in and out of cells. Another phospholipid, sphingomyelin, consists of glycerol, fatty acid, phosphate, choline, and an amino alcohol called sphingosine and is part of the tissues covering brain and nerve cells, as are the cerebrosides, phospholipids that contain galactose, fatty acid, and sphingosine.

Sterols/Cholesterol
Sterols, the third primary lipid, include cholesterol, phytosterols (plant sterols), and some of the steroid hormones. Cholesterol, the best known of the sterols, is the precursor of the bile acids and the sex hormones. Manufactured in the body, primarily in the liver, although all tissues of the body except the brain can make it, cholesterol is present in almost all cells and is particularly high in the liver, brain and nervous tissue, and the blood. Cholesterol, like lecithin, is also available in foods, such as egg yolk, meats, and other animal fats, including milk products. It is not readily available in most vegetable foods.

Cholesterol has been implicated in occlusive cardiovascular disease, causing plaque and obstruction of the arteries. The cholesterol in foods, however, is not really the villain. It is the oxidized cholesterol in the blood that causes the trouble, and the level of this is more a function of total dietary fat intake and genetically determined aspects of cholesterol metabolism, than of the amount of cholesterol in our food. In particular, a transport mechanism of cholesterol called the low-density lipoprotein (LDL) is likely the villain in our society’s rampant disease—atherosclerosis. This LDL is contrasted to the so-called “good” cholesterol-carrying high-density lipoprotein (HDL). The ratio of these two (LDL:HDL) is the blood test currently favored to evaluate our risk of cardiovascular disease.

Lipoproteins, the fat-protein combination molecules circulating in our blood and tissues, can move around the body only if they are surrounded by protein, because fats are not soluble in water (the basic makeup of blood and lymph). The fatty acids in these large lipoprotein molecules are positioned at the inside, as far away from the water as possible. The higher the protein portion in these molecules, the higher their density.

There are several important lipoproteins.

Chylomicrons are made in the intestines to transport digested fats (mainly triglycerides) into the circulation to be carried to the liver and other organs.

VLDLs (very-low-density lipoproteins) are made in the intestines and the liver to carry fats throughout the body. Though VLDLs carry mostly triglycerides, they carry a small component, maybe 5–15 percent, of the cholesterol to the tissues of the body.

LDLs (low-density lipoproteins) are made by the liver (and possibly by transformation of VLDLs in the blood) and are the primary molecular complexes that carry cholesterol in the blood to the organs and cells. LDL contains the highest percentage of cholesterol in most people.

HDLs (high-density lipoproteins) are large, dense protein-fat molecules that circulate in the blood, picking up already used or unused cholesterol and cholesterol esters and taking them back to the liver as part of a recycling process. Where in the body HDL is made is not certain (probably in the liver), but it may be the most protective form of lipoprotein in preventing buildup of cholesterol. People with higher HDL levels have less risk of cardiovascular disease because their cholesterol is cleared more readily from the blood. It also appears that HDL may be able to collect cholesterol from artery plaque, thus reversing the atherosclerotic process that leads to heart attacks. HDL will deliver cholesterol to the VLDL, converting them to LDL, which have more density; the liver then removes the LDLs from the blood and converts their cholesterol into bile acids, which are then eliminated.