Fatty acid composition and cholesterol physiology: From the health effects of saturation to medical advice on scientific intake.

The term "fat" usually refers to lipids and oils. Lipids are solid at room temperature, while oils are liquid. Animal fats are high in saturated fatty acids, are solid at room temperature, and are prone to rancidity. Foods rich in saturated fatty acids include butter, lard, cheese, fatty meat, and some vegetable oils such as coconut oil, palm oil, and cocoa butter. Vegetable oils, on the other hand, contain more unsaturated fatty acids, are less prone to rancidity, and have a longer shelf life at room temperature. Unsaturated fatty acids include monounsaturated and polyunsaturated fatty acids.

Monounsaturated fatty acids are mainly found in rapeseed oil and olive oil (although monounsaturated fatty acids can be synthesized in the body, the amount synthesized is small, so they still need to be obtained from food). Polyunsaturated fatty acids are abundant in vegetable oils such as flaxseed, sunflower seed, soybean, and peanut (polyunsaturated fatty acids cannot be synthesized in the human body and must be obtained from vegetable oils). Whether saturated or unsaturated, the primary function of fatty acids is as an energy source for the human body. Saturated fatty acids are the most unhealthy component of fatty acids. Consuming too much food rich in saturated fatty acids accelerates fat accumulation, leading to obesity and increasing the risk of heart disease.

Unsaturated fats have various benefits, including regulating blood lipids, clearing blood clots, improving eyesight, nourishing the brain, combating allergies, and enhancing immune function. They can also prevent arteriosclerosis, cardiovascular disease, and aging. According to the Chinese Nutrition Society, the recommended ratio of fatty acids for human consumption is monounsaturated fat: polyunsaturated fatty acids: saturated fatty acids = 1:1:1. However, for individuals with high cholesterol levels, the proportion of saturated fatty acids should be lower, and the proportion of unsaturated fatty acids should be appropriately increased. The optimal ratio of the three types of fats is 13:10:7, meaning that calories from saturated fats should be reduced to approximately 7% of total intake.

Besides fats, another type of lipid is the phospholipid. Pulses are fundamental components of biological membranes, making up about 5% of body weight. Cholesterol is one such phospholipid. Cholesterol is widely distributed in animal bodies, especially abundant in the brain and nerve tissue, and also found in high concentrations in the kidneys, spleen, skin, liver, and bile. The vast majority of cholesterol in the blood is in the form of cholesterol esters bound to fatty acids; less than 10% exists in a free state. Normal total cholesterol levels in the blood are 140–200 mg/100 ml, or 3.6–5.2 mmol/L.

Cholesterol is essential for the human body because it has a wide range of physiological functions: it is an important component of cell membranes and a raw material for their production. In the body, cholesterol can also be converted into steroid hormones, such as adrenocortical hormones, androgens, estrogens, and vitamin D3. Vitamin D3 promotes bone calcification; a deficiency can lead to rickets in adults and osteomalacia in children. Bile acids in bile are also synthesized in the liver using cholesterol as a raw material. Excessive avoidance of cholesterol-rich foods can easily cause anemia and lower the body's resistance. However, long-term excessive cholesterol intake is detrimental to health, as it can raise serum cholesterol levels and increase the risk of cardiovascular disease.

Therefore, a scientific dietary approach advocates for moderate cholesterol intake, with experts recommending a daily intake of 50-300 mg of cholesterol. Healthy adults and those with hypercholesterolemia without coronary heart disease or other atherosclerotic diseases should consume less than 300 mg of cholesterol daily, while those with hypercholesterolemia and coronary heart disease should consume less than 200 mg. For example, a 70 kg adult has approximately 140 grams of cholesterol in their body. About 1 gram is renewed daily, with only 1/5 needing to be obtained from food (4/5 produced through metabolism). Therefore, consuming 200 mg of cholesterol daily from food is sufficient to meet the body's needs.