Vitamin A

Vitamin A (Fig. 2) and its derivatives belong to a large class of structurally related compounds, the retinoids. The term vitamin A is generically used for all derivatives of 0-ionone that possess the biological activity of all-trans retinol or are closely related to it. The biological activities of the vitamin A derivatives are expressed in IU (international

units.) One IU corresponds to 0.3 |g retinol, 0.34 |g vitamin A acetate, and 0.55 |g vitamin A palmitate.

Vitamin A is best known for its involvement in maintaining normal vision. It exerts, however, a number of other functions in the human organism, of which its activity in the epidermis is of particular interest for cosmetics.

The architecture of the human epidermis is a complex stratified system, its renewal a complex process. Epidermal keratinocytes proliferate and differentiate in a multilayered pattern. These processes are balanced so that new basal cells are formed as the totally cornified cells are shed from the surface of the skin. Proliferation and keratinization of keratinocytes are the two key elements for the build-up of a healthy epidermis. In both processes, vitamin A plays the role of a regulator.

On cell proliferation, vitamin A has a stimulating effect, as has been shown in various studies [16-18]. As little as 10 |g vitamin A acetate suspended in 0.2 mL water applied to normal rat skin led to a clear increase in mitotic activity after only 4 hours. Much more pronounced and longer lasting was the effect with 100 |g vitamin A acetate. However, 24 hours after treatment, the mitotic index had returned to original levels with both concentrations [17]. As can be seen from this study, the effect of vitamin A is dose dependent and disappears after a certain time with decreasing concentration in the tissue. An increase in mitotic activity is the first step in an increase of the number of new keratino-cytes formed [19], which results in a thickening of the epidermis [20-23].

In the process of aging, many aspects of the skin structure are altered because of a decreased metabolic activity of the human organism. A thinning of the epidermis is one of the characteristics of aging skin. The skin thereby loses part of its barrier function, and as a consequence of reduced water retention capacity it is often dry, scaly, or even cracks. Vitamin A can counteract this development by stimulating the cell-renewal process.

The effect on the keratinization process was investigated by Fuchs and Green [24]. Removal of vitamin A from the culture fluid of human keratinocyte cell cultures resulted in a reduced cell motility, an increased adhesiveness of the cells, and a prevention of pattern formation. They conclude that the nature of the keratins synthesized by the tissues is regulated by the concentration of vitamin A. Another symptom of skin aging is a decrease in collagen in the connective tissue. Skin collagen decreases linearly by about 1% per year throughout adult life [25]. Topical administration of vitamin A has shown significant dose-related changes in collagen content of the dermis. 0.1% vitamin A palmitate applied to skin of hairless mice for 14 days increased the collagen content by 88%, 0.5% vitamin A palmitate by 101% [20].

Vitamin A not only improves the barrier function of the skin but also its appearance and elasticity. Application of a lotion with vitamin A palmitate to the temples of a group of 40- to 60-year-old volunteers has shown an increase in elasticity by 14% after 2 weeks and by over 22% after 6 weeks [26].

There is evidence that UV light strongly affects vitamin A concentration in epidermis and dermis as was shown in animals and humans [27,28]. Particularly low were the levels when test animals were exposed to UVA near the absorption maximum of vitamin A. The regeneration of normal levels in the depleted tissue is very slow and took more than 1 week in rabbit ear skin [28].

Cluver and Politzer measured the vitamin A concentration in blood serum of humans after 1 hour of exposure to the sun. Depletion was observed immediately after exposure, which lasted at least a further 21/2 hours [29]. It can be assumed that, under similar conditions, a depletion also takes place in the skin. The low blood levels could also be an

Figure 2 Structural formula of vitamin A alcohol.

Figure 2 Structural formula of vitamin A alcohol.

explanation for the slow restoration of vitamin A in the skin. It cannot be excluded that low vitamin A levels in the skin after regular and excessive sun exposure are implicated in the typical changes seen in photodamaged skin, such as the thickened horny layer and the relatively thin rest of the epidermis. A common practice in treatment of photodamaged skin is the use of vitamin A acid. Although it is not proven, it can be hypothesized that retinoids could be involved in the process of photoaging. In most countries, however, vitamin A acid is classified as a drug and cannot be used in cosmetic products. Whether vitamin A esters have a similar effect to vitamin A acid, and whether they could be used not in the cure but in the prevention of photoaging, is presently under investigation. Some first results are available and show promising results.

Although vitamin A was one of the first vitamins discovered, the molecular mechanism of its activity is still largely unknown. Many attempts have been made to define in biochemical terms the manner in which it induces the differentiation of cells. Uncertainties still exist, but one pathway increasingly seems to explain most of the effects of various retinoids on different cell types. This pathway includes an oxidation of retinol (vitamin A alcohol) to retinal (vitamin A aldehyde), and subsequently a further bioconversion in a controlled mechanism to retinoic acid [30].

In cosmetics, vitamin A is used mainly in the ester forms: vitamin A palmitate and vitamin A acetate, as well as retinol. None of these forms are very stable when exposed to light or warmth. Special attention has to be paid, therefore, to the stabilization of vitamin A-containing cosmetic products and their handling during the manufacturing process. This is particularly true for retinol.

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