Detailed Explanation of Dietary Sources and Metabolic Processes of Fat
Fat is an important nutrient and a basic component of food. Excessive intake of saturated fatty acids can easily induce cardiovascular and cerebrovascular diseases, lead to obesity, and also induce hypertension and diabetes. For people who use vegetable oil as their cooking oil, fat deficiency is generally not a problem. Supplementing the diet with a certain amount of omega-3 unsaturated fatty acids can prevent hyperlipidemia and Alzheimer's disease. Adequate omega-3 unsaturated fatty acid supplementation in the diets of infants, children, and adolescents can improve intelligence and memory. Reducing and controlling trans fatty acid intake is important because they can lower the activity of enzymes used by the body to fight cancer, reduce immunity, interfere with insulin receptor function, hinder the body's utilization of omega-3 fatty acids, increase the risk of asthma and allergies, reduce fertility, and decrease the activity of enzymes necessary for the production of sex hormones. Among the various fats ingested by the human body, trans fats are low in nutrients and unhealthy. They increase bad cholesterol and decrease good cholesterol, increasing the risk of cardiovascular disease, diabetes, and various cancers. They are a major cause of obesity, especially strong in inducing abdominal obesity. Vegetable oils break down into saturated fats at high temperatures, losing their original benefits. Therefore, we do not recommend consuming fried foods. For example, fresh corn is rich in nutrients; however, when it is processed into corn oil, it contains excessive amounts of polyunsaturated oils and fatty acids. Excessive consumption can lead to breast and prostate cancer. Therefore, it is better to eat fresh corn directly.
The main sources of fat are the oils and fats contained in the food itself and cooking oils. Edible oils contain approximately 100% fat, while animal-based foods and nuts are rich in fat. Nuts have the highest fat content; various meats are in the middle; rice, noodles, vegetables, and fruits contain very little. Among animal-based foods, meat is the richest source of fat, mostly saturated fatty acids. Generally, animal organs, except for the large intestine, have relatively low fat content but high protein content. Poultry generally has a low fat content, mostly below 10%. Fish generally has a fat content below 10%, mostly around 5%, and its fat is mostly unsaturated fatty acids. Eggs contain about 10% fat, with egg yolks containing about 30%, and whole eggs are mostly monounsaturated fatty acids. Besides animal-based foods, among plant-based foods, nuts have the highest fat content, reaching over 50%, but their fat composition is mainly linoleic acid, an important source of polyunsaturated fatty acids.
Among our daily foods, some are high in fat, while others are relatively low. High-fat foods fall into three categories: first, nuts, such as peanuts, sesame seeds, pistachios, walnuts, and pine nuts; second, animal products, such as fatty pork, pork skin, lard, butter, ghee, and vegetable oil; and third, fried foods, such as pasta, pastries, and cakes. Low-fat foods mainly include: fruits, such as apples, lemons, and oranges; vegetables, such as winter melon, cucumber, loofah, white radish, bitter melon, leeks, mung bean sprouts, and chili peppers; as well as seaweed, wood ear fungus, lotus leaf tea, and vinegar.
Excessive fat can lead to mobility issues, and high blood lipid levels are often a major contributing factor to hypertension and heart disease. Frenchman Chevreau first discovered that fat is composed of fatty acids and glycerol. Viewing fat as a mode of storing fatty acids, from a nutritional perspective, certain fatty acids are crucial for preventing birth defects in our brain, immune system, and even reproductive system. However, these fatty acids cannot be synthesized by the human body and must be obtained from the diet. Some studies suggest that consuming small amounts of these fatty acids, known as polyunsaturated fatty acids, can contribute to health and longevity.
**The Essential Fat Metabolism Process for Weight Loss**
Understanding the proper storage and elimination of fat is fundamental to health and weight loss. Only by understanding how fat is metabolized can we effectively use various weight loss methods to lose weight and develop a lean physique, ensuring that weight loss is not reversed. As the saying goes, "Know yourself and know your enemy, and you will never be defeated." For weight loss and body shaping, understanding the processes of fat synthesis, metabolism, and breakdown in the human body is essential.
As we've learned from the preceding reading, our bodies obtain fatty acids from two sources: synthesis by the body itself and dietary intake. Some unsaturated fatty acids cannot be synthesized by the body and must be obtained from food. There are two main types of these essential fatty acids. One is alpha-linolenic acid (ALA), a quorum-3 series fatty acid, which is abundant in oily plant foods such as flaxseed, perilla seeds, purple perilla seeds, hemp seeds, and walnuts, as well as in dark green plants like spirulina and deep-sea microalgae. It is only found in a very few animal foods, such as silkworm pupae and deep-sea fish. The other is linoleic acid (LAA), a quorum-6 series fatty acid, mainly found in soybean oil, corn oil, and sunflower oil.
Fat synthesis and metabolism mainly occur in the liver, adipose tissue, and small intestine. Among these three sites, the liver has the strongest synthetic capacity. Hepatocytes can synthesize fat but cannot store it. After synthesis in the liver, fat binds with carrier proteins, cholesterol, etc., to form very low-density lipoproteins (VLDL), which are then transported into the bloodstream to extrahepatic tissues for storage or utilization. If the triglycerides synthesized in the liver cannot be transported in time, fatty liver will develop. Adipocytes are the body's warehouse for fat synthesis and storage. The glycerol and fatty acids required for triglyceride synthesis are mainly provided by the metabolism of ingested glucose. From the process of fat synthesis and metabolism in the human body, it can be seen that the liver is key to fat reduction and protection. The basic processes and pathways of fat synthesis are: ① Monoglyceride pathway: This is the pathway for fat synthesis by small intestinal mucosal cells, where triglycerides are synthesized from monoglycerides and fatty acids. ② Diaglyceride pathway: The synthesis pathway of hepatocytes and adipocytes. Adipocytes lack glycerol kinase and therefore cannot utilize free glycerol, but can only utilize glycerol-3-phosphate provided by glucose metabolism.
Human fat metabolism ultimately involves the production of lipases, which biodegrade fat into metabolic waste products for excretion. Biological processes can directly synthesize lipases, while most chemically synthesized lipases cannot be directly absorbed by the body. Under the action of lipases, fats are hydrolyzed into glycerol and fatty acids. Glycerol undergoes phosphorylation and dehydrogenation to transform into dihydroxyacetone phosphate, which is then incorporated into carbohydrate metabolism. The digestibility of fats is related to their melting point. Generally, the higher the content of unsaturated fatty acids, the lower the melting point, and the easier it is to digest. Animal fats, such as tallow and mutton fat, contain a high amount of saturated fatty acids, with melting points above 40°C, resulting in relatively low digestibility, around 80% to 90%. Animal fats contain almost no vitamins, but animal livers are rich in vitamins A and D, as are the fats from milk and eggs. Raw vegetable oils contain a high proportion of unsaturated fats, and their digestibility can generally reach 100%. Vegetable oils are high in linoleic acid and linolenic acid, contain no cholesterol, and are rich in vitamin E, an essential element for maintaining human health, making them more nutritious than animal fats. In normal individuals, the fat content of vegetable oil excreted in feces accounts for 10% to 15% of the dry stool volume daily. This includes 5% to 15% conjugated fatty acids, 5% to 13% free fatty acids, and 1% to 5% neutral fats. However, the fat content in an infant's stool is about 50% higher than that in an adult's stool, and the fat content in a toddler's stool is about 30% higher than that in an adult's stool, with neutral fats being the main component.