In-depth analysis of fats (Part 1): Classification, fatty acids, and nutritional value

Fats are a general term encompassing oils, fats, and lipids, composed of carbon, hydrogen, and oxygen. They are commonly known as oils and fats. The fats in food are mainly oils and fats; generally, those that are liquid at room temperature are called oils, while those that are solid at room temperature are called fats. We can consider fat as a form of fatty acid storage in the body; therefore, the properties and characteristics of fat mainly depend on the fatty acids. Different foods contain different types and amounts of fatty acids. There are more than 40 kinds of fatty acids in nature, which can form various fatty acid triglycerides. Fatty acids are classified into saturated fatty acids, monounsaturated fatty acids, and polyunsaturated fatty acids. Fats are soluble in most organic solvents but insoluble in water. Fats have two sources: animal and plant. Animal fats mainly include lard, tallow, mutton fat, fish oil, bone marrow, fatty meat, and cod liver oil; animal milk fats mainly include butter. Plant fats are mainly extracted from the fruits of plants, such as sesame seeds, sunflower seeds, tea seeds, walnuts, pine nuts, and soybeans. It can be seen that fat is also a major component of edible oils. Fats are triglycerides synthesized from glycerol and three fatty acid molecules. There are four main types: neutral fats (triglycerides), which are the main components of lard, peanut oil, soybean oil, rapeseed oil, and sesame oil; lipids, including phospholipids, lecithin, cephalin, and inositol phospholipids; glycolipids, cerebrosides, and gangliosides; and lipoproteins, which account for 98% of dietary fat and more than 28% of body fat. All cells contain phospholipids, which are structural components of cell membranes and blood, and are particularly abundant in the brain, nerves, and liver. Lecithin is one of the most abundant phospholipids in the diet and in the body. The four types of lipoproteins are the main transport vehicles for lipids in the blood.

From the perspective of chemical bond arrangement, fatty acids without double bonds are called saturated fatty acids, which are one of the basic components of lipids and their main function is to provide energy for the human body. All animal oils, except fish oil, primarily contain saturated fatty acids. A few plant oils, such as coconut oil, cocoa butter, and palm oil, also contain a significant amount of these fatty acids. Saturated fatty acids can increase cholesterol and triglycerides in the body. Insufficient intake of saturated fat can make blood vessels brittle, increasing the risk of cerebral hemorrhage, anemia, tuberculosis, and neurological disorders. Fatty acids other than saturated fatty acids are unsaturated fatty acids. Unsaturated fatty acids are a type of fatty acid that constitutes body fat and are essential for the human body. Based on the number of double bonds, unsaturated fatty acids are divided into monounsaturated fatty acids and polyunsaturated fatty acids. Among dietary fats, monounsaturated fatty acids include oleic acid, while polyunsaturated fatty acids include linoleic acid, linolenic acid, and arachidonic acid. The human body cannot synthesize linoleic acid and linolenic acid and must obtain them from the diet. Based on the position and function of the double bonds, polyunsaturated fatty acids are further divided into ω-6 series and ω-3 series. Linoleic acid and arachidonic acid belong to the omega-6 series, while linolenic acid, DHA (docosahexaenoic acid, commonly known as "brain gold"), and EPA (eicosapentaenoic acid) belong to the omega-3 series. Unlike saturated fats, many unsaturated fats are liquid at room temperature and remain liquid even when refrigerated or frozen. Monounsaturated fatty acids are mainly oleic acid, and oils rich in monounsaturated fatty acids include olive oil, canola oil, and peanut oil. They have the effect of lowering LDL cholesterol and increasing the proportion of HDL cholesterol, thus helping to prevent arteriosclerosis. While polyunsaturated fatty acids also lower cholesterol, they lower both LDL and HDL cholesterol indiscriminately, and are less stable, making them unsuitable for heating. During heating, they easily oxidize, forming free radicals that accelerate cell aging and contribute to cancer. Linoleic acid is an essential fatty acid for the human body, helping to prevent high cholesterol, improve hypertension, prevent myocardial infarction, and prevent cholesterol-related gallstones and arteriosclerosis. However, excessive intake of linoleic acid can cause allergies, premature aging, and other symptoms. It can also suppress immunity, weaken the body's resistance, and, in large quantities, even trigger cancer. Alpha-linolenic acid (ALA) is also an essential fatty acid. Its main functions are the synthesis of EPA and DHA, dissolving blood clots, improving blood flow, lowering blood pressure, inhibiting cancer development, and eliminating symptoms of excessive linoleic acid intake. It also helps improve allergic dermatitis, hay fever, and bronchial asthma. However, while excessive ALA intake has no specific side effects, as a fat component, it can lead to excess energy. Arachidonic acid (RAA) is a semi-essential fatty acid, synthesized only in small amounts in the human body. It plays a role in regulating the immune system, improving and preventing various systemic diseases, protecting liver cells, promoting digestive motility, and promoting normal fetal and infant development. When consuming RAA, it is crucial to avoid excessive intake. For example, while RAA can lower blood pressure, excessive intake can cause elevated blood pressure; it can inhibit blood clotting, but excessive intake can promote blood clotting; and it can improve allergy symptoms, but excessive intake can trigger allergies.

Fat is a component and energy storage substance of the human body, mainly found in subcutaneous tissue. Lipids in the human body are divided into two parts: fats and lipids. Fats have already been described, while lipids refer to cholesterol, phospholipids, lecithin, etc. Lipids are important components of living organisms. For example, phospholipids are an important part of biological membranes; oils and fats are the storage and transport form of fuel needed for metabolism; lipids provide animals with essential fatty acids and fat-soluble vitamins. Certain terpenes and steroids, such as vitamins A, D, E, and K, bile acids, and steroid hormones, have nutritional, metabolic, and regulatory functions. Lipids on the surface of the body prevent mechanical damage and heat loss, protecting body tissues and organs. So how much fat should be consumed daily? Dietary habits vary in different regions, and economic development levels also differ, resulting in significant differences in actual fat intake. The Chinese Nutrition Society recommends that dietary fat intake should not exceed 30% of total daily energy, with the ratio of saturated, monounsaturated, and polyunsaturated fatty acids being 1:1:1. Linoleic acid provides 1% to 2% of the total energy, which can meet the body's need for essential fatty acids.

In nutrition, the nutritional value of a fat is evaluated based on the following three indicators: First, digestibility. The digestibility of a fat is related to its melting point; the more unsaturated fatty acids it contains, the lower its melting point, and the easier it is to digest. Therefore, vegetable oils generally have a digestibility of 100%. Animal fats, such as beef tallow and mutton tallow, contain more saturated fatty acids, and their melting points are all above 40℃, resulting in a lower digestibility of 80% to 90%. Second, essential fatty acid content. Vegetable oils have a relatively high content of linoleic acid and alpha-linolenic acid, making them more nutritious than animal fats. Third, fat-soluble vitamin content. Animal stored fat contains almost no vitamins, but liver is rich in vitamins A and D, milk and egg fats are also rich in vitamins A and D, and vegetable oils are rich in vitamin E. These fat-soluble vitamins are essential for maintaining human health.