#1-1. Moisturizer: Theoretical Concept, Actions and Effects

A moisturizer is a component contained in the cosmetic cream or lotion that provides hydration to the skin. In the past, the key purpose of a moisturizer was to provide hydration but now it is expected to provide both hydration and a protective barrier to the skin. In this two-article series, Dr. Ahn Sungku of Dermatology, Wonju Severance Christian Hospital, discusses the science of moisturizers. He will cover basic theoretical concepts as well as ingredients and types of products available. We hope the readers interested in moisturizers will get helpful information from these articles.

Ahn Sungku, M.D., Ph.D.

Professor of Dermatology, Wonju Severance Christian Hospital

e-mail: ahnsk@yonsei.ac.kr

Basic concepts of a moisturizer

A moisturizer is a component contained in a cosmetic cream or lotion that provides hydration to the skin. In the past, the key purpose of a moisturizer was to provide hydration but now it is expected to provide both hydration and a protective barrier to the skin.

Moisturizers improve the skin’s dryness. Although the etiological factors of dry skin are not clearly understood, damage of the stratum corneum, the protective barrier of the skin, is thought to be a key cause of skin’s dryness. The water content from the stratum basale to stratum spinosum is similar to the water content of the cells. However, the water content drops drastically between the stratum granulosum and stratum corneum, which means the stratum corneum is left with very low water content. In normal skin, the stratum corneum has about 12-15% water content. A dry skin can be defined as this water content being less than 10%.

Metabolites of epidermal differentiation such as natural moisturizing factor(NMF), sebum, and intercellular lipids, and keratin turnover play a key role in maintaining the right level of hydration in the stratum corneum. Imbalance between these factors can lead to xeroderma.

First, a problem in the metabolism of proteins expressed during normal epidermal differentiation can disrupt the hydration of stratum corneum, leading to dry skin. During epidermal differentiation, profilaggrin is released to be converted to filaggrin. It is proteolyzed in the last stage of differentiation to produce hydrophilic amino acids or endogenous NMF(lactate, pyrrolidone carboxylic acid, urea, sugar, etc.). NMF maintains skin’s elasticity in dry surroundings and protects the skin from external irritants. It affects the flexibility of the stratum corneum. Atopic dermatitis and dry skin have reduced NMF compared to normal skin. The correlation between water content of the stratum corneum and NMF has been established.

Second, impaired biosynthesis, structural degradation and abnormal distribution of epidermal lipids cause xeroderma. An electron microscope study reported that degradation or destruction of the lamella structure of stratum corneum lipids, morphological differences in the lipid membrane arrangement(hexagonal vs. orthorhombic) contribute toward water loss in the stratum corneum. Dry skin lesions of atopic dermatitis are found to have lower content of ceramide, a major component of epidermal lipids. Animals on long-term diet lacking essential fatty acids have damaged skin barrier. Therefore, imbalance of epidermal proteins and lipids negatively affect production of NMF and is the primary cause of skin barrier dysfuntion as water loss occurs through degraded lipid membrane.

Third, abnormal water loss in the stratum corneum disrupts the moisture balance of the skin. Normal water content of the skin enables elasticity of the stratum corneum and healthy keratin turnover. Here, proteases mediated by water and pH play an important role. The right pH level for activation of proteases is maintained by appropriate water content. The pH level of healthy stratum corneum is about 5.0~5.5. However, the pH level drastically drops from the normal level(7.2~7.4) through the stratum spinosum, stratum granulosum and stratum corneum. The lower pH of the stratum corneum inhibits bacterial proliferation on the skin surface and maintains the activity of proteases. In normal skin, appropriate water content plays an important role in protein-lipid interaction that maintains the skin barrier’s homeostasis. In this light, a moisturizer should go beyond simple replenishment of water to maintaining or fortifying the skin barrier.