• External sources, in other words the diet;
• Internal sources, in other words that which is manufactured/recycled by the body.

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The diet
In terms of the diet, cholesterol and triglycerides come mainly from eating animal products, fish, dairy products and various oils, which are then absorbed through the gut.

Eggs and shrimp have a very high cholesterol content. Plants have trace amounts of cholesterol, so that even vegans will consume some cholesterol. Plant products also have cholesterol-like substances called phytosterols, which may compete with cholesterol for absorption.

In the intestine, the ingested cholesterol and triglycerides are assembled into special spherical packages called chylomicrons, which travel through the bloodstream to the liver. Here, the chylomicrons are systematically converted into different lipoproteins, carrying more and more cholesterol. These lipoproteins then circulate to deliver cholesterol to the tissues where they are either used or stored.

Of all the lipoproteins, only high-density lipoprotein (HDL) does not deposit cholesterol in tissues. Instead, it removes cholesterol, taking it back to the liver for excretion. For this reason, HDL is known as "good" cholesterol.

The liver is an important organ in the body, one of its main jobs being to ensure that cholesterol from the diet is delivered to the parts of the body where it is needed. But it also synthesises (makes) cholesterol and triglycerides that are distributed along with the dietary cholesterol.

The amount of cholesterol and triglyceride made in the liver is influenced by the total energy (calorie) intake in the diet and the amount and kind of fat eaten. That is one reason why diet is so important.

Diets high in saturated (animal) fats will cause the total cholesterol to rise, and this excess may be deposited in vital organs and arteries. High cholesterol levels are a strong risk factor in the development of heart disease.

Production in the body
Cholesterol is made mainly from Acetyl-CoA, which is a breakdown by-product formed during fat digestion. An important enzyme, called HMG-CoA reductase, is involved in this process.

The more fat a person eats, the more AcetylCoA is produced, and thus more is available for cholesterol production. Decreasing the amount of fat eaten will decrease the amount of AcetylCoA available, and thus lower the total cholesterol produced.

However, in certain individuals, cholesterol levels remain high regardless of intake due to genetic abnormalities in the LDL receptor. Here, limiting the intake of fats may have very little influence on cholesterol levels.

Apart from making new cholesterol, the body recycles that used for bile production. Between 92% and 97% of the cholesterol in bile is reabsorbed from the gut and recycled back to the liver, where it can be reused.

Reviewed by Dr A.G. Hall (B.Soc.Sc.(SW), MB,Ch.B)
Last updated: 2008/05/19
 

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