الفهرس 
TOPICAL VITAMINSOnly 14 pages are availabe for public view
 |  | from | 115 |  |  |
| | | from | 115 | | |
Abstract
Since human life span has expanded, and people more and more desire a youthful and attractive appearance, a tremendous public and scientific interest has been generated on antiaging products.
Aging of the skin can be the result of a normal process that involves a lower rate of metabolic activities and is called chronologic aging. Chronologically aged skin remains smooth and demonstrates only subtle changes, such as slight atrophy, loss of elasticity, fine wrinkling, and deepening of expression wrinkles. Those changes are also evident histopathologically. The epidermis appears atrophic and flattened. In the dermis fibroblasts, inflammatory cells and the microvasculature are significantly reduced and the collagen bundles become thick and disorientated.
On the contrary, photoaging is the consequence of chronic sun exposure and manifests clinically with fine and coarse wrinkling, roughness, dryness, laxity, shallowness, pigmentary mottling, telangiectasia, and in some cases with preneoplastic and neoplastic changes. Photoaged skin is typified histopathologically by acanthosis, loss of epidermal polarity, a basket weave appearance of the epidermis, keratinocyte atypia, irregularly dispersed melanocytes, reduction and alteration in collagen, twisted and dilated microvasculature, and most of all by deposition of abnormal elastoid material in the upper dermis (elastosis).
There is increasing evidence that reactive oxygen species play a pivotal role in the process of ageing. The skin, as the outermost barrier of the body, is exposed to various exogenous sources of oxidative stress, in particular UV-irradiation. These are believed to be responsible for the extrinsic type of skin ageing, termed photo-ageing. It therefore seems reasonable to try to increase levels of protective low molecular weight antioxidants through a diet rich in fruits and vegetables or by direct topical application.
Various procedures have been used for skin rejuvenation including dermabrasion, chemical peels, ablative and non ablative laser resurfacing, intensified pulsed light, photodynamic therapy, Radio frequency devices (Thermage®), botulinium toxin (Botox) and fillers.
While all vitamins are essential for our survival as a part of food intake, only a few have been shown to reduce or retard the signs of skin aging when applied to the skin. This includes fat soluble vitamins (vitamin A and vitamin E), water soluble vitamins (vitamin C and nicotinamide which is a form of B3), and vitamin-like conditionally-essentially nutrients (coenzyme Q10 and lipoic acid), showing promise in anti-aging skin care.
Retinoids are natural and synthetic vitamin A derivatives. They are lipophilic molecules and penetrate the epidermis easily. Their biologically active forms can modulate gene expression by binding to nuclear receptors and then to specific DNA sequences. Because of their ability to modulate genes involved in cellular differentiation and proliferation, they appear as good candidates to treat and prevent photoaging. Hyaluronate and collagen, two major constituents of the dermis, are progressively decreased and altered during aging. Various retinoids were shown to increase their synthesis and concentration in the skin and reduce their rate of degradation.
Furthermore, retinoids share a common chemical structure containing several conjugated double bonds that enable them to trap free radicals and absorb UV radiations from the sun, thereby protecting cellular targets such as DNA, lipid membranes, or proteins by preventing direct photochemical damage or UV-induced oxidative stress. Therefore, retinoids may be beneficial in treating skin aging and photoaging because of their biologic, chemical, and physical properties, which act at several levels.
Over recent years much has been learned about the mechanisms by which retinoids exert their diverse effects on cellular proliferation and differentiation, thereby permitting the development of new retinoid derivatives with a narrower spectrum of activity in the hope of reducing side-effects of therapy.
The protective effects of vitamin E against photoaging have been demonstrated in various animal and in vitro skin models. When applied before UV exposure, this vitamin can reduce oedema, the formation of sunburn cells, lipid peroxidation, the formation of DNA adducts, immunosuppression and the formation of UVA-induced sensitizing substances. It has also been shown that topical application of vitamin E can reduce some late effects of UV radiation, like the formation of wrinkles and the incidence of cutaneous tumours.
Some esters of vitamin E (acetate, succinate, linoleate) have shown a certain ability to reduce cutaneous damage induced by UV rays. However, α-tocopherol has a more pronounced photoprotective effect than its esters, which must be hydrolyzed during cutaneous absorption in order to show antioxidant activity; these metabolic aspects have still not been completely clarified.
Although vitamin E has thus far yielded promising results for the treatment of both intrinsic and extrinsic ageing, very few of these studies have used human subjects.
Treatment of photodamage with topical preparations of vitamin C has become very popular over the past several years, based on animal studies demonstrating the ability to block UV-induced erythema by topical application of vitamin C preparations, as well as in vitro studies demonstrating stimulation of collagen production by fibroblasts. However, there are few clinical studies demonstrating a clinical benefit in reversal of photodamage by topical application of vitamin C or its analogs. The most common forms of vitamin C found in cosmetic products are L-ascorbic acid, and its ester form ascorbyl palmitate.
Nicotinamide has a potent immunomodulatory effect and may have great potential for treatment of human inflammatory diseases. It is gaining increasing interest in different parts of medicine. Numerous studies suggest a successful use in different diseases. Moreover beneficial effects of topically applied nicotinamide in aging skin, such as improvement in barrier functions and decreased appearance of signs of facial photoaging, e.g. skin texture changes and hyperpigmentation, have been noted. There is still a need for evidence based on in vivo studies in this field as in the care for atopic dry skin. The efficacy of topical nicotinamide in the management of acne-prone skin should be a stimulus for further work.
Coenzyme Q10, otherwise known as ubiquinone, is an endogenous cellular antioxidant present in almost all the tissues of the body, including the skin. It is believed that ubiquinone may have roles in both the extrinsic and chronological processes of ageing.
A substance widely used as an antioxidant principle in skin care products is lipoic acid. However, there is still a lack of convincing evidence of its clinical efficacy.
In order to maximize protective effects against different types of photodamage, e.g., sunburn reaction, photoimmunosuppression, photoaging, and photocarcinogenesis, synergistic effects of several vitamins and antioxidants at sufficient concentrations have to be taken into consideration.