The sebum film of the skin is mainly composed of lactic acid, amino acids, urea, neutral fatty acids and fats. The sebum film composed of these substances can protect the loss of skin moisture and nutrients. If the skin is dehydrated, it will cause dryness and itching on the skin surface. To keep the skin hydrated, it is necessary to replenish the moisturizing ingredients of the skin's sebum film. Among the moisturizing ingredients, the most commonly used moisturizing agents, no matter what skin care products are used, are more or less: hyaluronic acid, collagen, amino acids, glycerin, urea, these five golden moisturizing ingredients.

Hyaluronic acid
Moisturizing effect: ★★★★ ★
The scientific name is hyaluronic acid, which is a natural moisture regulator of the stratum corneum. It has a very strong water absorption capacity and can absorb water several times higher than itself, filling up the keratin water in a short time. Hyaluronic acid originally exists in human skin, and proper supplementation of hyaluronic acid in human skin can not only help the skin absorb a lot of water from the skin surface, but also enhance the long-term water retention capacity of the skin. It's by far the most effective and expensive moisturizer, widely used in luxury moisturizers, and has also started appearing on affordable moisturizer lists in the past two years.
Collagen
Moisturizing: ★★★★
It is ubiquitous in human skin, tissue, blood vessels and bones, and there are as many as 21 known collagens. The macromolecular collagen is not easily absorbed by the human body, and should be hydrolyzed into small molecules so that it has an amino acid structure similar to keratin to enhance its skin-friendly properties. Amino acids can maintain the integrity of the stratum corneum moisture and fiber structure, improve the living environment of skin cells, and promote the metabolism of skin tissue, thereby achieving the effect of moisturizing the skin and delaying aging.
Glycerin
Moisturizing: ★★★

Glycerin is not a special ingredient. The main role of glycerin in skin care products is to moisturize. It was used 50 years ago. Compared with well-known moisturizing ingredients such as hyaluronic acid and collagen, glycerin can only be regarded as a natural supporting role. .The price is also very cheap, almost all skin care products have it. Glycerin can absorb water vapor in the air. Of course, too high a concentration of glycerin can be irritating, and since it is very hygroscopic, it may directly absorb moisture from the skin when applied directly to the skin. Therefore, we can only use diluted glycerin.

Urea
Moisturizing effect: ★★★★
Urea is also known as carbamide, carbonamide, urea. It is an organic compound composed of carbon, nitrogen, oxygen and hydrogen, also known as urea (sound urine). For the skin, urea has the effect of moisturizing and softening the stratum corneum, so it can also prevent the stratum corneum from clogging pores, thereby improving acne problems. It is used to add moisturizing ingredients to masks, body lotions, face creams, hand creams and other products. The addition ratio is 3-5%.
Amino acids
Moisturizing effect: ★★★★
The protein with the smallest molecular structure is more easily absorbed by the skin, which can strengthen the keratin function and make the skin itself have moisturizing ability. When the moisture content in the stratum corneum decreases, the amino acid content also decreases, so the addition of amino acids is necessary. While it doesn't have the high water absorption of other moisturizers, it has a conditioning effect on moisture.