cholesterol

Cholesterol and nutrition

Importance of cholesterol and its functions

Every day we take a certain amount of cholesterol through food, which goes alongside the one produced by the body (especially in the liver).

Cholesterol is often considered in negative terms, when in reality it is a fundamental molecule for the human organism. It is indeed:

a structural component of plasma membranes, in particular it ensures a certain degree of fluidity;

the precursor of bile acids (in the liver), essential for the digestion of dietary fats;

the precursor of steroid hormones (in the reproductive organs and in the adrenal cortex);

the precursor of vitamin D (in the skin).

Cholesterol becomes harmful to the body only when its concentration in the plasma exceeds certain levels.

In such situations it tends to form so-called atherosclerotic plaques, which represent the starting point of numerous and serious cardiovascular pathologies.

In free cholesterol, the carbon in position three is bound to a hydroxyl (OH); for this reason it is not 100% hydrophobic. On the contrary, when this hydroxyl is esterified with a fatty acid, a cholesterol ester totally insoluble in water is obtained.

Blood cholesterol transport: lipoproteins

In the plasma most of the cholesterol is in esterified form, that is in its less soluble form. For this reason it must be transported in a special way, through the lipoproteins.

The main lipoproteins (LP) responsible for transporting blood cholesterol are:

chylomicrons

VLDL (very low density lipoprotein)

LDL (low density lipoprotein)

HDL (high density lipoprotein)

These lipoproteins are classified according to their size (see figure), composition and density.

Each lipoprotein is composed of a hydrophobic lipid heart, rich in triglycerides and esterified cholesterol. This central part is surrounded by a peripheral portion made up of proteins and phospholipids, which orient their polar head towards the outside. Between these two substances there is also a small amount of free cholesterol, which directs its hydroxyl towards the external portion.

Proteins do not completely cover these molecules, but tend to be in patches.

The quantity of lipids, unlike the size, tends to progressively decrease passing from chylomicrons, to VLDL, to LDL and to HDL. As the lipoprotein gets depleted of fat, it increases its protein content and along with it its own density.

Lipoproteins maintain the lipids in solution in the plasma and carry them from one tissue to another. In particular:

the chylomicrons are formed in the enterocytes and convey to the peripheral tissues the products of lipid digestion (triglycerides, cholesterol, cholesterol esters, fat-soluble vitamins) of food origin first in the lymph and then in the blood.

VLDLs are synthesized by hepatocytes. They transport triglycerides from the liver (where they were synthesized, for example, from glucose) to other tissues (especially the adipose and muscular tissues).

LDLs derive from VLDL, due to the progressive depletion of their triglyceride content. They are loaded with cholesterol that they carry to the peripheral tissues, where it carries out all those fundamental functions listed above.

HDL, secreted in the blood from the liver and intestine, transports cholesterol from peripheral tissues to the liver (they carry out the so-called reverse cholesterol transport).

Levels higher than the norm of LDL cholesterol predispose to the development of atherosclerosis; for this reason the cholesterol transported by the LDL is called "bad". In contrast, HDLs are called "arterial scavengers", since the cholesterol bound to them, called "good", is transported to the liver after the "vessel cleaning" operation.

The liver can thus direct cholesterol transported by HDL to the synthesis of bile salts; if present in excess it can also eliminate it through the bile as free cholesterol.

The cardiovascular risk is lower the higher the HDL cholesterol level and the lower the LDL cholesterol level. In other words, within certain limits, it is better to have high levels of total cholesterol associated with equally high HDL cholesterol levels, rather than having low levels of total cholesterol associated with low HDL cholesterol levels.

How much cholesterol do you take daily?

As for cholesterol, it is advised not to take more than 300 mg a day. In the presence of cardiovascular diseases or high family predisposition to these diseases, the cholesterol intake should be more contained.