Cholesterol is a waxy substance known as a sterol that is manufactured primarily by the liver or to a lesser extent by other cells in the body (endogenous cholesterol) and is also absorbed from food in the gut (exogenous cholesterol). Cholesterol does not contain fatty acids but since it is made from fatty acid molecules, it is highly fat soluble. Cholesterol can therefore not travel on its own in the blood stream and has to be bound to lipoproteins.
Cholesterol in Foods
When cholesterol is absorbed from the gut, it is transported in the form of a chylomicron from the intestinal lacteals, through the lymphatic system and emptied into the blood stream. These chylomicrons do not only carry cholesterol but it also carries other types of fats like triglycerides and phospholipids. The adipose tissue and liver break down these chylomicrons by the action of the enzyme lipoprotein lipase. Some fat is stored in the adipose tissue and the rest is taken to the liver for further processing.
Cholesterol in the Liver
Fats in the body are broken down to form acetyl coenzyme A (acetyl-CoA) which can be metabolized to release large amounts of energy. Acetyl-CoA molecules can form a sterol nucleus which is essentially cholesterol or its derivatives. Most of this cholesterol is used for the production of bile salts but it may also enter the blood stream and travel to other parts of the body where it can be used in various ways as discussed below under the functions of cholesterol.
Cholesterol in the Blood
Lipids (cholesterol, phospholipids and triglycerides) travel in the blood in the form of lipoproteins, which are a combination of alipoproteins and lipids. It is produced in the liver and while a chylomicron is also a form of lipoprotein, there are 4 other types of lipoproteins which play an important role in fat metabolism, transport and utilization.
- VLDL – very low density lipoprotein
- IDL – intermediate density lipoprotein
- LDL – low density lipoprotein
- HDL – high density lipoprotein
These lipoproteins carry triglycerides, phospholipids and cholesterol – VLDL has the most amount of triglycerides while LDL has very little or sometimes no triglycerides. It is the effect of these different types of lipoproteins on cholesterol that is of the greatest interest for diseases associated with hypercholesterolemia.
LDL is a source of cholesterol for cell membranes and other functions of cholesterol that are discussed below. HDL is the body’s mechanism to guard against excess cholesterol by taking up the cholesterol from tissues and transporting it to the liver where it is passed out in bile salts.
LDL is commonly referred to as “bad” cholesterol while HDL is known as “good cholesterol”.
Factors that Increase and Decrease Cholesterol
- Diet. A diet high in fats, not only cholesterol, will increase blood cholesterol since the fats are metabolized to form acetyl-CoA which in turn makes up cholesterol. Unsaturated fatty acids in the diet however, reduces blood cholesterol. A diet high in cholesterol (exogenous cholesterol) will increase the blood cholesterol only slightly because dietary cholesterol inhibits liver cholesterol production (endogenous cholesterol).
- Hormones. Low levels of insulin as seen in diabetes mellitus and thyroid hormones in hypothyroidism increases blood cholesterol. High levels of thyroid hormones seen in hyperthyroidism decreases blood cholesterol levels.
- Genes. Familial hypercholesterolemia is due to an overproduction of atherogenic Apo B-containing lipoproteins which is seen as high levels of LDL’s.
- Liver diseases which affect the hepatocytes (liver cells) or the emptying of bile into the duodenum of the small intestine.
- Kidney diseases like nephrotic syndrome and chronic renal disease.
- Eating disorders like anorexia.
- Drugs like corticosteroids, anabolic steroids and diuretics.
Functions of Cholesterol
Cholesterol has important roles in the body.
- It is an integral component of cell membranes and the membranes of internal organelles.
- It is used to make cholic acid in the liver which is a part of the bile salts that breaks down fat and assist with fat absorption in the gut.
- It helps to form a waterproof barrier along with lipids in the skin. This prevents water loss from the skin (evaporation) or the entry of solvents into the body.
- It is used to make important hormones in the body :
- Progesterone and estrogen in the ovaries.
- Testosterone in the testes.
- Adrenocorticol hormones in the adrenal glands.