Cholesterol is the principal modified steroid or sterol, that is synthesized in human body. Cholesterol is produced in liver. Cholesterol was first identified as a solidified form in gallstones by François Poulletier de la Salle in 1769 and in 1815 Michel Eugène Chevreul named it cholesterine. Cholesterol is also considered as a lipid due to it’s presence in animal cell membrane.

The molecular formula of cholesterol is C27H46O and the structural formula is

Functions : Cholesterol is an essential component of cell membrane that is needed to maintain proper membrane permeability and fluidity. It is also needed for the synthesis of Steroid Hormones, Vitamin D and Bile Acids.

  1. Cholesterol is required for building and maintaining animal cell membranes. It modulates membrane fluidity over the range of physiological temperatures. It prevents the Hydrocarbons present in the cell membrane from crystallization through the interaction with the Phospholipid fatty-acid chains thus modulating the membrane fluidity over the range of temperatures. The Tetracyclic ring structure of cholesterol molecule also helps to decrease membrane fluidity.
  2. Cholesterol reduces the permeability of Hydrogen Cations and Sodium Ions through the Plasma Membrane.
  3. Myelin Sheath or the protective covering of the Axon (the elongated part of the nerve cell or neuron) are rich in Cholesterol. Myelin Sheath acts as an insulator to the Axons and protect them from electrically charged atoms and molecule thus helping in the propagation of nerve impulses properly and with more efficiency (nerve impulse propagates faster through the Myelinated nerve fibres than the Non-myelinated ones).
  4. In the liver Cholesterol is converted to bile and then stored in the Gallbladder; Bile Acids and Bile Salts are essential for the absorption and digestion of fat molecules and fat soluble Vitamins – Vitamin A, D, E and K.
  5. Cholesterol is an important precursor molecule essential for the biosynthesis of Vitamin D and various steroid hormones like Cortisol and Aldosterone, and also the sex hormones Estrogens, Progesterone, and Testosterone.
  6. Some recent researches suggest that Cholesterol may act as an antioxidant but this is yet to be confirmed.

Sources : All animal fats contain cholesterol to varying extents. The main sources of cholesterol are as
follows :-

Butter, Cheese, Egg Yolks, Beef, Pork, Poultry, Fish, and Shrimp, Breast milk also contain significant amounts of cholesterol.

Plant sources generally contain a cholesterol-like compound named Phytosterols.

Plasma transport of cholesterol ; LDL, HDL and Triglycerides

Since cholesterol is a lipid so it is insoluble in polar solvent like water, that is the base of blood plasma so cholesterol is carried by the Lipoproteins through blood (lipids combined with proteins). The Lipoproteins are Amphiphilic in nature. The outer portion of the Lipoprotiens are water soluble whereas the inner surface is fat soluble. The cholesterol is carried internally by the Lipoprotein through the Blood Plasma.

There are various types of Lipoproteins present in blood. The Blood Lipoproteins are classified according to their densities (amount of lipid present in a lipoproteins determines it’s density, the amount of lipid is inversely proportional with the density). According to the decreasing order of density the Lipoproteins are – High Density Lipoprotein (HDL), Low Density Lipoprotein (LDL), Intermediate-density Lipoprotein (IDL), Very-low-density Lipoprotein (VLDL) and Chylomicrons.

The most important Lipoproteins for human are LDL and HDL they play an important role in cholesterol metabolism. The main features of LDL and HDL are given below:

  • LDL : LDL is the carrier of cholesterol from the liver to cells. Excess LDL is oxidized and form foam cells, these foam cells then get trapped in the walls of blood vessels and lead to the formation of Atherosclerotic plaque (swelling in arterial walls due to the accumulation of macrophage cells, excess cholesterol and fatty acids, calcium etc). These plaques are considered as the main cause of stroke, heart attacks and various other cardiovascular diseases. That is why LDL is often referred as Bad Cholesterol.
  • HDL : The function of HDL is to carry cholesterol back to liver for excretion or to the other tissues for further use like production of hormones. This process is known as Reverse Cholesterol transport. HDL is considered good for health so is termed as good cholesterol.

It is important to maintain low level of LDL and high level of HDL in blood.

Triglycerides : Although not directly related to cholesterol yet the article will remain incomplete without the mention of Triglycerides. Cholesterol and Triglycerides both are considered as important blood lipids, that needed for human metabolism.

Triglycerides molecules are derived by the Esterification of Glycerol molecules with three Fatty Acids. The function of Triglycerides is the both way transport of Adipose fat and Glucose. High level of Triglycerides may lead to heart diseases and stroke.

Biosynthesis and metabolism of cholesterol : Most of the body Cholesterols are produced in liver, intestine, adrenal gland, and reproductive organs. Biosynthesis of cholesterol is regulated by the level of cholesterol present in blood. The homeostatic mechanisms responsible for the regulation of the production of cholesterol are not fully understood. It is seen that the higher intake of cholesterol from external source leads to a decrease in cholesterol production in body.

Cholesterol metabolism : Cholesterol is readily oxidized to form Oxysterols. Cholesterol can be oxidized in three ways as follows : Autooxidation, Secondary Oxidation and Cholesterol-metabolizing Enzyme Oxidation. Cholesterol is oxidized in liver to form various bile acids, after that the bile acids and un oxidised cholesterols are secreted from the liver into the bile. Most of the bile acids are reabsorbed from the intestines, and the remainder are eliminated with feces.

Oxysterols show inhibitory actions on Cholesterol Biosynthesis and this is called Oxysterol Hypothesis.

Cholesterol level in blood : Normal cholesterol, HDL, LDL and Triglycerides levels are given below :

LDL Cholesterol (mg/dl)
<100 Normal
100-129 Near Optimal
130-159 Borderline High
160-189 High
> 190 Very High
HDL Cholesterol (mg/dl)
<40 Low
>60 High
Total Cholesterol (mg/dl)
<200 Normal
200-239 Borderline
>=240 High risk
Triglycerides (mg/dl)
<150 Normal
150-199 Borderline High
200-499 High
>500 Very High

The total cholesterol is to HDL ratio is better indicator of the risk of having cardiovascular disease than total cholesterol.

Ratio of total Cholesterol to HDL
Men Women
Very low <3.4 <3.3
Low risk 4.0 3.8
Average risk 5.0 4.5
Moderate risk 9.5 7.0
High risk >23 >11
Ratio of LDL to HDL
Risk Men Women
Very low 1 1.5
Average risk 3.6 3.2
Moderate risk 6.3 5.0
High risk 8 6.1

Hypercholesterolomia : Hypercholesterolomia is the condition of having elevated level of cholesterol in blood. This generally happens due to the imbalance of HDL and LDL ratio. As stated before, low level of LDL and high level of HDL is needed to maintain proper cholesterol level, the reverse leads to Hypercholesterolomia.

Causes and management of Hypercholesterolomia : Most of the cases lifestyle is the cause behind this condition. The frequent causes of Hypercholesterolomia are given
below :-

  • Too much intake of food with saturated fat
  • Smoking
  • Lack of exercise
  • Overweight
  • Some diseases like coronary disease or high blood sugar

Changing the lifestyle like regular exercise, weight loss often leads to the reduction of blood cholesterol level. In some cases medication is also needed to control Hypercholesterolomia.

Hypecholesterolomia : Very low level of cholesterol in blood is termed as Hypocholesterolomia. This condition may happen due to some illness like internal hemorrhage or may be genetic like having Smith-Lemli-Opitz syndrome or 7-dehydrocholesterol reductase deficiency syndrome ( inability to synthesize cholesterol properly).