Skin And Nutrition
General nutrition requirements
The primary fuel for skin cells is glucose; glucose oxidation rates in skin are similar to those observed in resting skeletal muscle. Glucose also provides carbohydrate backbones for modification of proteins (glycoproteins) and lipids (glycoplipids) that comprise the extracellular environment of the epidermis (35). Aberrant glucose handling drastically affects skin structure and appearance (36).
Specialized lipids required for the development of the stratum corneum, such as sterols and ceramides, are synthesized in the epidermis from amino acids, carbohydrates, and phospholipids. However, differentiating keratinocytes also utilize fatty acids from circulating stores or dermal fat layers for energy. The extruded fatty acids that make up the mortar of the stratum corneum can absorb lipid-soluble materials placed on the external surface of this outermost skin layer. This is especially the case for sebum, a waxy substance secreted from the sebaceous glands that are attached to hair follicles, but it is also true for topically applied materials (35).
Amino acids are essential for the production of both dermal and epidermal structures, producing the extracellular matrix proteins and enzymes needed for the synthesis of the epidermal barrier. The amino acid requirements of skin are poorly understood (37).
Much of the role of nutrition in skin health focuses on the effects of deficiency, since the structural components of the skin are supported by a variety of nutritive factors, such as small peptides, minerals, and vitamins, which serve as enzyme cofactors, activators, or inhibitors (4). The skin is also constantly exposed to high concentrations of oxygen, UV light, and oxidizing chemicals, highlighting a role for antioxidant vitamins in skin function (38, 39). Further, nutritional support of the skin is important for inflammatory response during wound healing (32).
Oral nutrition vs. topical supplementation
The architecture of skin deserves special consideration in regard to nutrition. The epidermal layers of the skin do not contain blood vessels that supply the cells with nutrients; blood vessels are found only in the dermis. Additionally, as the epidermis develops, its unique protein and lipid structure (the aforementioned “bricks and mortar” model) prevents the circulation of extracellular fluids (7). Therefore, the outer layers of the epidermis are provided with less nutritional support than the underlying cells. Dietary interventions to affect skin health may be limited in effect and slow to occur.
Unique among organs, the skin may be directly affected by the use of topical applications. The stratum corneum prevents the passage of many different types of molecules, but some compounds pass through to the underlying layers. In general, uncharged or lipid-soluble molecules pass through the epidermis and may also penetrate the dermis. Concentrations of nutrients in the skin may be comparable to that achieved through oral ingestion. Yet, topical application may be a more efficient, targeted method for supplying nutrients to the skin, especially to the epidermis (12, 39).
Micronutrients important for skin health
Changes in the appearance of skin were associated with poor nutrition long before any biochemical relationships were discovered. Among the first to make these associations was the Scottish physician James Lind, who described the bleeding gums, skin discoloration, abnormal hair growth, and poor wound healing associated with the vitamin C deficiency disease known as scurvy (see the article on Vitamin C and Skin Health). Other investigators would soon find skin abnormalities in vitamin deficiency diseases (pellagra, ariboflavinosis, etc.) that could be corrected with an appropriate diet. Thus, many micronutrients were initially identified for their impact on skin health.
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