The Roles of Vitamin C in Skin Health
Juliet M. Pullar, Anitra C. Carr and Margreet C. M. Vissers *
Abstract: The primary function of the skin is to act as a barrier against insults from the environment, and its unique structure reﬂects this. The skin is composed of two layers: the epidermal outer layer is highly cellular and provides the barrier function, and the inner dermal layer ensures strength and elasticity and gives nutritional support to the epidermis. Normal skin contains high concentrations of vitamin C, which supports important and well-known functions, stimulating collagen synthesis and assisting in antioxidant protection against UV-induced photodamage. This knowledge is often used as a rationale for the addition of vitamin C to topical applications, but the efﬁcacy of such treatment, as opposed to optimizing dietary vitamin C intake, is poorly understood. This review discusses the potential roles for vitamin C in skin health and summarises the in vitro and in vivo research to date. We compare the efﬁcacy of nutritional intake of vitamin C versus topical application, identify the areas where lack of evidence limits our understanding of the potential beneﬁts of vitamin C on skin health, and suggest which skin properties are most likely to beneﬁt from improved nutritional vitamin C intake.
Keywords: ascorbate; dermis; epidermis; skin barrier function; vitamin C status; skin aging; wound healing; collagen; UV protection
The skin is a multi-functional organ, the largest in the body, and its appearance generally reﬂects the health and efﬁcacy of its underlying structures. It has many functions, but its fundamental role is to provide a protective interface between the external environment and an individual’s tissues, providing shielding from mechanical and chemical threats, pathogens, ultraviolet radiation and even dehydration (functions reviewed in). Being in constant contact with the external environment, the skin is subject to more insults than most of our other organs, and is where the ﬁrst visible signs of aging occur. The skin is composed of two main layers with quite different underlying structures—the outermost epidermis and the deeper dermis. The epidermis fulﬁls most of the barrier functions of the skin and is predominantly made up of cells, mostly keratinocytes. The keratinocytes are arranged in layers throughout the epidermis; as these cells divide and proliferate away from the basal layer, which is closest to the dermis, they begin to differentiate. This process is called keratinization, and involves the production of specialized structural proteins, secretion of lipids, and the formation of a cellular envelope of cross-linked proteins. During differentiation, virtually all of the subcellular organelles disappear, including the nucleus. The cytoplasm is also removed, although there is evidence that some enzymes remain. Thus, the uppermost layer of the epidermis that interacts with the outside environment is composed of ﬂattened metabolically ‘dead’ cells (the terminally differentiated keratinocytes). These cells are sealed together with lipid-rich domains, forming a water-impermeable barrier. This layer is known as the stratum corneum and fulﬁls the primary barrier function of the epidermis, although the lower epidermal layers also contribute.
In contrast, the dermal skin layer provides strength and elasticity, and includes the vascular, lymphatic and neuronal systems. It is relatively acellular and is primarily made up of complex extracellular matrix proteins, being particularly rich in collagen fibres, which make up ~75% of the dermis dry weight. The major cell type present in the dermis is fibroblasts, which are heavily involved in the synthesis of many of the extracellular matrix components. Blood vessels that supply nutrients to both skin layers are also present in the dermis. Between the two main layers is the dermal–epidermal junction, a specialized basement membrane structure that fixes the epidermis to the dermis below. The major cell type present in the dermis is fibroblasts, which are heavily involved in the synthesis of many of the extracellular matrix components. Blood vessels that supply nutrients to both skin layers are also present in the dermis. Between the two main layers is the dermal–epidermal junction, a specialized basement membrane structure that fixes the epidermis to the dermis below.
Role of Nutrition in Skin Health
It is accepted that nutritional status with respect to both macronutrients and micronutrients is important for skin health and appearance. Evidence of this is provided by the many vitamin deficiency diseases that result in significant disorders of the skin. Dermatological signs of B vitamin deficiency, for example, include a patchy red rash, seborrhoeic dermatitis and fungal skin and nail infections. The vitamin C deficiency disease scurvy is characterized by skin fragility, bleeding gums and corkscrew hairs as well as impaired wound healing.
Nutritional status is vital for maintaining normal functioning of the skin during collagen synthesis and keratinocyte differentiation. Additionally, many of the components of our antioxidant defenses such as vitamins C and E and selenium are obtained from the diet, and these are likely to be important for protection against UV-induced damage.
Nutrition Issues Specific to the Skin
The epidermis is a challenged environment for nutrient delivery, as it lacks the blood vessels that normally deliver nutrients to cells. Delivery of nutrients is dependent on diffusion from the vascularized dermis, and this may be particularly limited for the outermost layers of the epidermis. Delivery is further compounded by the chemical nature of these outer epidermal layers in which there is little movement of extracellular fluid between cells due to the complex lipid/protein crosslink structure forming the skin barrier. All of this makes it likely that dietary nutrients are not easily able to reach the cells in the outermost layers of the epidermis, and these cells receive little nutrient support.
The skin can be targeted for nutrient delivery through topical application. However, in this case the delivery vehicle is influential, as the stratum corneum functions as an effective aqueous barrier and prevents the passage of many substances. Although some uncharged and lipid-soluble molecules can pass through the surface layer, it is unlikely that nutrients delivered via topical application would easily penetrate into the lower layers of the dermis. The dermal layer functions are therefore best supported by nutrients delivered through the bloodstream.
Normal skin contains high concentrations of Vitamin C, with levels comparable and well above plasma concentrations, suggesting active accumulation from the circulation. Most of the vitamin C in the skin appears to be in intracellular compartments, with concentrations likely to be in the millimolar range. It is transported into cells from the blood vessels present in the dermal layer. Skin vitamin C levels have not often been reported and there is considerable variation in the published levels, with a 10-fold range across a number of independent studies. Levels are similar to that found in numerous other body organs. The variation in reported levels most likely reflects the difficulty in handling skin tissue, which is very resilient to degradation and solubilisation, but may also be due to the location of the skin sample and the age of the donor.
Vitamin C Deficiency
One of the most compelling arguments for a vital role for vitamin C in skin health is the association between vitamin C deficiency and the loss of a number of important skin functions. In particular, poor wound healing (associated with collagen formation), thickening of the stratum corneum and subcutaneous bleeding (due to fragility and loss of connective tissue morphology) are extreme and rapid in onset in vitamin-C-deficient individuals. It is thought that similar processes occur when body stores are below optimal, although to a lesser extent.
Functions of Vitamin C in the Skin
- Promotion of Collagen Formation
- Scavenge Free Radicals and Dispose of Toxic Oxidants
- Inhibition of Melanogenesis
- Interaction with Cell Signaling Pathways
- Modulation of Epigenetic Pathways