Most discussions of heart disease risk, sooner or later, include the mention of an infamous waxy substance called cholesterol. In spite of its bad reputation, cholesterol plays many roles, making it just as essential to your health as phospholipids. Like phospholipids, cholesterol is a lipid found in virtually all your body's cells. However, they have differences worth noting.
Cholesterol is a steroid alcohol with a structure that makes it very hydrophobic, or insoluble in liquids. It consists of four fused carbon rings forming a steroid nucleus, with attachments of an eight-carbon hydrocarbon chain, a hydroxyl group and double bond between carbon atoms. Phospholipids, on the other hand, are amphipathic, meaning they are both water-loving and water-hating. Indeed, phospholipids contain two hydrophobic fatty acid tails attached to a hydrophilic, or water-loving, head. The hydrophilic head is attached to a phosphate group and may be based on glycerol or an amino alcohol named sphingosine.
Specialized tissues in your body require cholesterol as a precursor to make vitamin D, steroid hormones and the bile acids needed for digestion. The main function of phospholipids is to form the membrane that surrounds your cells and their internal structures. Although cholesterol is also a component of cell membranes, it mainly helps maintain their fluidity. Phospholipids can also act as the following: reservoir for chemical messengers within the cell, anchors for some proteins to attach to a cell surface and components of lung surfactant, a substance that reduces tension on the inner surface of your lungs. As essential components of bile, phospholipids also help make cholesterol soluble in water.
Although the liver plays a comparatively greater role, the synthesis of phospholipids and cholesterol occurs in nearly all human tissues. Phosphatidyl-ethanolamine and phosphatidylcholine are the most abundant phospholipids in your body. Their production begins with pre-existing choline and ethanolamine. Choline is considered an essential nutrient because your body does not produce enough of it to meet all its needs. However, your body's needs in phosphatidylcholine are so important that, if choline levels become too low, your liver uses another substance called phosphatidylserine as a precursor. In contrast, cholesterol synthesis begins with two compounds called acetate and NADPH.
Because of its inability to dissolve in blood, cholesterol must either be transformed in the bile, or associated with special carrier molecules known as lipoproteins. The major types of lipoproteins are low-density lipoprotein, or LDL, and high-density lipoprotein, or HDL. You may recognize LDL as the so-called "bad cholesterol," which carries cholesterol away from the liver, to various organs. In contrast, HDL's tendency to remove excess cholesterol from arteries to return it to the liver has earned it the name "good cholesterol." (See Reference 2) Although lipoproteins also contain phospholipids, scientist James Burnette's description of their behavior in liquids indicates that they do not require lipoproteins to circulate in the blood.
Your body cannot break the ring structure of cholesterol down to its single components. Rather, it first has to convert the intact steroid nucleus to bile acids and bile salts, which are then excreted in the stool. Your body also secretes some of the cholesterol into the bile, which then carries it to the intestine for elimination. In contrast, a group of enzymes called phospholipases, along with pancreatic juice, are responsible for degrading phospholipids. All human tissues contain phospholipases.
According to the American Heart Association, your body typically produces all the cholesterol it needs daily, mainly through the liver. Although you do not need to consume it, you may also get cholesterol from your diet. Foods of an animal origin contain cholesterol, while plant-based foods do not. Examples of cholesterol-rich foods include: egg yolks, poultry, meats, shellfish and dairy products. Eggs, organ meats, lean meats, fish, shellfish, cereal grains and oilseeds are also rich in phospholipids, according to scientists John Weihrauch and Young-Sun Son. In contrast, leafy vegetables, fruiting parts, roots and most tubers have a low phospholipid content.
The hydrophobic character of cholesterol makes it potentially dangerous because excess cholesterol that accumulates in the bloodstream can form plaque and block blood vessels unless it is removed from the bloodstream. According to the American Heart Association, high blood cholesterol—or hypercholesterolemia—increases your risk of developing heart disease, and may lead to a heart attack.
- American Heart Association: Cholesterol
- "Lippincott's Illustrated Reviews: Biochemistry"; Pamela Champe and Richard Harvey; 2006
- Journal of the American Oil Chemists' Society; "Phospholipids"; John Weihrauch; 1983