What is Cholesterol

What is Cholesterol?

The word cholesterol is derived  from Greek: chole-(bile) stereos(solid) and -0l (chemical suffix denoting alcohol). It was first identified in its solid state as gallstones. It is responsible for being the precursor for the synthesis of Bile Acids, Steroid Hormones, Vitamin D and  in the Cell Membrane helps keep the integrity of the cell membrane and decreases the permeability to small water-soluble molecules as well as prevents crystallization of the hydrocarbons that could lead to rigidity. Despite being an important and necessary molecules, high levels of it in the body lead to what it is more well known for; atherosclerosis, heart diseases and stroke.

It is introduced into our bodies either by being synthesized in varying rate through out cells in the body depending on the type of cell and organ. Another major source is through dietary intake. The cholesterol that is used in bile and excreted into our other intestines to help with digestion and absorption of fat molecules and fat-soluble Vitamin A, Vitamin D, Vitamin E and Vitamin K gets recycled. This happens due to the fact that it is  reabsorbed  in the small intestines back into the bloodstream. Approximately 50% of the cholesterol that is excreted as bile gets reabsorbed. So how is it regulated from accumulating in our bodies. When dietary intake far exceeds the necessary amount need in our body, the liver stops synthesis of cholesterol, but most of the time this is not enough and a  change in diet along with exercise and in most cases the use of cholesterol lowering medications is necessary to prevent the formation of atherosclerotic plaques and heart disease.

Sources of Cholesterol: Synthesis and Dietary Sources

Cholesterol Synthesis

The major site of production of cholesterol is the liver, accounting for approximately 25% of the daily production. Virtually all cell in the body produce it to be used in cell membranes and intracellular cell signaling and intracellular transport, but a few other sites that are higher in production, not a much as the liver, are the adrenal glands for the production of cortisol, aldosterone and in the sex organs to produce testosterone, estrogen and progesterone. The cholesterol produced the liver is either turned into bile and stored in the gall bladder or it is transported out to other site. Since it is insoluble in blood the transport mechanism involves the use of lipoproteins.

The steps in synthesis are outlined below; it starts with the dehydration of Acetyl-CoA with Acetoacetyl-CoA to form 3-Hydroxy-3-Methylglutaryl-CoA (HMG-CoA) which is then reduced to mevalonate by a rate-limiting enzyme HMG-CoA Reductase. The whole process and diagram of synthesis is discussed on its own page. However the importance of these first few steps needs to be emphasized here, to point out that the irreversible rate-limiting step involving the enzyme HMG-CoA Reductase is the target of Statin drugs which are competitive inhibitors of the enzyme.

Dietary Sources

Major dietary source of cholesterol come from animal fat and products. Beef, Pork, Shrimp, Poultry, Egg yolk, Cheese are all examples of foods containing higher levels of cholesterol. It is hard to precisely calculate the exact amount of cholesterol as it would depend on the size of the serving, the amount of fat left on the cut of meat. However compared to plant-based food the amount in amount of cholesterol is significantly higher.