Oil-Soluble Vitamin C (THD) vs. L-Ascorbic Acid: Stability, Delivery, and Evidence

The thing that makes vitamin C useful in skincare is also what makes it hard to keep in a bottle. It reacts readily with other molecules, which is how it works as an antioxidant, but that reactivity also means it can break down before it reaches your skin.¹⁰ L-ascorbic acid supplies active vitamin C directly, while THD ascorbate is a modified, oil-soluble form that can release L-ascorbic acid once it's in skin.¹ So when you compare vitamin C products, the form matters, and so do the base carrying it and the ingredients protecting it.

What does vitamin C do, and how does THD become it?

Vitamin C helps skin make collagen, the protein that gives it strength and structure. It also helps limit pigment formation and acts as an antioxidant, reacting with molecules that can damage cells.^{4, 5} L-ascorbic acid dissolves in water and THD dissolves in oils, and that difference shapes how a serum keeps the vitamin intact and carries it into skin in a form cells can use.^{1, 2, 6}

Tetrahexyldecyl ascorbate, or THD, is L-ascorbic acid with four fatty-acid chains attached. The chains let it dissolve in oils and help protect it from breaking down, and proteins called enzymes can remove them to release the active vitamin. Think of the chains as packaging that protects the vitamin in the bottle and can come off once it's in skin. In the studies discussed here, the same form also goes by ascorbyl tetraisopalmitate or VC-IP.^{1, 7}

Researchers have found that THD releases active vitamin C within a skin model. In collagen-making skin cells grown in a lab, it raised the amount of active vitamin C inside the cells more than L-ascorbic acid did, and in a separate test, an enzyme found in skin removed THD's fatty-acid chains and released L-ascorbic acid.^{3, 7, 1} It's a striking result for a form that has to be unpacked before it works.

How does each form get into skin?

L-ascorbic acid is already active vitamin C, so getting it through the skin barrier is the formulation challenge. Skin's outer barrier is rich in lipids, or fat-like substances, and water-soluble L-ascorbic acid passes through it more easily when it carries no electrical charge. A more acidic formula keeps more of the vitamin uncharged.^{1, 2}

Acidity is measured by pH, with lower numbers meaning more acidic. In a pig-skin study, L-ascorbic acid got into skin when the formula's pH was below 3.5, and absorption rose with concentration up to 20 percent but not beyond it.² So a higher percentage on the label doesn't always mean more vitamin C gets in, and the acidity and the base carrying the vitamin matter alongside its concentration.

THD doesn't need an acidic, water-based formula to be carried. In a test using a synthetic membrane that models skin, THD from an emulsion passed through more readily than L-ascorbic acid from the serum tested alongside it.^{6, 8}

Both forms need protecting, in different ways

L-ascorbic acid dissolved in water can react with oxygen, and heat, light and traces of metals can speed up its loss.¹⁰ A lower pH helps preserve it as well as helping it get into skin, and supporting ingredients can slow its breakdown too. Adding ferulic acid, another antioxidant, to a solution of vitamins C and E, for example, improved the vitamins' stability.^{10, 11}

THD stays stable in a much less acidic formula. In one formula study, less than 15 percent of THD was lost after six months at room temperature, at a pH of 5.5, less acidic than the conditions used for L-ascorbic acid.⁶ For vitamin C, six months with that little loss is a good showing.

THD can still benefit from supporting antioxidants. It broke down more slowly when combined with vitamin A and E derivatives in a formulation study, and in a separate test, a highly reactive form of oxygen broke THD down while another antioxidant helped protect it.^{9, 1}

What has each form done on skin?

L-ascorbic acid formulas have improved fine wrinkles, roughness and other signs of sun-related skin aging in controlled studies.^{12, 13} A THD cream reduced pigmentation after UV exposure, and another THD formula improved uneven pigmentation compared with its base alone.^{3, 14} Researchers also compared a serum containing THD and supporting ingredients with a different moisturizer, and the serum improved fine lines, uneven tone and smoothness more than the moisturizer did.¹⁵ Because the serum was compared with a different product rather than its own base, that result belongs to the whole serum.

Research on L-ascorbic acid shows what the active vitamin can do, and studies of THD show how an oil-soluble form can make that vitamin available to skin. Keeping the modified form intact preserves the vitamin C available for release in skin, so stability, delivery and conversion all shape how much active vitamin C reaches cells.^{1, 3, 6, 7}

In practice, the two forms differ in what's applied, how they fit into a formula and what keeps them stable:

^{1, 2, 6, 9, 11}

Why we chose THD for Omnia

Omnia is a waterless, lipid-based solid serum. We chose THD for its stability, its ability to dissolve in our lipid base and its release of active vitamin C in skin.^{1, 6} Squalane, jojoba esters and other lipids carry THD while helping skin feel softer, and the anhydrous serum guide explains how the format works.^{1, 6}

We also chose antioxidants to help protect the formula. Mixed tocopherols, forms of vitamin E, interrupt the chain reactions that spread oxidation through oils, and rosemary extract supplies carnosic acid, which reacts with molecules that would otherwise oxidize lipids.^{16, 17} Our aim is to limit vitamin C loss before application and keep it available for release once it reaches skin.^{1, 6}

The Omnia page has more about the formula and its ingredients, and The Science of Biomimetic Design explains the design thinking behind it.

Sources

1. Swindell WR, et al. Tetrahexyldecyl ascorbate (THDC) degrades rapidly under oxidative stress but can be stabilized by acetyl zingerone to enhance collagen production and antioxidant effects. International Journal of Molecular Sciences, 2021.
2. Pinnell SR, et al. Topical L-ascorbic acid: percutaneous absorption studies. Dermatologic Surgery, 2001.
3. Ochiai Y, et al. A new lipophilic pro-vitamin C, tetra-isopalmitoyl ascorbic acid (VC-IP), prevents UV-induced skin pigmentation through its anti-oxidative properties. Journal of Dermatological Science, 2006.
4. Stamford NPJ. Stability, transdermal penetration, and cutaneous effects of ascorbic acid and its derivatives. Journal of Cosmetic Dermatology, 2012.
5. Murad S, et al. Collagen synthesis in cultured human skin fibroblasts: effect of ascorbic acid and its analogs. Journal of Investigative Dermatology, 1983.
6. Maia Campos PMBG, et al. Application of tetra-isopalmitoyl ascorbic acid in cosmetic formulations: Stability studies and in vivo efficacy. European Journal of Pharmaceutics and Biopharmaceutics, 2012.
7. Xiao L, et al. Cytoprotective effects of the lipoidic-liquiform pro-vitamin C tetra-isopalmitoyl-ascorbate (VC-IP) against ultraviolet-A ray-induced injuries in human skin cells together with collagen retention, MMP inhibition and p53 gene repression. Journal of Cellular Biochemistry, 2009.
8. Maloney ME, et al. A Closer Look at Penetration: The Efficacy Gap in Vitamin C Products. Journal of Cosmetic Dermatology, 2026.
9. Gianeti MD, et al. Benefits of combinations of vitamin A, C and E derivatives in the stability of cosmetic formulations. Molecules, 2012.
10. Yin X, et al. Chemical stability of ascorbic acid integrated into commercial products: a review on bioactivity and delivery technology. Antioxidants, 2022.
11. Lin FH, et al. Ferulic acid stabilizes a solution of vitamins C and E and doubles its photoprotection of skin. Journal of Investigative Dermatology, 2005.
12. Traikovich SS. Use of topical ascorbic acid and its effects on photodamaged skin topography. Archives of Otolaryngology–Head & Neck Surgery, 1999.
13. Humbert PG, et al. Topical ascorbic acid on photoaged skin. Clinical, topographical and ultrastructural evaluation: double-blind study vs. placebo. Experimental Dermatology, 2003.
14. D'Angelo Costa GM, Maia Campos PMBG. Efficacy of topical antioxidants in the skin hyperpigmentation control: a clinical study by reflectance confocal microscopy. Journal of Cosmetic Dermatology, 2021.
15. Maloney ME, et al. Hydroquinone-free, tetrahexyldecyl ascorbate antioxidant serum for hyperpigmented and photodamaged skin to achieve skin health. Journal of Cosmetic Dermatology, 2026. Figure-caption correction.
16. Burton GW, Ingold KU. Autoxidation of biological molecules. 1. The antioxidant activity of vitamin E and related chain-breaking phenolic antioxidants in vitro. Journal of the American Chemical Society, 1981.
17. Loussouarn M, et al. Carnosic acid and carnosol, two major antioxidants of rosemary, act through different mechanisms. Plant Physiology, 2017.