Sources, pathways, normal ranges, and physiological functions of triglycerides and cholesterol in blood lipids.

Sources and fates of blood lipids

The sources and pathways of blood lipids are as follows:

Sources: lipids in food, lipids synthesized in the body, and lipid depot mobilization and release.

Fate paths: oxidation for energy, storage in lipid stores, formation of biological membranes, and transformation into other substances.

Normal range of blood lipids

During a health check-up, blood lipid tests typically include total cholesterol, high-density lipoprotein cholesterol, low-density lipoprotein cholesterol, triglycerides, and the ratio of apolipoprotein A to apolipoprotein B.

The ideal total cholesterol level is <5.2 mmol/L (200 mg/dL).

Edge elevation values: 5.23–5.69 mmol/L (201–219 mg/dL).

Elevated value: >5.72 mmol/L (220 mg/dL).

High-density lipoprotein (HDL-C): >40 mg/dL or 1.04 mmol/L.

The ideal value for low-density lipoprotein cholesterol is <3.12 mmol/L (120 mg/dL).

Edge elevation values: 3.15–3.61 mmol/L (121–139 mg/dL).

Ideal value elevation: >3.61 mmol/L (140 mg/dL).

The ideal triglyceride level is <1.70 mmol/L (150 mg/dL).

Elevated value: >1.70 mmol/L (150 mg/dL).

The ratio of apolipoprotein A to apolipoprotein B should be higher than 1.30 in a normal blood lipid level; when blood lipids are abnormal, the ratio often drops below 1.0.

Physiological functions of triglycerides

1. Body composition and energy intake

When the body ingests energy that cannot be utilized in time or when there is excess energy, it is converted into fat and stored. All bodily activities require energy expenditure, and fat is the nutrient that produces the most energy; each gram of fat in the body produces approximately 39.7 kilojoules of energy. At rest, 60% of a person's energy comes from body fat, and during exercise or prolonged periods of fasting, body fat provides even more energy.

2. Maintain normal body temperature

Human skin contains a layer of fat that prevents and regulates body heat loss, acting as insulation to maintain a normal and constant body temperature. Lipids also protect skin epithelial cells and accelerate the healing of skin injuries. Dietary lipids also have specific nutritional functions, such as providing fat-soluble vitamins, increasing satiety, and improving the sensory properties of food. Therefore, the human body needs fat and cannot live without it; completely rejecting fat in daily life is harmful to health.

The main physiological functions of cholesterol

As early as the beginning of the 18th century, scientists first extracted a substance from gallstones and named it cholesterol. Cholesterol is composed of a steroidal portion and a long side chain. The total amount of cholesterol in the human body is approximately 0.2% of body weight, with varying amounts in different tissues. For example, bone contains the least amount of cholesterol. Per 100g, bone contains about 10mg, skeletal muscle about 100mg, internal organs mostly between 150-250mg, with slightly higher amounts in the liver and skin, around 300mg. The brain and nerve tissue have the highest content, about 2g per 100g of tissue, accounting for about one-quarter of the total body cholesterol.

Cholesterol has three main physiological functions.

1. Formation of bile acids

Bile is produced in the liver and stored in the gallbladder. It is then released into the small intestine and mixes with digested fats. The function of bile is to break down large fat particles into smaller ones, making them easier for enzymes in the small intestine to interact with. In the tail of the small intestine, 85%–95% of the bile is reabsorbed into the bloodstream. The liver reabsorbs bile acids, keeping the system in constant circulation. The remaining bile (5%–15%) is excreted in feces. The liver needs to produce new bile acids to compensate for this 5%–15% loss, which requires cholesterol.

2. Constituting the cell membrane

Cholesterol is an essential component of cell membranes, which surround every cell in the human body. It has been found that feeding animals a cholesterol-deficient diet increases the fragility of their red blood cells, making them more prone to rupture. Therefore, it's conceivable that without cholesterol, cells cannot maintain normal physiological functions, and life would cease.

3. Synthetic hormones

Hormones are chemical messengers that coordinate the metabolic processes of different cells in a multicellular organism. They participate in the metabolism of various substances in the body, including carbohydrates, proteins, fats, water, electrolytes, and minerals, and are crucial for maintaining normal physiological functions. Various hormones released by the adrenal cortex and gonads, such as cortisol, aldosterone, testosterone, estradiol, and vitamin D, are steroid hormones, and their precursor is cholesterol.