine. Sequestering bile salts in the intestine so that they are excreted in waste and not recycled will force the liver to increase LDL clearance from the blood to replenish the cells' internal cholesterol stores (used for making new bile). Another class of cholesterol - lowering drugs works in this fashion. An important concept to keep in mind is that there is only one primary route out of the body for cholesterol (in the form of either free cholesterol or bile salts), and that is through the feces by way of the intestinal tract. If you block the absorption of free cholesterol or interrupt the recirculation of bile salts by trapping them in the intestine and excreting them, the end result is a drop in circulating LDL cholesterol. Several steps in the Cholesterol Down Plan work to lower LDL in this manner. High LDL? Don't blame it all on genes Some people do have the unfortunate luck of inheriting a genetic predisposition to make truckloads of cholesterol internally. Most LDL is removed from the circulation by LDL receptors on the liver. Certain individuals inherit a tendency to produce fewer receptors, resulting in less LDL removal from the blood (predisposing them to excess LDL and atherosclerosis). With regard to receptors, we can't blame it all on the genes. The food you eat can greatly affect the density of the receptors on the liver. Ingestion of dietary cholesterol, saturated fat, and trans fat has unhealthy consequences. These foods suppress the manufacturing of cholesterol - clearing liver receptors and raise your "bad" cholesterol level. For a one-two punch against heart disease, eat a heart-healthy diet (low in saturated fat and cholesterol and devoid of trans fat) and follow the Cholesterol Down Plan, which includes foods that help augment the number of LDL receptors. The body's main LDL regulatory organ
By clearing excess LDL from the bloodstream, LDL receptors help protect against atherosclerosis. After the liver, the cells of the adrenal gland and ovaries have the most receptors because these are the cells in the body with the greatest need for cholesterol. Any agent of medication capable of increasing the number of receptors on the liver cells will reduce the concentration of blood-borne "bad" LDL cholesterol. The density of LDL receptors plays a critical role in atherosclerosis. The fewer LDL receptors you have on your liver, the more circulating LDL particles and the greater your risk of heart disease. You now understand the basics of cholesterol mechanics and the importance of maintaining normal lipoprotein levels in your bloodstream. The ten simple steps in the Cholesterol Down Plan capitalize on the ability of certain foods plus exercise to exploit the body's LDL - lowering mechanisms in order to promote heart health and prevent the development or recurrence of heart and blood vessel disease. Source: Cholesterol Down, Foreword by Jennifer H. Mieres, M.D. | ||||||
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