Ascorbic Acid

What is ascorbic acid?

Ascorbic acid or vitamin C is a water-soluble vitamin found in a wide variety of plants and animals, especially citrus fruits. The Australian Kakadu plum has the highest content of vitamin C discovered so far, varying from 3.5% to 5.5% of wet weight (R), in comparison to orange, grapefruit and lime, which contain only approximately 0.5% (R). Camu-camu, acerola, chili peppers and blackcurrants are also rich sources of vitamin C (R, R, R, R).

Although many plants and mammals can synthesise ascorbic acid, humans cannot and must obtain ascorbic acid from their diet (R). This is important as ascorbic acid plays a crucial role as a major antioxidant and functions as an essential cofactor for many enzymes (R).

Is ascorbic acid stable?

Ascorbic acid is extremely unstable. Its stability is affected by light exposure, concentration, pH and other characteristics of the formulation. It is oxidised under aerobic conditions, degrades 70x slower in darkness than in light, and is more stable at higher concentrations and at lower pH (R, R).

The storage stability of ascorbic acid is also markedly increased when encapsulated in a solid-in-oil nanosuspension, owing to its low moisture content (10).

Palmitic acid as an emulsifier and glycerin as a humectant have been shown to impart the most stabilising effects on ascorbic acid compared to other emulsifiers such as myristic acid and stearic acid, and other humectants like ethylene glycol and propylene glycol (R).

Does ascorbic acid get absorbed?

Ascorbic acid is highly hydrophilic, making its penetration through the hydrophobic stratum corneum of the skin a challenge. When dissolved in water, its absorption through the skin depends critically on pH. In a study on pig skin, tissue levels of ascorbic acid were enhanced only at pH levels less than 3.5. Since the pKa of ascorbic acid is 4.2, it must apparently be unionized to be delivered across the stratum corneum (R). At a pH of 3.2, absorption is optimal when ascorbic acid is applied at a concentration of 20%, and tissues were saturated at 20x the ordinary levels of ascorbic acid after daily application of 15% ascorbic acid for 3 days (R).

Interestingly, skin resurfacing modalities have been shown to increase the topical delivery of ascorbic acid into the skin. In a study on nude mouse skin, both erbium:yttrium-aluminium-garnet (Er:YAG) and carbon dioxide lasers, as well as microdermabrasion, greatly increased the flux and skin deposition of ascorbic acid (R). This is not surprising as these techniques all remove layers of the stratum corneum (R, R), which is the biggest barrier to the permeation of ascorbic acid into the skin.

Incorporating ascorbic acid in microemulsions, particularly gel-like microemulsions, also improves its delivery compared to solutions (R, R). The increased absorption of such microemulsions is attributed to the use of penetration enhancers in the oil phase (R).

After reaching the dermis, ascorbic acid remains stable in the tissue for several hours before being metabolised. However, exposure to UV light depletes up to two-thirds of cutaneous ascorbic acid stores (R). It has therefore been suggested that ascorbic acid may need to be topically applied every 8 hours for optimal effect (R).