Vitamin C forms compared
L-ascorbic acid, ascorbyl glucoside and 3-O-ethyl ascorbic acid are not interchangeable. Stability, conversion and evidence all differ.
The stability problem
L-ascorbic acid is the biologically active form and the one with the strongest evidence, but it is also the least stable cosmetic ingredient in common use — it oxidises in water on exposure to air, light and metal ions, turning yellow then brown, and the oxidation products are inactive or worse. Every vitamin C derivative exists to solve that problem, and each solves it by sacrificing something.
The forms in this range
Ascorbyl glucoside is L-ascorbic acid with a glucose molecule attached. It is stable and non-irritating, and requires enzymatic cleavage in skin to release active vitamin C — so the effective dose is lower than the label percentage suggests, and depends on enzyme activity. Youlief uses it at 2% in DIA Toning; Vegalab uses it in the Bright Micro-Bubble Cleanser.
3-O-ethyl ascorbic acid is ethylated at the 3 position, giving good stability with better penetration than the glucoside and no conversion step for part of its activity. Vegalab uses it cyclodextrin-encapsulated in Lumina Brightening Cream — belt and braces on stability.
L-ascorbic acid appears in the Youlief WP Sun product. In an anhydrous or well-buffered system that is defensible; in a water-based one it needs a stability rationale.
How to read a vitamin C claim
Three questions. Which form, since the derivatives are not equivalent to L-ascorbic acid on a percentage basis. What is the pH, since L-ascorbic acid needs to be below about 3.5 to penetrate. And is the packaging opaque and airless, because a clear jar of vitamin C serum is a countdown.
What vitamin C does
Reasonably well supported: antioxidant activity, a role as a cofactor for collagen synthesis, and inhibition of tyrosinase contributing to a brightening effect. Less well supported: the dramatic transformations claimed in marketing, and any specific superiority of one derivative over another in finished products, which is rarely tested head to head.