Vitamin C is the most common antioxidant in skin. Unfortunately, humans do not have the enzyme (L-gluconogamma lactone oxidase) to make vitamin C, therefore it must be obtained from external sources.
L ascorbic acid is the active form of vitamin C that carries out the biologic effects. Besides retinoids, vitamin C is probably the next most studied ingredient in skincare. Ascorbic acid has many benefits. Repeat studies have shown it helps to prevent photodamage from UVA and UVB radiation. It stimulates collagen production, serving as co-factor for enzymes in collagen production. Lastly, vitamin C is a great lightening ingredient. It blocks tyrosinase, key enzyme in melanin production.
However, vitamin C is highly unstable. It easily oxidizes in light, heat, pH change, and presence of other other ions. This has led to development of various vitamin C derivatives that are more stable while trying to retain efficacy. However formulation continues to be a challenge. Demonstrating ingredient effectiveness needs to occur not just in the laboratory setting or on animals, it also needs assessment on human skin. Unfortunately, there’s a paucity of data regarding skincare ingredients in general (compared to medical dermatology), and some publications are conflicting. One study may show benefit of a derivative, while another demonstrates no efficacy. This inherently speaks to the challenges of manufacturing L ascorbic acid, as formulation is everything.