Let’s start with the physiological functions of the skin. The skin is divided into epidermis, dermis and subcutaneous fat, and the outermost layer of the epidermis is the stratum corneum, which is composed of inactive keratinocytes. The keratinocytes are 30 microns wide, 0.8 microns thick, and hexagonal in shape, and they are closely connected and interlaced in layers, with about 10-20 layers forming the stratum corneum, which is about 10-15 microns thick. The interstitial space of keratinocytes is about 0.02-0.2 microns, which is filled with lamellar structured lipid material. The cell membrane of keratinocytes in the stratum corneum, the amount of cell contents and the amount of water bound to the intercellular matrix determine the softness of the skin. The following three substances play an important role in maintaining skin elasticity: first, water; second, water-soluble substances in the skin that retain water (the key players are lactate and pyrrolidone hydroxy acid-PCA), which are called natural moisturizing factors, and which contain hydroxyl groups in their molecular structure that hold water molecules like hands, thus keeping them in the stratum corneum. Thirdly, lipids in the skin, they cover the surface of the skin in a thin layer to prevent water evaporation and prevent water molecules from escaping to the surrounding environment, and also have an impact on skin moisturization by affecting the adhesion of stratum corneum cells. The amount of water, natural moisturizing factors and lipids in the skin and the balanced relationship between them have a great impact on the skin, which is often referred to as the moisturizing balance. Under normal circumstances, due to the water absorption and barrier function of the stratum corneum, the covering effect of oil secreted by sweat glands and sebaceous glands, and the effect of natural moisturizing factors contained in the stratum corneum, the moisture content in the stratum corneum can be maintained at 10-20% and is not easily dissipated. When water-moisturizing factor-lipid is in balance, the skin is smooth, tender and elastic. When the moisture content of the stratum corneum is below 10%, the skin will be dry, taut, rough and flaky. So what are the causes of skin moisture loss, resulting in dry skin and loss of luster? Generally speaking, the internal and external conditions that cause the natural moisturizing structure of the skin to lose balance are: 1, with ageing, the skin ages, its moisturizing effect and barrier function gradually weaken, and the content of natural moisturizing factors decreases; 2, dry and cold climate with low humidity, such as autumn and winter; 3, certain skin lesions, such as psoriasis, ichthyosis, etc.; 4, environmental and chemical factors, such as frequent use of alkaline relatively large detergents to wash the face, wash the sebum clean as well, as well as long hours of air conditioning environment; 5, diet and sleep habits, such as partial diet, little water, insomnia, etc.; 6, dry skin. As the skin is always in direct contact with the external environment, if not protected, more or less dehydrated phenomenon, directly affecting the appearance of the skin. In order to maintain the elasticity and moisture of the skin, we should: 1. Replenish water: drink enough water every day, so that it penetrates between the tissue cells and maintain the acid-base balance of the body. Ensure the normal operation of the body’s metabolism, and effectively discharge human waste out of the body, so as to maintain the cleanliness and vitality of the skin. Drink water can drink plain water, fruit juice, mineral water and so on. Among them, plain water is the best “natural beverage”, should be preferred. Chinese people like to drink green tea has the effect of clearing heat and fire, often drinking, can prevent certain skin diseases, such as acne, acne and other occurrences. Generally speaking, drink 6-8 cups of water a day, that can meet the internal needs of the skin. 2, balanced nutrition: malnutrition will make the person’s skin dry, coarse, wrinkled, hard. If too much animal fat is consumed, the skin will be oily or flaky, which makes it easy for acne and other skin diseases to occur. Therefore, usually should pay attention to the diversity of diet, nutrition, should eat more food rich in collagen protein, collagen is the main component of the skin, accounting for more than 71% of the protein content of skin cells, collagen makes the cells become plump, so that the skin is full, to keep the skin elastic and moist, collagen has a very important role. The main foods that contain collagen are meat skin, pig’s feet, cow’s feet, fresh fish, etc. Eat more vitamin-rich foods, such as animal liver, milk, eggs, red and white radish, apples, bananas and pears. Eat less animal food containing high levels of saturated fatty acids. 3, pay attention to skin cleansing: air pollutants are very easy to block the pores, thus causing skin diseases. In addition, a large number of keratin layer shedding, not timely cleaning of the skin will also cause severe dryness and roughness. Therefore, regardless of makeup or not, carefully removing dirt with cleanser every morning and evening should be a must-do job. The skin should be cleansed with a strong bactericidal power and good cleansing effect face wash (weak acidic products). 4, bedtime skin care: bedtime skin care is very important, because the number of facial cell division is much higher than during the day (more than 10 times), the new cells need to be prepared for care. You should always use a moisturizing lotion on your face and wipe your forehead, nose, chin and other areas with high sebum secretion with lotion. In addition to a normal healthy diet to ensure protein and vitamin supplementation, as well as avoiding the influence of external factors and actively treating skin disorders, you should also use appropriate moisturizing cosmetics as needed. Moisturizing cosmetics are cosmetics that contain moisturizing ingredients to maintain a certain water content in the skin’s stratum corneum to restore the skin’s luster and elasticity. Moisturizing cosmetics are mainly available according to their characteristics: one is to use water-retaining substances that can strongly combine with water on the skin surface to moisturize the stratum corneum, called humectants, commonly used moisturizing ingredients are: glycerin, urea, amino acids, lactic acid, etc.; the other is to use water-insoluble substances to form a lubricating film on the skin surface to play a closed role and prevent water loss, so that the stratum corneum can maintain a certain amount of water, called emollient or conditioning agent, commonly used are: vantuoline and a variety of fats, oils, waxes, including lanolin, fatty acids, fatty alcohols, wax esters, etc., the disadvantage is that it is too greasy and easy to dirty; the last is a deep moisturizer, its mechanism of action is through penetration into the skin epidermis or even dermis, after a series of biological effects, and ultimately the ability to absorb water and barrier function of the stratum corneum to maintain and strengthen the role of maintaining Skin stratum corneum water content, its common moisturizing ingredients are: unsaturated linoleic acid, linolenic acid, arachidonic acid, various vitamins, etc.