Vitamin C serums have become the Little Black Dress of skincare: It seems like everyone has one, they flatter just about every skin tone, and they just plain work.
That said, some vitamin C serums work better than others. Sometimes this is due to the concentration of vitamin C in the serum, but with most savvy consumers regularly looking for vitamin C in 10%-25% concentrations on the label, this usually isn’t the case. Instead, most informed skincare consumers know it is the form of vitamin C that matters most.
L-Ascorbic Acid: Best for Using in the Recommended Concentrations (15% or Higher)
L-ascorbic acid is the most common form of vitamin C used in skin care. It is water-soluble. The biggest advantage to L-ascorbic acid is that its concentrations are well-established in skin care products; most studies that show efficacy use L-ascorbic acid in concentrations of 15% or higher. For this reason, products like the patented Skinceuticals CE Ferulic contain 15% L-ascorbic acid.
The effects of L-ascorbic acid also last several days: It has been shown that topical application of Cellex C, with 10% L-ascorbic acid, results in the presence of L-ascorbic acid in the skin for 2-3 days after application, with an increase in the level of collagen as well.
The final benefit of L-ascorbic acid is that it, in and of itself, has value to the skin, without having to be converted to another form within the skin. An issue with vitamin C derivatives are that they typically need to be converted to vitamin C within the skin. For instance, ascorbyl glucosamine needs to be broken down into L-ascorbic acid and glucosamine by enzymes found naturally within the skin in order to have any activity at all. On the other hand, L-ascorbic acid on its own has been shown to have effects, without needing to have any sort of breakdown or “activation” within the skin.
The issue with L-ascorbic acid is that it is notoriously unstable, which is exacerbated in the presence of light, heat, and air (Die Pharmazie). In particular, L-ascorbic acid is reversibly oxidized to L-dehydroascorbic acid, at which point it may then be irreversibly oxidized to diketogluconic acid, which is inactive. For this reason, numerous substitutes to L-ascorbic acid are used in skin care.
Tetrahexyldecyl Ascorbate: Best for Really Tired, Aging Skin
Tetrahexyldecyl ascorbate is a form of vitamin C that is oil-soluble. What makes THD ascorbate special is that it has been shown in peer-reviewed studies to penetrate both the epidermis (the uppermost layer of skin) and dermis (the deepest layer of skin) (Clinics in Dermatology, 2008). I cannot find any studies that demonstrate other forms of vitamin C penetrate the skin this far.
Like all of the other vitamin C derivatives on the skin, THD ascorbate has to be converted to L-ascorbic acid within the skin to be active. Which means that 20% THD ascorbate is more likely to effectively be like (I’m guessing) 10-15% L-ascorbic acid. That said, it’s also more stable than L-ascorbic acid, so you’re likely getting less potency from a 20% THD ascorbate serum than a 15% L-ascorbic acid serum when you first open the bottle, and more potency when you get towards the bottom of the bottle. (Tradeoffs, I tell you, trade-offs.)
The product with the highest concentration of THD ascorbate I know of is Peter Thomas Roth Potent-C Serum, with a whopping 20% THD ascorbate.
Sodium Ascorbyl Palmitate: Best for Budget
It’s more stable and bioavailable than Magnesium Ascorbyl Phosphate (MAP), Ascorbyl Palmitate, and Ascorbic Acid (among other Vitamin C derivatives) (Journal of the Chemical Society of Pakistan, International Journal of Pharmaceutics). For this reason, it’s best for the budget — if you’re the type of skincare consumer who likes to draw out a bottle of serum for two or three months, choosing one with SAP is better than one with LAA or THD ascorbate.
Cosmetic chemists like sodium ascorbyl palmitate because it is stable in both water-in-oil emulsions and oil-in-water emulsions, unlike ascorbyl palmitate. It also is more active at a higher range of pH levels than L-ascorbic acid (International Journal of Cosmetic Science), which has to be formulated at a pH under 4 for maximum results. For these reasons, you’ll see sodium ascorbyl palmitate in a number of products, even though its concentrations aren’t competitive (I’ve never seen a 20%+ SAP serum).
3-0-Ethyl Ascorbic Acid and Ascorbyl Glucosamine: Best for Skin Brightening and Sunspots (Tie)
The studies on each of these stabilized vitamin C derivatives are limited, but promising.
In a clinical skin lightening test, a solution containing just 2% 3-0-ethyl ascorbic acid was found to improve skin whitening and radiance after just 28 days of twice-daily application (Cosmetics and Toiletries). I like Vitabrid and Dr. Dennis Gross C+ Collagen Brighten & Firm Vitamin C Serum.
On the other hand, ascorbyl glucosamine is less common. The combination of ascorbyl glucosamine and niacinamide was shown to reduce facial hyperpigmentation in Japanese and Caucasian subjects with facial hyperpigmentation in two double-blind, vehicle-controlled, split-face, left-right randomized clinical studies (source: Dermatology)
When considering a vitamin C serum, the right form of vitamin C will be available in concentrations in which it has been proven to work; have clinical data to support its collagen-building and/or sunspot-fighting and/or UV-free radical-scavenging abilities; penetrate the skin well; and meet your lifestyle in terms of stability. Meaning, if you’re the type of person who spends ample money on skincare and uses it frequently, go ahead with the L-ascorbic acid. On the other hand, if you’re more frugal and spread out a bottle to last 2-3 months (or more), sodium ascorbyl phosphate makes more sense.