The metabolism of cholesterol has the purpose of preventing excessive variations, in defect or in excess, of the levels of this lipid in the organism. In fact, we are talking about a fat essential for the cells of our body, essential - for example - for the synthesis of steroid hormones and bile acids, but also for the structural integrity of plasma membranes.
and dietary intakeThe metabolism of cholesterol involves multiple organs. Upstream we find the intestine - which absorbs food-borne cholesterol from the digestion of animal foods - and the liver, an organ capable of synthesizing significant quantities of cholesterol starting from the "vinegar-CoA derived from the metabolism of various nutrients (carbohydrates, proteins and in particular fats). To a lesser extent, cholesterol can also be synthesized in the intestine and skin.
Compared to a dietary intake of about 300 mg / day, the body of an adult person synthesizes about 600-1000 mg of cholesterol every day. This means that the influence of dietary cholesterol on total cholesterol (amount of cholesterol in the blood) is, after all, less relevant than one might think, estimated at an average of 30%. For this reason in some people with high cholesterol diet therapy, alone, it is unable to bring cholesterol to normal; the body of these people, in fact, synthesizes excessive quantities of cholesterol, so even by correcting eating habits, cholesterol in the blood remains elevated. In this regard, it is important to note that the dietary intake of cholesterol and its hepatic synthesis are closely linked by a regulatory feed-back mechanism: it means that the synthesis of cholesterol in the liver is enhanced when the dietary intake is low, while slows down when the person introduces high amounts of cholesterol with food.
lipid surrounded by a protein shell called apoprotein.
The cholesterol absorbed by the intestine is released into the lymphatic circulation in the form of chylomicrons; these are lipoprotein aggregates formed by a heart rich in triglycerides, phospholipids, cholesterol and fat-soluble vitamins, surrounded by a protein shell. At the level of the subclavian veins, the chylomicrons they pour into the bloodstream in a still incomplete form (nascent chylomicrons); only following the interaction with other plasma lipoproteins, in particular HDL, the chylomicrons acquire the apoproteins, C-II and E: the former are necessary for their recognition by the cells, to which they distribute their lipid content, while the apoproteins E serve to recognize them in the liver. The residual chylomicrons (called remnants) are in fact intercepted by the liver and reprocessed for endogenous lipid synthesis. Inside the hepatocytes (liver cells) the triglycerides are partly used as a reserve and partly degraded for energy purposes. Unlike the latter, cholesterol cannot be transformed or degraded for energy purposes; any excess can be eliminated only through the bile which, when released into the liver, favors its elimination with the faeces.
Cholesterol Metabolism - Video
Watch the video
- Watch the video on youtube
The cellular use of cholesterol is mediated by a series of events whose description is beyond the scope of this article; what we want to underline, in a healthy way, is that a high intracellular level of cholesterol blocks both the intake of new cholesterol from LDL that its endogenous synthesis. In other words, if the cell contains sufficient quantities of cholesterol, it does not synthesize any more and no longer absorbs it from the LDL, which consequently accumulate in the circulation, increasing cholesterol levels in the blood. We speak of hypercholesterolemia. Since excess cholesterol can infiltrate the walls of the arteries, undergoing oxidative and inflammatory processes that lead to very serious damage to the health of the organism (atherosclerosis and related diseases), the human body has developed a defensive strategy. The liver, in fact , synthesizes HDL (high density lipoproteins), responsible for the transport of cholesterol from peripheral tissues to the liver by implementing the so-called reverse transport of cholesterol. In the liver, excess cholesterol from HDL can be eliminated through bile and bile acids which In addition to the liver, HDLs are also synthesized at the enteric level (in the intestine) and in part derive from the catabolism of lipoproteins rich in triglycerides (chylomicrons and VLDL). The reverse transport of cholesterol is mediated by the HDL activity and essentially consists in the return to the liver of the excess cholesterol present in the peripheral blood. HDL, in fact, represent an "important reserve of apoproteins: anticipated coma, it is precisely thanks to the" acquisition of apoproteins C and E from HDL that chylomicrons and VLDL acquire the proteins necessary for cell recognition and their hepatic catabolism. In addition to apolipoproteins, HDL also release the cholesterol extracted from the cells thanks to a receptor system that recognizes apoprotein A-1 , immediately esterified by means of the enzyme LCAT (plasma lecithin-cholesterol acyltransferase). By discharging its cholesterol content to the VLDL and LDL at the peripheral level, and acquiring triglycerides in exchange, the HDL can accept new cellular cholesterol, and the cycle begins again We have therefore seen the indirect way through which HDLs deliver cholesterol to the liver; alongside this path there is also a direct path, through which HDL are intercepted by the liver, which recycles their protein portion and, at least in part, eliminates the surplus of cholesterol through the bile.
The bile with its bile acids, essential for the correct absorption of fats in the intestine, is partly reabsorbed and partly eliminated with the faeces. There are drugs, called resins sequestering bile acids, which limit the intestinal reabsorption of bile acids by stimulating their ex-novo synthesis; since this process uses the cholesterol present in the body, these drugs lower cholesterol. The percentage of reabsorbed bile salts is conveyed to the liver where the metabolism of cholesterol restarts.
.jpg)
