"Sodium Ascorbyl Phosphate": what is it?

Key points to remember about “Sodium Ascorbyl Phosphate.”

“Sodium Ascorbyl Phosphate” (SAP) is the INCI name for sodium ascorbyl phosphate, which is a stable, water‑soluble form of pure vitamin C, also known as ascorbic acid.
The stability of pure vitamin C has been improved by theaddition of a phosphate group to its chemical structure.
Unlike conventional vitamin C, sodium ascorbyl phosphate does not need a low pH to be effective.
Once applied to the skin, sodium ascorbyl phosphate is converted into the active form of pure vitamin C under the action of skin enzymes.
SAP exerts an antioxidant effect, helping protect skin cells against oxidative damage. Benefits for skin radiance, firmness, and reduction of imperfections have also been reported, although clinical data still need to be consolidated.
Sodium ascorbyl phosphate is a milder alternative to L-ascorbic acid, particularly suitable for sensitive skin.

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"Sodium Ascorbyl Phosphate": why is it used in cosmetic formulations?

Simply put, “Sodium Ascorbyl Phosphate” (SAP) refers to a water‑soluble, stable precursor of vitamin C. In fact, L-ascorbic acid, although effective, is known for its instability. It is easily oxidized when exposed to sunlight, to high oxygen levels (aerobic conditions), to the presence of metal ions (copper or heavy metals in general catalyze this reaction), to an elevated pH, and to temperature fluctuations. It first degrades reversibly into dehydro-L-ascorbic acid, then into the irreversible formation of diketogulonic acid, a biologically inactive form, accompanied by a yellowish discoloration of the formula.

However, this instability often limits its effectiveness in skincare formulations and requires specific packaging to protect it. To address this issue, most cosmetic brands have turned to its derivatives, which are more stable and easier to formulate. Ascorbic acid has been chemically modified by esterification of the hydroxyl (-OH) group with long-chain organic or inorganic acids, such as sodium ascorbyl phosphate.

It consists of a stabilized sodium salt of L-ascorbic acid, which has also undergoneesterification of a phosphate (P) group at position 2 of the ring, thereby protecting the enediol system from oxidation. This stability not only extends the product’s shelf life, but also ensures a constant supply of vitamin C to the skin, maximizing its benefits. It also offers formulators greater flexibility in designing skincare products, since sodium ascorbyl phosphate can be incorporated into a wider range of formulations, from serums to creams, without concern about its degradation.

“Sodium Ascorbyl Phosphate” is present in two of our products, at different concentrations: the Serum for Wrinkles and Firmness A34 at 10% and the Glow Drops T31 at 5%.

Structure chimique du "Sodium Ascorbyl Phosphate". — Chemical structure of “Sodium Ascorbyl Phosphate.”
Source: PubChem.

KADRI A. and colleagues (2013) showed that sodium ascorbyl phosphate (2%) was significantly more stable in an emulsion after 28 days at 8°C, 25°C, and 40°C in an incubator, with more than 95% of the compound remaining undegraded, compared with an emulsion containing magnesium ascorbyl phosphate (2%) and L-ascorbic acid (2%). Another study showed that sodium ascorbyl phosphate maintained its stability at approximately 60–70%, even after 365 days of storage in the dark at room temperature.

Identification	Chemical and Physical Characteristics
Chemical formula	C6H6Na3O9P
Synonym	Sodium ascorbyl phosphate, trisodium salt, 2-phospho-L-ascorbic acid, L-ascorbic acid 2-monophosphate, ascorbyl phosphate salt
Appearance	White to off‑white powder, practically odorless
Process for obtaining	Phosphorylation reaction of ascorbic acid with sodium and phosphate compounds
Molar mass	322.05 g/mol
Solubility	Soluble in water and water-based solvents (glycerin, etc.), insoluble in ethanol
Stability	Good stability, superior to that of conventional vitamin C

As a precursor, however, it needs to be converted into free ascorbic acid to exert these topical benefits. It has been observed that sodium ascorbyl phosphate is converted into active vitamin C within the epidermis by enzymes (alkaline phosphatase). In fact, the phosphate group and the salt to which it is bound act synergistically with skin enzymes to cleave the molecule within the physiological pH range, thereby releasing ascorbic acid. This characteristic not only enables a gradual release of its beneficial properties but also minimizes the risk of irritation.

KRISTL J. et al. have shown that SAP penetrates the stratum corneum more effectively from liposomal dispersions than from an aqueous solution.

The main benefits of Sodium Ascorbyl Phosphate.

Used in a large number of cosmetic products, sodium ascorbyl phosphate provides many beneficial effects on the skin.

Protect the skin from the harmful effects of free radicals.
Just like its counterpart ascorbic acid, sodium ascorbyl phosphate (SAP) appears to exhibit antioxidant activity by eliminating reactive oxygen species, thereby protecting cellular components from oxidative damage. The protective effect of sodium ascorbyl phosphate against UVB-induced skin damage in mice in vitro is due to the maintenance of normal ascorbic acid levels through the conversion of SAP into ascorbic acid in skin tissues. However, topically applied sodium ascorbyl phosphate has been found to be less effective than similarly applied ascorbic acid in reducing oxidative stress in human skin in vivo, highlighting the importance of enzymatic conversion efficiency when evaluating its actual effectiveness.
Helps support skin firmness and elasticity.
In 2020, a comparative clinical study conducted on 12 women (40–50 years old) with crow’s feet wrinkles evaluated an emulgel formulation containing 5% sodium ascorbyl phosphate applied for 8 weeks, twice daily. The results show a significant improvement in skin elasticity and a reduction in wrinkle depth around the eyes, with an efficacy comparable to that of 5% ascorbic acid. However, these findings still need to be confirmed.
Additional studies, involving more participants and other areas of the face, will be necessary to confirm the relevance of sodium ascorbyl phosphate for visible signs of skin aging. Thus, these initial results indicate that sodium ascorbyl phosphate may strengthen the dermal matrix and help maintain the structure and function of the skin, although the underlying biological mechanism remains to be elucidated.
Calming skin rashes.
Studies in humans have reported the effectiveness of sodium ascorbyl phosphate in the prevention and treatment of acne. It appears to act by reducing significantly the peroxidation of lipids. Squalene, present in sebum, is an unstable molecule that is highly sensitive to oxidation reactions. When it oxidizes under the influence of environmental factors, particularly UV radiation, it is converted into squalene peroxide, a highly comedogenic form. The accumulation of this peroxide is involved in the processes underlying the development of acne (hyperplasia of sebaceous glands, proliferation of keratinocytes, inflammatory response).
In an in vivo study conducted in 2005 (n = 20), SCHEHLMANN V. and colleagues showed that an O/W formulation of sodium ascorbyl phosphate reduced squalene peroxide formation by 30–40%, preventing UVA‑induced oxidation of sebum. Therefore, treating acne with ascorbyl phosphate could help prevent comedogenesis. In addition, some studies also report that sodium ascorbyl phosphate has strong antimicrobial activity and inhibits the growth of Propionibacterium acnes, the main bacterium involved in the development of acne. However, these findings are contradicted by other studies. Further research is therefore needed to clarify the potential role of sodium ascorbyl phosphate as an antibacterial agent.

Références	Cohorte	Conditions expérimentales	Résultats
OHMORI K. & al. (2003)	60 femmes sujettes à l'acné faciale, présentant au moins 10 mais moins de 50 lésions inflammatoires, au moins 10 mais moins de 100 lésions non-inflammatoires et pas plus de 2 lésions nodulo-kystiques	Application deux fois par jour d'une lotion à 5% de phosphate d'ascorbyle de sodium pendant 12 semaines consécutives	Réduction du taux d'acné inflammatoire et non-inflammatoire chez 75% des sujets avec le phosphate d'ascorbyle de sodium, supérieure au gel d'adapalène à 1% (64%)
SCHEHLMANN V. & al. (2005)	60 sujets souffrant d'acné	Application deux fois par jour d'une lotion à 5% de phosphate d'ascorbyle de sodium pendant 12 semaines (visage)	Diminution du nombre de lésions inflammatoires et non-inflammatoires chez 75,9% des patients ayant appliqué pendant 12 semaines la lotion au phosphate d'ascorbyle de sodium contre 60,9% pour ceux ayant utilisé une crème à 5% de peroxyde de benzoyle.
NISHIKAWA T. & al. (2008)	40 personnes présentant de l'acné	Application deux fois par jour d'une lotion à 5% de phosphate d'ascorbyle de sodium pendant 12 semaines	Taux d'efficacité de 78,9% pour la lotion à 5% de phosphate d'ascorbyle de sodium contre 38,9% pour le gel à 1% de clindamycine
NATAKANKITKUL S. & al. (2009)	45 patients présentant une acné faciale de grade II - III, avec 10 à 50 lésions inflammatoires (papules, pustules et nodules)	Application deux fois par jour d'une lotion à 5% de phosphate d'ascorbyle de sodium pendant 8 semaines	Réduction du nombre de lésions inflammatoires de 20,14% et 48,82% après respectivement 4 et 8 semaines d'utilisation.
IKENO H. & al. (2010)	50 sujets atteints d'acné légère à sévère	5% de phosphate d'ascorbyle de sodium	Amélioration de l'acné chez 61% des sujets

Furthermore, a 2017 study (n = 11 women) showed that the combined use of ascorbyl palmitate and SAP in a multiple (O/W/O) emulsion made it possible to reduce facial sebum secretion after 90 days of application, suggesting a potential role of these vitamin C derivatives in the control of seborrhea.

The safety profile of Sodium Ascorbyl Phosphate.

Sodium ascorbyl phosphate is generally considered safe and well tolerated for most skin types, a conclusion shared by the Cosmetic Ingredient Review (CIR) Expert Panel which, after a thorough evaluation of the available scientific literature, determined that the ingredient is safe as used in cosmetic formulations. However, some individuals have reported dryness, itching, flaking, and/or mild erythema. A slight, temporary redness was reported in an animal dermal irritation study following semi-occlusive application, which resolved spontaneously in less than 3 days. Nevertheless, it is still less likely to trigger skin reactions than ascorbic acid. As with any active ingredient, it is therefore recommended to perform a patch test before incorporating it into a skincare routine.

And for pregnant and breastfeeding women, can sodium ascorbyl phosphate be used safely?

The CIR Expert Panel has not issued any specific warnings, although no targeted studies have yet evaluated its safety in these particular populations. Sodium ascorbyl phosphate thus appears to be regarded as a vitamin C derivative compatible with pregnancy and breastfeeding.

Sodium ascorbyl phosphate versus L-ascorbic acid (vitamin C): a functional equivalent?

While sodium ascorbyl phosphate and L-ascorbic acid share the same biological precursor, their respective profiles—with regard to stability, skin penetration, tolerability, and demonstrated clinical efficacy—show notable differences.

Parameters	Sodium ascorbyl phosphate	Vitamin C (L-ascorbic acid)
INCI Name	Sodium Ascorbyl Phosphate	Ascorbic Acid
Stability	More stable, meaning less prone to oxidation and degradation, but still sensitive to light, humidity, and oxygen, requiring appropriate packaging – Should be formulated at a pH > 5, not compatible with acidic environments	Unstable in light and air – Must be formulated at a pH < 3.5
Solubility	Water-soluble (hydrosoluble) – Low skin penetration	Water-soluble (hydrosoluble) – Low skin penetration, optimized by a low pH (< 3.5) to reduce its charge density and favor the use of the non-ionized form
Dosage	Between 0.5% and 15%	Between 8 and 20% (a concentration higher than 20% leads to a decrease in tissue concentrations of ascorbic acid, for unknown reasons)
Tissue elimination half-life	Undefined	Approximately 4 days
Antioxidant protection	Less potent than L-ascorbic acid – No effect on lipid peroxidation	Powerful recognized antioxidant – Exhibits low pro-oxidant activity following the electron transfer reaction – Regenerates oxidized vitamin E molecules
Brightening properties	Combined with the tetrapeptide PKEK	Interferes with the melanogenesis process (reduces the ortho-quinones generated by tyrosinase) – demonstrated on cultured melanocytes and in vivo
Regenerative properties	Not proven – An inconclusive study on wrinkles around the eye area	Essential cofactor for two enzymes (lysyl hydroxylase and prolyl hydroxylase) that enable the stabilization and maturation of collagen fibers; Increases the proliferation and migration of dermal fibroblasts; Stimulates the synthesis of collagen, elastin, and glycosaminoglycans; Modulates keratinocyte differentiation (mechanism of action yet to be elucidated)
Antibacterial properties	Tested on acne-prone skin – In vitro antibacterial activity demonstrated on Propionibacterium acnes, pending confirmation	Demonstrated, but mechanism of action unresolved – No study of a direct effect on Propionibacterium acnes
Anti-inflammatory properties	Unproven	Limits activation of pro-inflammatory signaling pathways (indirect effect)
Moisturizing properties	Unproven	Promotes the synthesis and organization of lipids in the skin barrier and increases formation of the cornified envelope during differentiation
Skin tolerance	Softer on the skin – Suitable for dry and sensitive skin	Likely to cause irritation, tingling, and/or redness – Not suitable for sensitive skin.

Comparison between the characteristics of sodium ascorbyl phosphate versus ascorbic acid.

Although ascorbic acid remains the most extensively tested and validated form of vitamin C, SAP stands out for its stability and excellent skin tolerance.