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Since you will see the word "cholesterol" on just about every food package you buy, it's important to know what it is, what it does, and how much is safe.
What cholesterol is. Cholesterol is not a fat. Biochemically it's called a "sterol." It contains no calories, so the body cannot derive any energy from it. Cholesterol forms an integral part of the cell membranes throughout your body, sort of like the mortar that holds the brick wall together. It is particularly important in the cellular structure of the brain and central nervous system, and is an important component of the myelin sheath that provides insulation to the nerves. The body uses cholesterol to make bile acids, which are necessary for proper food digestion. It's also a vital part of adrenal and sex hormones (estrogen, progesterone, testosterone), and it helps the body manufacture vitamin D.
Only the cell membranes of animal tissue contain cholesterol. Cell membranes of plants are composed of fiber, not cholesterol. When you see "no cholesterol" on a package of fruit, vegetables, grains, or even vegetable oil, don't believe that the manufacturer has done you a favor by removing the cholesterol. There was no cholesterol in these foods to begin with. While cholesterol is essential to life, the body makes all the cholesterol it needs. You can live quite well, even better, with eating little or no cholesterol.
What cholesterol does. Cholesterol enters the body from saturated fats in animal sources, such as meat, poultry, egg yolks, liver, butter, cheese, and other dairy products. The cholesterol goes to the liver where it joins the cholesterol that is made there. The cholesterol is transported from the liver to the cells by low density lipoproteins (LDL), which acts like a nutritional ferry boat, loading up the cholesterol and navigating through the bloodstream, stopping at cells and depositing cholesterol to the cells that need it. If a cell already has enough cholesterol, it "refuses delivery" of the cholesterol cargo. The excess LDL stays in the blood where the cholesterol is deposited in the walls of arteries, causing atherosclerotic plaque. The more plaque that builds up, the narrower the arteries become, until eventually the blood supply to vital organs is reduced. This is why LDLs are known as the "bad cholesterol."*
But take heart, a nutritional rescuer is also present in the bloodstream, the high density lipoproteins, or HDLs. These are known as "good cholesterol," since they travel like a vacuum cleaner through the bloodstream, picking up excess cholesterol in the bloodstream, and also possibly sucking the cholesterol from the fat-laden plaques. The HDLs carry this excess cholesterol back to the liver, which converts it to bile, which is eliminated into the intestines. How your liver handles cholesterol is determined primarily by genetics, and secondarily by your diet.
While this is an oversimplification of a complicated biochemical process, it helps us understand two conclusions:
To remember which cholesterol is "good" and which is "bad," think of LDL as "lousy" cholesterol, and HDL as "healthy" cholesterol. As a further reminder, "lousy fats," the ones that are saturated or hydrogenated, contribute to lousy cholesterol.
How much cholesterol do you need? If your body has just the right amount of cholesterol, HDL, and LDL, it is in cholesterol balance. But how much is the right amount?
Homemade versus dietary cholesterol. For most people, about eighty percent of the cholesterol in their blood is made by their own body, with the rest coming from their diet. In fact, your body needs cholesterol so much that it makes around 3,000 milligrams per day that's ten times the maximum recommendation for daily dietary cholesterol. It is estimated that around thirty percent of people are sensitive to the cholesterol-raising effects of dietary cholesterol. Normally, when a healthy person eats high cholesterol foods, the liver reduces its own cholesterol production to keep blood cholesterol at a healthy level. In cholesterol-sensitive individuals, this internal monitoring mechanism doesn't operate, so that their blood cholesterol level goes up when they eat high-cholesterol foods.
One theory that explains cholesterol sensitivity is humans are by nature vegetarians. Originally, human bodies were not genetically equipped to metabolize dietary cholesterol, since plants are cholesterol-free. As the human diet began to include animal products, some people's bodies developed metabolic ways to dispose of excess cholesterol and some didn't. People who descended from the ones that didn't adapt are the cholesterol-sensitive ones.
Gender differences of cholesterol. Women tend to have higher levels of HDL than men, since female sex hormones release HDL and male sex hormones lower HDL. At menopause, estrogen production drops, and so does HDL. Just another mid-life biochemical quirk that should stimulate menopausal-age women to start an HDL-raising exercise program.