August 2014




Cholesterol {kuh-les'-tur-awl} is the best-known member of an important group of chemicals called Lipids. Cholesterol is specifically found only in animal tissues. Cholesterol, however, rarely occurs in any significant amounts plants, although some plants contain several varieties of related chemical compounds. 

What Is the Purpose and Function of Cholesterol?

Cholesterol is found in virtually all animal cells. It is primarily a component of membranes of these cells. The cholesterol content of any specific membrane, relative to other lipids, varies from tissue to tissue and is often dependent on the specific function of the membrane itself.

That is, the amount of cholesterol often determines the function of the membrane. The relationship of cholesterol to other lipids (particularly phospholipids and glycolipids) determines the most important characteristics of the membrane itself. This ratio determines such characteristics as its stability (ability to breakdown, strength, resistance to chemical attack and temperature) and its ability to allow other chemicals and proteins to move in and out of the membrane. Membranes with high ratios of cholesterol to other lipids, such as the myelin membranes that make up the sheath (cells that cover) the central nervous system, have high stability and relatively limited ability to let chemicals, nutrients and proteins in or out. As you might guess their primary function is protective and the membrane acts as a barrier keeping in what the tissues need to be kept in and keeping out what it does not want to enter.

Membranes with many intracellular organelles (such as microsomes, mitochondria) have low cholesterol ratios and are consequently fluid and less protective allowing many chemicals, nutrients and proteins to move in and out on a much more random basis. Most cellular membranes have intermediate cholesterol-lipid ratios and therefore have more balanced protective and metabolite-transport functions.


Cholesterol and the Sex Hormones

In addition to its role in membrane structure and function, cholesterol has another extremely important function. Cholesterol is used by the body as one of the basic building blocks of the stress and sex hormones. Cholesterol is stored in the adrenal glands, testes, and ovaries where it is available for whenever it is need to be converted to steroid hormones. Steroid hormones include the male and female sex hormones (androgens and estrogens) as well as the adrenal corticoid hormones (cortisol, corticosterone, aldosterone, and others which are also the hormones of the Stress Mechanism).


Cholesterol Is Also Used to Make Bile Acids

Cholesterol is also found in the liver where it is used in the creation of Bile Acids. Bile acids are a group of some 24 similar compounds manufactured by the body that are used in the digestive process. Bile acids when linked with the amino acids glycine or taurine, form bile salts. Bile acids are secreted into the intestine to aid in the digestion of food, especially other lipids or fats. Once the bile acid has done its job it is often reabsorbed by the intestinal tract. This is important as dietary fiber bind bile acids and ties them up keeping them from being reabsorbed and hence requiring that additional cholesterol be used to make new bile acids. This is used as an important strategy to lower serum cholesterol levels.


Where Does Cholesterol Come From?

There are two main sources of cholesterol. The first, we have already discussed. Cholesterol is ingested whenever we eat foods made with animal parts, or by products. Since cholesterol makes up cellular and tissue membrane, and it is stored in organs and tissues of the body when we eat meats we are ingesting some cholesterol. Cholesterol is also obtained from foods having saturated fatty acids or animal fat.

The second source as it is also made by our liver. That's right, we actually make cholesterol in our body. This should be no great surprise for we too, are animals and require cholesterol for our membrane and cellular tissues and our sexual and stress hormones.

Normally, the total amount of cholesterol from these two sources remains constant because the rate of cholesterol synthesis in the liver is under feedback control. When the dietary intake is high, liver synthesis is low; when intake is low, synthesis increases.


Cholesterol and the Digestive System

Dietary cholesterol is carried from the intestinal tract into our body and to the liver by means of large lipoprotein molecules known as chylomicrons. After entering into the liver these chylomicrons are broken down and the liver then secretes cholesterol in the form of Very Low Density Lipoprotein (VLDL)--which contains, cholesterol as one of its primary ingredients. The VLD Lipoprotein molecules are then excreted into the blood stream to be transported to whatever tissues need it.

Upon entering into fat tissues storage areas of the body, VLD Lipoproteins are converted into Low Density Lipoprotein (LDL). LDL is the major transport form for cholesterol. In the LDL form cholesterol is transported and then supplied to many different bodily tissues.

High Density Lipoprotein (HDL or HDL-Cholesterol) is involved in the transport of cholesterol in the opposite direction, that is, from the tissues to the liver.

The average American adult, the total amount of lipoprotein-bound cholesterol circulating in the blood is approximately 200 mg per 100 ml of serum. If an individual's dietary intake of fat is high, the level of serum cholesterol will generally increase above this number and move up into the high cholesterol range.

Since elevated blood cholesterol is associated with a greater risk of atherosclerosis and coronary atherosclerotic heart disease, control of dietary intake of cholesterol is essential. When dietary intake of cholesterol is high then LDL-Cholesterol is also elevated. This is important information in protecting against atherosclerotic heart disease.

Researchers have demonstrated that increased levels of HDL-Cholesterol can actually decrease the risk of atherosclerosis and heart disease. In a report issued in 1984 reductions of LDL-Cholesterol ratios also lowered the risk of heart disease. In a 1987 it was demonstrated that reduction of dietary cholesterol had a positive effect in reducing elevated cholesterol levels. A 1989 report indicated that a third of all American adults had cholesterol levels and that they were at high risk for coronary artery disease.

In further sections we will look at the role of cholesterol screening, the drugs and natural treatments available for lowering cholesterol levels and risk of heart disease.

To Learn About The Role of Diet in Reducing Cholesterol in Heart Disease and Stroke.

To Learn About Other Ways To Lower Your Serum Cholesterol and Hence Prevent a Heart Attack and Stroke.