Cholesterol biosynthesis pathway shows how our body makes cholesterol from simple units like Acetyl Coenzyme A, which can be obtained either from carbohydrates or fats. There is no way that fats be directly converted to good or bad cholesterol. Good or bad cholesterol in blood stream depends upon the nutritional status of the individual.
Cholesterol is complex molecule the structure of which is shown here:Cholesterol is a complex molecule having 27 carbon atoms and a four ring structure. Compare it with the structure of fats. Fats is a triglyceride where three fatty acid molecules are esterified with glycerol.
If saturated fats have to be converted to cholesterol, fat molecule, which is actually a triglyceride, has to be hydrolyzed to fatty acids of different chain length. Fats are catabolized in the body to simple units and ultimately converted into energy.
The hydrolysis products of fats are different fatty acids of different chain length. The fats enter the intestine from where they are absorbed and hydrolyzed to fatty acids by “Chylomicron pathway”. There is no connection whatsoever between cholesterol in food and cholesterol in blood as postulated in “lipid hypothesis”. Our liver regulates the quantity of cholesterol required by our body.
As soon as the requirement is met it stops making cholesterol. In spite of this information being known known the lipid hypothesis states that intake of dietary fats and cholesterol increases LDL cholesterol. Intake of low-fat diet is advocated as a measure to lower cholesterol levels.
Then what is the connection between saturated fats and cholesterol?
Does saturated fat act as a building block for cholesterol? Chylomicron pathway describes very well how the fat is hydrolyzed after it reaches the intestine. There is absolutely no way that you can turn saturated fat (or any other saturated fat) into cholesterol. Human biochemistry is very complicated. There is a particular pathway that our body follows to make cholesterol. This is called as cholesterol biosynthesis pathway.
Cholesterol biosynthesis pathway: The liver can make cholesterol from carbohydrates as well. Even if you do not happen to eat saturated at all or any other unsaturated fat, liver will still make cholesterol. This is because it needs a simple molecule to start building a complex molecule like cholesterol and that simple starting raw material is “Acetyl coenzyme A”.
The flow chart below shows how the body synthesizes cholesterol starting from acetyl co enzyme A. The major steps in the synthesis of cholesterol are:
- 1.0 Acety-Coenzyme A’s are converted to 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA)
- 2.0 HMG-CoA is converted to mevalonate.
- 3.0 Mevalonate is converted to isoprene based molecule, isopyrophosphate (IPP), with the concomitant loss of CO2.
- 4.0 IPP is converted to squalene
- 5.0 Squalene is converted to cholesterol.
If you look at the structures of the intermediates involved in cholesterol biosynthesis you will not find any saturated fatty acid structure acting as an intermediate of cholesterol synthesis.
From cholesterol biosynthesis pathway shown above we cannot justify that fats are converted to cholesterol. The fundamental building block of cholesterol is a substance called as Acetyl CoA. If fats cannot be converted to cholesterol how does consumption of fats increases LDL cholesterol.
How to lower cholesterol?
The lipid theory proposes that one should avoid eating food containing saturated fats, eat low-fat diet and low-fat diet recipes but these suggestions do not seem to work in lowering LDL cholesterol levels.
How to lower cholesterol
Recent research shows that our blood plasma contains some emerging risk factors which if we try to control will keep our cholesterol levels under control. The appearance or rise of these emerging risk factors can also be attributed to un optimum nutrition and unbalanced diet.