Cholesterol is a lipidic, waxy alcohol. It is a fat-like substance produced by all animals, including humans. Cholesterol is needed for many bodily functions and serves to insulate nerve fibers, maintain cell walls and produce vitamin D, various hormones and digestive juices. Cholesterol is produced by the liver.
The name cholesterol originates from the Greek chole - (bile) and stereos (solid), and the chemical suffix -ol for an alcohol.
There is a difference between dietary cholesterol (the cholesterol we consume in foods) and blood cholesterol (the cholesterol in our bloodstream, also called serum cholesterol). Dietary cholesterol is present in varying amounts in some foods, such as meat, poultry, seafood, eggs, and dairy products. Dietary cholesterol does not automatically become blood cholesterol when we eat it. Most of our blood cholesterol is made by our body. Individuals vary in how much cholesterol their body makes. There is little doubt that elevated blood cholesterol levels increase heart disease risk. But the effect of dietary cholesterol on blood cholesterol levels is the subject of debate among health professionals. That's because research does not show that food cholesterol significantly boosts blood cholesterol levels in everyone.
Although cholesterol is essential for life, high levels in circulation are associated with atherosclerosis. For a person of about 150 pounds (68 kg), typical total body cholesterol content is about 35 grams; typical daily dietary intake is 200–300 mg in the United States and societies with similar dietary patterns. If you have heart disease, limit your daily intake to less than 200 milligrams.
People with severe high blood cholesterol levels may need an even greater reduction. Since cholesterol is in all foods from animal sources, care must be taken to eat no more than six ounces of lean meat, fish and poultry per day and to use fat-free and low-fat dairy products. High-quality proteins from vegetable sources such as beans are good substitutes for animal sources of protein.
Function
Cholesterol is required to build and maintain cell membranes; it regulates membrane fluidity over the range of physiological temperatures.
Within the cell membrane, cholesterol also functions in intracellular transport, cell signalling and nerve conduction. In many neurons a myelin sheath, rich in cholesterol since it is derived from compacted layers of Schwann cell membrane, provides insulation for more efficient conduction of impulses.
Within cells, cholesterol is the precursor molecule in several biochemical pathways. In the liver, cholesterol is converted to bile, which is then stored in the gallbladder. Bile contains bile salts, which solubilize fats in the digestive tract and aid in the intestinal absorption of fat molecules as well as the fat soluble vitamins, Vitamin A, Vitamin D, Vitamin E and Vitamin K. Cholesterol is an important precursor molecule for the synthesis of Vitamin D and the steroid hormones, including the adrenal gland hormones cortisol and aldosterone as well as the sex hormones progesterone, estrogens, and testosterone and their derivatives.
Dietary Sources
People get cholesterol in two ways. The body - mainly the liver - produces varying amounts, usually about 1, 000 milligrams a day. Food also contains cholesterol. Animal fats are complex mixtures of triglycerides, with lesser amounts of phospholipids and cholesterol. Consequently all foods containing animal fat contain cholesterol to varying extents. Major dietary sources of cholesterol include cheese, egg yolks, beef, pork, poultry, and shrimp. Human breast milk also contains significant quantities of cholesterol. Cholesterol is not present in plant based food sources unless it has been added during the food's preparation. However, plant products such as flax seeds and peanuts contain healthy cholesterol-like compounds called phytosterols, which are suggested to help lower serum cholesterol levels.
A change in diet may help reduce blood cholesterol in addition to other lifestyle modifications. Avoiding animal products may decrease the cholesterol levels in the body not through dietary cholesterol reduction alone, but primarily through a reduced saturated fat intake. Those wishing to reduce their cholesterol through a change in diet should aim to consume less than 7% of their daily calories from saturated fat and less than 200mg of cholesterol per day.
The view that a change in diet (specifically, a reduction in dietary fat and cholesterol) can lower blood cholesterol levels, and thus reduce the likelihood of development of, amongst others, coronary artery disease (CHD) has been challenged. An alternative view is that any reductions to dietary cholesterol intake are counteracted by the organs such as the liver, which will increase or decrease production of cholesterol to keep blood cholesterol levels constant.
Synthesis
About 20–25% of total daily cholesterol production occurs in the liver; other sites of high synthesis rates include the intestines, adrenal glands and reproductive organs.
Regulation of cholesterol synthesis
Biosynthesis of cholesterol is directly regulated by the cholesterol levels present, though the homeostatic mechanisms involved are only partly understood. A higher intake from food leads to a net decrease in endogenous production, while lower intake from food has the opposite effect. The main regulatory mechanism is the sensing of intracellular cholesterol in the endoplasmic reticulum by the protein SREBP (Sterol Regulatory Element Binding Protein). Cholesterol synthesis can be turned off when cholesterol levels are high as well.
