Cholesterol is an essential component of every cell in the body and is beneficial for proper brain and nerve function (such as coordination and speech). Cholesterol makes up to 50% of the insulation cover around nerve cells. It is also the basis for the manufacture of sex hormones (estrogen, progesterone, testosterone, aldosterone etc.) which define the sex characteristics of an individual. Cholesterol is also necessary for the synthesis of vitamin D and bile acids. Bile acids’ function is to digest and absorb fats and fat soluble vitamins.

There are two types of choelsterol, namely Low-density lipoproteins (LDLs) and high-density lipoproteins (HDLs).

Low-Density Lipoproteins (LDLs) and High-Density Lipoproteins (HDLs)

Lipoproteins are composed of protein and triglycerides: with low-density lipoproteins containing 21% protein whereas high-density lipoproteins contain 50% protein. Low-density lipoproteins transport cholesterol from the liver to the rest of the body, and therefore low-density lipoproteins (LDLs) seem to encourage the deposit of cholesterol in the arteries,  which is the reason for the definition “bad cholesterol”. High-density lipoproteins (HDLs) on the other hand, are considered to be “good cholesterol” because they carry unneeded cholesterol away from the cells back to the liver, where it is broken down for removal from the body. If there are not enough High-density lipoproteins (HDLs) to pick up the excess cholesterol, cholesterol can form plaque that sticks to the arteries.

When is Cholesterol Bad?

A high blood cholesterol level, especially elevated low-density lipoproteins (LDL), is considered a contributor to plaque build up in the arteries and impeded blood flow to the brain, kidneys, genitals, extremities, and heart. It is among the primary causes of heart disease, because cholesterol produces deposits in arteries. High cholesterol levels may also be implicated in gallstones, erectile dysfunction (although it is often the drugs prescribed to deal with high cholesterol that cause this), mental impairment, and high blood pressure.

Table: LDL-Cholesterol, HDL-Cholesterol and Total Cholesterol Levels(mg/dl).

(Cholesterol levels source: Janice R. Hermann, PhD, RD/LD, http://osufacts.okstate.edu)

 
Cholesterol Sources

There are two sources of cholesterol in the body: cholesterol from the diet and also cholesterol produced in the body. The body produces its cholesterol, so even if one does not eat cholesterol containing food, there is cholesterol in the blood stream. Causes of high serum cholesterol can be genetic, diet high in cholesterol and/or saturated fat. Factors that may lower cholesterol are vegetarian diet, regular exercise, the nutrients niacin and vitamin C.

LDL cholesterol related links:
"How low should LDL cholesterol be lowered?"
LDL cholesterol lowering and atherosclerosis

Fat related content
Fat: List of Food high in total fat and saturated fatty acids
Unsaturated fat: Food high in polyunsaturated and monounsaturated fat content?

Cholesterol: benefits and how to raise HDL cholesterol level
List of cholesterol free foods
Cholesterol content in foods of plant-origin
Cholesterol in beef, chicken, pork, lamb comparison
Cholesterol content in seafoods (tuna, salmon, shrimp)
Cookbooks for low cholesterol diet