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Recently, a reader asked me if it was better to use vitamin C or AHA first.
My answer? Usually AHA. Here’s why:
- AHAs are usually more acidic than vitamin C products. This advances desquamation and exfoliation of the skin, preparing the skin for other skin care products.
- AHAs are usually based in alcohol or very thin solutions. Remember: the rule of thumb is thinner products first, then thicker products.
- AHAs are always somewhat acidic. On the other hand, vitamin C is not always acidic. In fact, tetrahexyldecyl ascorbate (THD) or magnesium ascorbyl phosphate (MAP) are acidic-neutral, at a slightly higher pH range that will not prep the skin for AHA solutions. But when used the other way around, AHAs will prep the skin for vitamin C.
AHAs come in various forms, ranging from the small, ultra-exfoliating structure of glycolic acid to the larger, more hydrating structure of lactic acid. Typically, the more AHA that is in a solution, the more acidic and exfoliating the solution will become. Most super-effective AHA solutions with high concentrations of glycolic, lactic, mandelic, and/or malic acids are around the pH range as low as 3.0 (Skin Therapy Letter) to as high as 4.5-5.0.
A strong AHA formula can be thought of as a type of “prep” solution for the skin. It acidifies the skin and immediately promotes desquamation — that “flaking” of old, dead skin cells. This in turn helps other skin care products to be absorbed into the skin better. The lowering of the pH of the skin also puts the skin in the pH range where L-ascorbic acid can actually be effective.
On the other hand, vitamin C is less effective as a prep than AHA solutions. Yes, vitamin C is acidic as L-ascorbic acid, but it is usually around the pH range of 3.5-5.5. This pH range is still typically somewhat higher than AHAs in the 3.0-5.5 range, making vitamin C less effective as a skin care prep and more effective as a skin care treatment. However, at this pH range, studies show that L-ascorbic acid can still do great things, as it can be absorbed into the skin and create amazing effects, including lightening, tightening, and brightening (Cosmetic Dermatology).
The use of AHAs before vitamin C is especially important if you use non-acidic forms of vitamin C. Non-acidic forms of vitamin C are almost always of a higher pH than AHAs, so be sure that you acidify the skin first so that the THD, MAP, or ascorbyl glucoside (AG) can actually be broken down or transformed/activated into L-ascorbic acid to begin with.
Unless you’re using the rare highly acidic and thin vitamin C product (like Skinceuticals CE Ferulic) with the equally rare, not-so-acidic and thick AHA product (like LacHydrin), then use the AHA first, and the vitamin C second.