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Vitamin C
Vitamin
C (L-Ascorbic Acid)
Vitamin C is one of the body's primary protectants from
reactive oxygen damage, but it is depleted during ultraviolet injury. Vitamin C
serves as the major aqueous phase antioxidant in skin. It not only neutralizes
reactive oxygen species destructive to the skin but also actively recycles
vitamin E, which serves to protect lipids and cell membranes from these
oxidative insults. Human beings cannot synthesize vitamin C; they must ingest
it, and body control mechanisms limit absorption and subsequent delivery to
tissues. Skin comprises about 8% of body tissues and gets approximately the
same amount of oral vitamin C. Half-life after ingestion is 12 to 20 days.
Topical
L-Ascorbic Acid
The Duke-patented technology solved the problem of how to
stabilize the very unstable molecule, l-ascorbic acid, and get it into skin,
which usually is an excellent barrier to foreign substances.2 L-ascorbic acid
has been formulated in high concentration in a stable aqueous formulation.
L-ascorbic acid is un-ionized at acid pH (<3.5). In its un-ionized form,
vitamin C applied topically passes into skin in higher concentrations than ever
is possible by oral ingestion.
Because body control mechanisms limit the amount of
ingested vitamin C available to skin, applying stable L-ascorbic acid topically
yields higher concentrations that provide additional protection. And unlike
most sunscreens, once vitamin C gets into skin, it cannot be rubbed or washed
off or run off with perspiration. The protection seems to last unchanged for
days. Topical vitamin C also prevents ultraviolet immunosuppression, a reaction
that has been implicated in both melanoma and nonmelanoma skin cancers.
Photoprotection
The added levels of L-ascorbic acid achieved by topical
application provide photoprotection to skin. Photodamage, which is measured
histologically as sunburn cells, can be lessened by L-ascorbic acid (whether
produced by UVB or UVA), with protection against UVA damage appearing
especially good.8 Because L-ascorbic acid does not absorb UVB or UVA light, its
mechanism of action is different from that of sunscreens. L-ascorbic acid seems
to protect by neutralizing reactive oxygen species generated by ultraviolet
light. Topical ascorbic acid can even be used to treat sunburn, presumably by
neutralizing inflammation. Results to date indicate that topical vitamin C is a
useful adjunct to sunscreens.
Ultraviolet
Immunosuppression
Immunosuppression by ultraviolet light for both contact
and delayed-type hypersensitivity occurs in one of three individuals. However,
almost everyone with melanoma or nonmelanoma skin cancer has this reaction. In
animals, ultraviolet immunosuppression is associated with more aggressive
metastatic behavior. Unfortunately, sunscreens don't protect very well against
ultraviolet immunosuppression. In contrast, topical L-ascorbic acid prevents
ultraviolet immunosuppression.9
Collagen
Synthesis
Vitamin C is the only antioxidant that has been proven to
increase collagen synthesis. In human skin fibroblasts in culture, L-ascorbic
acid stimulates collagen synthesis without affecting other protein synthesis.
L-ascorbic acid is known to be necessary for prolyl hydroxylase, an enzyme
essential for producing a stable collagen molecule. In addition, L-ascorbic
acid is necessary for lysyl hydroxylase, an enzyme necessary for cross-linking
one collagen molecule to another collagen molecule, which is required for
tissue strength. Finally, L-ascorbic acid signals collagen genes to synthesize
collagen, a reaction that is important in wound healing.
Effective
Topical Vitamin C Products
For a topical vitamin C formulation to work, it must
first penetrate skin and then remain stable and be available in high enough
concentrations to have a biologic effect. Research to date indicates that the
gold standard is stable L-ascorbic acid at high concentration (more than 10%)
and low pH (<3.5).
Clinical
Studies
Topical vitamin C is used for its photoprotective and
anti-inflammatory effects. Because L-ascorbic acid is essential for collagen
synthesis, it is also used for its wound healing effects. In photoaged skin,
anecdotal improvement has been reported; physicians and their patients
particularly note improved skin clarity and color. The results of such
observations are encouraging; double-blind studies are underway.
Topical vitamin C also has been used in patients
undergoing CO2 laser resurfacing. In a published half-face study, the side
receiving topical l-ascorbic acid demonstrated less erythema.
Summary
A stable aqueous solution of L-ascorbic acid has been
developed that delivers higher doses of vitamin C into skin than ever can be
achieved by diet. Topical application augments the normal reservoir present in
skin and neutralizes reactive oxygen species, adding photo- and anti-inflammatory
protection and preventing ultraviolet immunosuppression. Because UVA has been
identified as an important photocarcinogen and photoaging influence, additional
antioxidant protection should be beneficial. Properly formulated, topical
vitamin C seems to be a useful adjunct to sunscreens and other forms of sun
protection (such as hats, protective clothing, and sun avoidance when
feasible). Topical vitamin C also seems to be useful in speeding wound healing,
reducing wrinkling, and preventing the erythema associated with laser
resurfacing
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