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There are two main sources of cholesterol and triglycerides:
- External sources: the diet
- Internal sources: manufactured/recycled by the body, especially the liver
Fats from our diet
In terms of 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 very high cholesterol content. For practical purposes, plants have no cholesterol. Plant products have cholesterol-like substances called phytosterols, which may compete with cholesterol for absorption. When consumed in very large amounts, they can lower cholesterol absorption and consequently lower blood cholesterol by about 5-10%.
In the intestine, the ingested cholesterol and triglycerides are assembled into special spherical packages called chylomicrons, which travel through the bloodstream to the liver. Most of the cholesterol in the diet stays in the chylomicrons and is taken up by the liver where it mixes with newly made cholesterol, made by the liver itself. Chylomicrons only remain in the blood stream for a short while. The liver is where fats are broken down and made from metabolic products of sugars and proteins. Not only does the liver make cholesterol, it also makes fatty acids, triglycerides and phospholipids for export to the rest of the body. The fats made in the liver are assembled with protein compounds called apolipoproteins to make a new, fat-rich lipoprotein called VLDL – very-low-density lipoproteins.
Again, the VLDLs lose most of their triglycerides as fatty acids to muscle and adipose tissue where they are either used or stored. Some of these are taken up by the muscles as fuel while the rest is laid down as fatty (adipose) tissue in the body. The fats in adipose tissue act as energy reserves – all too often never utilised!
But instead of excess being taken up by the liver, they are converted in the plasma (the liquid, non-cellular part of the blood) into a new form of lipoprotein called LDL or low-density lipoprotein. LDL is very rich in cholesterol and is known as the “bad” form of cholesterol. It is these particles which, in excess, can lead to atherosclerosis and coronary artery disease.
LDL remains in the blood stream for around three days before it is removed by the liver.
The mechanism whereby LDL is removed by the liver is important. LDL is linked to a protein called apoB. This recognises a specific receptor on the liver cell – the apoB-receptor also known as the LDL receptor. This allows LDL to “dock” at the receptor and so be taken up by the liver.
This is important because if the receptor is faulty, as happens in the case of familial hypercholesterolaemia, the LDL is not taken up. When this happens, an excess of LDL circulates in the blood stream, penetrating the vascular wall and leading to coronary artery disease.
There is another lipoprotein in this story – HDL or high-density lipoprotein. 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.
HDL has the ability to pick up excess free cholesterol from peripheral (non-liver) cells, including those accumulating in the arterial wall which predispose to coronary artery disease.
HDL returns the excess cholesterol directly to the liver. HDL also carries important protective antioxidant enzymes and other molecules which lessen the risk of coronary artery disease. It seems also to limit the adverse response of the arterial lining to lipoproteins, cells and clotting processes.
All these properties make HDL an effective anti-coronary disease agent. The cholesterol measured in HDL is therefore called "good" cholesterol.
This means that not only are your levels of cholesterol and triglycerides important, but also the levels of LDL and HDL.
High levels of LDL and low levels of HDL mean that you have a greater risk of coronary artery disease, and the opposite means that you’re better protected.
The amount of cholesterol and triglycerides made in the liver is influenced by the total energy (kilojoule) intake in the diet and the quantity and kind of fat consumed. That is one reason why diet is so important.
- Eating saturated fats raises levels of LDL (“bad”) cholesterol; hence too much of these in the diet is harmful.
- Mono- and polyunsaturated fats are much better for you than saturated fats. In fact eating certain unsaturated fats can actually decrease your total cholesterol levels.
- When unsaturated vegetable fats are hydrogenated, the process produces yet another type of fatty acid as a side product, called a trans fatty acid. Not only do trans fatty acids raise LDL levels, and so the total levels of cholesterol, but they also lower the levels of HDL – known to protect against coronary artery disease.
Production in the body
Cholesterol is made mainly from acetyl-CoA, which is a breakdown by-product formed during fat digestion and also an intermediate in carbohydrate and amino acid metabolism. An important and rate-controlling enzyme called HMG-CoA reductase, is involved in this process.
The liver is an important hub of cholesterol metabolism in the body. The liver balances the daily requirements for cholesterol by sensing the amount it contains. It receives dietary and biliary cholesterol by taking in the remnants of chylomicrons and can also take in LDL cholesterol from the blood by expressing LDL receptors when necessary, and can draw on a stored form of cholesterol in its cells called cholesterol ester.
The liver exports cholesterol directly into bile, but also indirectly: it secretes bile acids made from cholesterol into the bile. The other form of export is as VLDL. When there is too little cholesterol in the liver, there is a programmed response to turn on HMGCoA reductase as well as LDL receptors to make good the deficit. When there is an excess of cholesterol in the liver, as in the case of a very cholesterol-rich diet, these two processes are switched off. The switching off of LDL receptors means less LDL is removed from the blood and the blood LDL cholesterol level rises by small amounts over a few weeks to reach a new level when the liver achieves a new, balanced state.
However, in some people cholesterol levels remain high, regardless of intake due to genetic abnormalities in the LDL receptor. For them, limiting fats in their diet 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.
