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The Inside Scoop on THD and Every Other Type of Vitamin C You Should Know About

The Inside Scoop on THD and Every Other Type of Vitamin C You Should Know About featured image
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“Vitamin C is a necessary component for many aspects of skin health,” says New York dermatologist Rachel Nazarian, MD. “It’s useful in maintaining healthy collagen fibers, but can also be used topically for various skin benefits because it is a wonderful antioxidant.” This gives it the ability to protect skin from free-radical damage. “Aside from improving the appearance of wrinkles, pores and fine lines, it can also improve pigmentation and sun damage,” Dr. Nazarian adds. “That’s why it’s so loved.”

However, there are many different types of vitamin C available in skin-care products today. And as Dr. Nazarian points out, this can make it a bit confusing to understand which is superior or the right choice for your skin. “Some are more potent; others are less potent and need to be converted in the skin,” she explains. “Some work at a lower pH; some at a higher pH. Then there are types that love oil, while others love water. And lastly, some are more stable or irritating than others.” Here, we tap top experts in the field to share their insight on the vast and varied world of vitamin C derivatives.


Ascorbic Acid: The Pure Form of Vitamin C

Ascorbic acid (or L-ascorbic acid) is the pure, water-soluble form of vitamin C, and the most researched and tested form of vitamin C. “It is not a derivative and does not need to be converted or modified for the body and skin to use it as a nutrient or antioxidant,” explains cosmetic chemist Stephen Alain Ko, founder of Poems From the Lab.

“Ascorbic acid is proven to do three main things,” says cosmetic chemist Ron Robinson, founder of BeautyStat Skincare. “First, it works as a powerful antioxidant. Second, it helps stimulate collagen, which reduces the look of lines and wrinkles and firms skin. Third, it helps prevent the overproduction of melanin, which helps even out skin tone and fade hyperpigmentation.”

However, despite its many benefits, ascorbic acid is a very unstable ingredient. “This means it oxidizes quickly when exposed to light, water or air,” says Robinson. If you’ve ever had a vitamin C serum that turned brown over time, this is why. Robinson also points out that when ascorbic acid is used at the effective levels of 10 to 20 percent (research shows that vitamin C requires a concentration of at least 5 percent to work), it can be irritating to those with sensitive skin. “This is why many derivatives of vitamin C have come to market,” he explains. “Some are more stable than the pure form and gentler on the skin, but may not be as effective.”

Where to Find It:

BeautyStat’s beloved Universal C Skin Refiner ($85) has developed a cult following since its launch. “It uses 20-percent pure, patented stable ascorbic acid, so it does not oxidize,” says Robinson. “It’s also clinically tested to reduce lines and wrinkles, firm and tighten skin, visibly fade hyperpigmentation and reduce the look of pores.”

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The Difference Between Oil-Soluble and Water-Soluble Vitamin C

Each form of vitamin C is either oil-soluble or water-soluble. Oil-soluble ingredients require an oil-based delivery system, whereas water-soluble ingredients need a water-based formula. Oil-soluble vitamin C “neutralizes the types of free radicals that damage skin’s lipids, such as ceramides and cholesterol,” explains Paula Begoun, founder of Paula’s Choice Skincare. “This is primarily how it works as an antioxidant. Water-soluble ascorbic acid helps neutralize other types of free radicals before they get too far past skin’s surface.”

Begoun says oil-soluble ascorbic acid can also penetrate further into skin. “It works with vitamin E that’s naturally in our skin or in skin-care products to target skin’s underlying support system via a different pathway than water-soluble ascorbic acid,” she explains. However, when it comes to which one—water or oil—is more effective, Ko says we don’t really know. “The scientific literature on this is very scant.”

8 Types of Vitamin C Used in Skin Care Today

Ethyl Ascorbic Acid

“This is a derivative of ascorbic acid where an ethyl group is bound to ascorbic acid,” explains Ko. “This slightly increases stability in comparison to ascorbic acid. There’s some cell and animal data to show that it is converted back into ascorbic acid by the skin, and potentially provides skin benefits.” Robinson adds that this type of vitamin C (aka 3-O-ethyl ascorbate) is both water- and oil-soluble.

Begoun adds, “As a modified and more stable version of pure vitamin C, ethyl ascorbic acid must be converted to vitamin C within skin by sodium-dependent proteins. This process within skin’s surface layers helps ensure skin benefits. Due to the conversion to pure vitamin C being a slower process, 3-O ethyl ascorbic acid is considered more tolerable.”

Where to Find It:

Alpha-H Vitamin C Serum with 10% Ethyl Ascorbic Acid ($65) is quick to absorb and great for dull skin. The addition of this type of vitamin C helps protect skin from environmental aggressors. We love the citrusy scent as well.

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Tetrahexyldecyl Ascorbate (THD)

Some experts have referred to this ingredient as “supercharged vitamin C” because it is highly active. “Tetrahexyldecyl ascorbate is an oil-soluble derivative of ascorbic acid where four lipids are attached to the ascorbic acid,” Ko explains. Dr. Nazarian says it’s used more often in skin care now because “it’s much more stable and less likely to irritate skin.” If you have dry or sensitive skin, pure ascorbic acid can be too much. “For these skin types, it’s imperative that they use a more stabilized and less-irritating version of vitamin C like this one.”

Some researchers theorize that THD has a greater affinity for skin, Begoun notes. “This is because its fatty acid component helps aid penetration. It also pairs well with other forms of vitamin C, vitamin E, green tea and retinol for enhanced effectiveness and anti-aging benefits. Additionally, it can be used in water-based formulas, but must be combined with silicones or oils for ideal bioavailability.”

Where to Find It:

We love the Paula’s Choice 25% Vitamin C + Glutathione Clinical Serum ($62), which contains tetrahexyldecyl ascorbate⁠ to brighten and firm without irritation. It also contains ascorbyl glucoside (more on this derivative below).

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Ascorbyl Palmitate

Ko says ascorbyl palmitate is an oil-soluble derivative of ascorbic acid where palmitic acid is bound to the ascorbic acid. “The stability is like ascorbic acid, but there is little human evidence that it provides benefits greater than ascorbic acid.”

In fact, he notes that “one skin cell study found that ascorbyl palmitate increased lipid peroxidation [the oxidative degradation of lipids] in response to UVB exposure. While that is a negative effect, it hasn’t been definitively proven to occur in human skin in a normal use case. However, its use in cosmetic formulations is now less popular.”

Although ascorbyl palmitate has antioxidant benefits for skin, Begoun says research has shown it doesn’t penetrate as well as ascorbic acid. “Additionally, its conversion to vitamin C isn’t as efficient as other forms, such as tetrahexyldecyl ascorbate. However, research has shown that a modified form of this ingredient, known as trisodium ascorbyl-6 palmitate 2-phosphate, overcomes this conversion issue.”

Where to Find It:

A cult-classic with rave reviews, Jan Marini Skin Research C-Esta Face Serum ($129) combines ascorbyl palmitate with DMAE and hyaluronic acid. Not only does it help reduce the appearance of fine lines and wrinkles and uneven skin texture, but it also protects from free radicals.

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Sodium Ascorbate and Calcium Ascorbate

“Sodium ascorbate is the salt formed when ascorbic acid is combined with a base like sodium hydroxide or sodium bicarbonate,” explains Ko. “Once dissolved, a proportion of it will dissolve and become ascorbic acid, depending on the solution pH. Calcium ascorbate is like sodium ascorbate in that it is a salt, but it is formed when combined with a calcium containing base like calcium bicarbonate. This is more often used in food and supplements to provide calcium, as well as ascorbic acid.” Both of these are water-soluble derivatives, adds Robinson.

With calcium ascorbate specifically, you may see it referred to as ester-C in skin care as well. According to Begoun, both of these ingredients are stable forms of vitamin C that help even skin tone and provide antioxidant benefits.

Where to Find It:

Alastin C-RADICAL Defense Antioxidant Serum ($192) contains sodium ascorbate in a proprietary, encapsulated form. This keeps it from irritating the skin as it helps to defend against free-radical damage and support elastin conservation.

Award Photo: C-Radical Defense Antioxidant Serum

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Ascorbyl Glucoside

“This is a stable, water-soluble derivative of vitamin C combined with the sugar glucose,” says Robinson. “It might be the best of the derivatives to show efficacy,” says Robinson. Begoun adds that “the glucose is derived from a natural starch source, while the vitamin C portion is synthetic. When properly formulated and absorbed into skin, it breaks down to ascorbic acid. Once absorbed, its breakdown to vitamin C is gradual, creating what’s known as a reservoir effect within skin. This leads to longer-lasting benefits.”

Begoun also points out that ascorbyl glucoside is sometimes referred to as AA2G. “Research on ascorbyl glucoside’s ability to improve uneven skin tone and hyperpigmentation is encouraging. But, it’s not as extensive when compared with the decades of research amassed for ascorbic acid. However, ascorbyl glucoside plus niacinamide is a viable combination to consider, and several skin-care products pair these ingredients.”

Where to Find It:

The Ordinary Ascorbyl Glucoside Solution 12% Serum ($15) is not only a bargain, but also an effective brightener. It’s lightweight and a great option for anyone concerned with uneven tone or dullness, or those looking for antioxidant support.

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Magnesium Ascorbyl Phosphate and Sodium Ascorbyl Phosphate

“Magnesium ascorbyl phosphate (MAP) and sodium ascorbyl phosphate (SAP) are water-soluble derivatives of ascorbic acid where the ascorbic acid has been bound to a phosphate group,” Ko explains. The difference is the counterion used: magnesium or sodium. Both are claimed to be more stable than ascorbic acid, but the data is scant.”

Begoun says that unlike other water-soluble forms of vitamin C, MAP is also lipophilic, or oil-loving. “This improves penetration into skin, but how well this form converts to ascorbic acid within skin is up for debate. MAP is considered one of the most hydrating forms of vitamin C. This is because of its oil-loving nature and ability to promote hydration deeper in skin when compared to ascorbic acid.”

Sodium ascorbyl phosphate works in both water- and oil-based formulas without breaking down, Begoun says. One study compared a 5-percent concentration with the same amount of ascorbic acid. Results showed an equivalent wrinkle-smoothing and elasticity-increasing benefit to skin around the eyes. “Amounts above 3 percent are considered necessary to target discoloration. This is similar to the range research has shown ascorbic acid works within to target this concern.”

Where to Find It:

Naturium Vitamin C Complex Serum ($21) combines sodium ascorbyl phosphate with ascorbic acid and a bioactive fruit blend. Layer this with your sunscreen for double-duty defense against harmful environmental stressors. In a clinical study, the serum improved the appearance of fine lines and wrinkles in just four weeks.

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Which types of vitamin C are best for those with acne-prone skin?

Sodium ascorbyl phosphate (SAP) and magnesium ascorbyl phosphate (MAP) are often marketed as the best ascorbic acid derivatives for acne-prone skin, says Ko. “However, these suggestions are based on one very small study that hasn’t been replicated. It also highlights the confusion between statistically significant and clinically relevant. Participants using a sodium ascorbyl phosphate lotion saw a statistically significant reduction in acne. This amounted to an average reduction of 1 inflammatory acne lesion and 3 non-inflammatory acne lesions after 12 weeks compared to placebo. I don’t think many people would consider a potential reduction of one pimple and a few blackheads/whiteheads after 12 weeks an effective acne treatment.” Clearly, more studies are needed here.

Anecdotally, there are many positive reviews from people with acne-prone skin using products that contain SAP. “This form of vitamin C may play a special role in reducing the likelihood of blemishes. This is because its soothing action helps quell an of oil-based saccharide that can irritate skin,” adds Begoun. “However, research on this matter was not comparative, meaning we don’t know how a range of other forms of vitamin C would’ve fared.” Dr. Nazarian recommends TruSkin Vitamin C Super Serum+ ($30) for her patients with acne-prone skin, but also all skin types. “It uses sodium ascorbyl phosphate, a derivative that is stable and gentle. It also includes retinol. I find vitamin A derivatives useful for acne-prone patients because they can decrease the dark marks and spots acne can leave behind.”

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