Basic knowledge of blood lipids: Analysis of the metabolism and function of cholesterol and triglycerides

Blood lipids: Blood lipids refer to the total lipid components contained in the blood.

The main lipid components in blood are cholesterol, triglycerides, phospholipids, free fatty acids, trace amounts of steroid hormones, and fat-soluble vitamins (such as vitamins A, D, and E). These are normal components of blood and have important physiological functions.

How blood cholesterol is metabolized: Except for brain tissue, all other tissues in the human body can synthesize cholesterol, synthesizing 1-2 grams daily. The liver is the organ with the strongest synthesis capacity, accounting for more than 3/4 of the total synthesis in the body, followed by the small intestine. The synthesis of cholesterol in the body is complex, requiring the participation of many enzymes and energy.

Another source of cholesterol in the human body is the small amount ingested from food. In a normal person's diet, cholesterol mainly comes from animal foods such as egg yolks, animal organs, cream, and meat. Plant-based foods do not contain cholesterol, and excessive consumption of plant-based foods can inhibit cholesterol absorption.

Is lower blood cholesterol always better? Most blood cholesterol (70%) is found in low-density lipoprotein (LDL) and is carried into tissue cells for further metabolism, playing an important physiological role. Therefore, lower cholesterol is not always better.

Besides being a structural component of cells, cholesterol's main physiological functions include being converted into various important steroid compounds in the body, such as cortisol and aldosterone released by the adrenal cortex, and testosterone, estradiol, and vitamin D released by the gonads. Cholesterol is a precursor to these compounds. In the liver, some cholesterol is converted into bile acids, and bile plays a crucial role in the digestion and absorption of lipids.

Therefore, cholesterol is an essential substance for human life. Atherosclerosis is only induced when cholesterol is too high or abnormal LDL is present.

What are triglycerides and what are their physiological functions? Triglycerides (TG), or triglycerides, are organic substances synthesized from one molecule of glycerol and three molecules of fatty acids. They are a major component of fats and a storage substance in the body.

Like cholesterol, triglycerides have two sources: exogenous and endogenous. Exogenous triglycerides are ingested from food, digested and absorbed in the small intestine, and assembled with apolipoproteins synthesized by the small intestine itself to form chylomicrons. These chylomicrons then enter the bloodstream via the lymphatic system, and contain 90% triglycerides. Endogenous triglycerides are synthesized by the liver, which is the most efficient organ for synthesizing them, followed by adipose tissue. Triglycerides synthesized by the liver assemble with apolipoproteins to form very low-density lipoprotein (VLDL), which is secreted into the bloodstream. VLDL contains 60% triglycerides.

When triglycerides from chylomicrons and VLDL pass through capillaries, they release a significant amount of energy. Some of this energy is dissipated as heat, and some is stored as adenosine triphosphate (ATP) to meet the body's vital functions.