"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, hydrophilic 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 require a low pH to be effective.
Once applied to the skin, sodium ascorbyl phosphate is converted into the active form of pure vitamin C through the action of skin enzymes.
SAP exerts an antioxidant effect, helping to protect skin cells against oxidative damage. Effects on complexion radiance, firmness and the reduction of imperfections have also been reported, although clinical data still need to be strengthened.
Sodium ascorbyl phosphate is a gentler alternative to L-ascorbic acid, particularly suitable for sensitive skin.

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

Put simply, “Sodium Ascorbyl Phosphate” (SAP) refers to a water‑soluble, stable precursor of vitamin C. Indeed, L‑ascorbic acid, although effective, is well known for its instability. It is readily oxidised when exposed to sunlight, to elevated oxygen levels (aerobic conditions), to the presence of metal ions (copper or heavy metals in general catalyse this reaction), to a high pH, and to temperature fluctuations. It first degrades reversibly into dehydro‑L‑ascorbic acid, then proceeds to the irreversible formation of diketogulonic acid, a biologically inactive form, accompanied by a yellowish discolouration of the formulation.

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

It consists of a stabilised sodium salt of L-ascorbic acid, which has also undergone esterification of a phosphate (P) group at position 2 of the ring, thereby protecting the enediol system against oxidation. This stability not only prolongs the product’s shelf-life, but also ensures a constant supply of vitamin C to the skin, maximising its benefits. It also offers formulators greater flexibility in the development of skincare products, as 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 emulsion form 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 based on magnesium ascorbyl phosphate (2%) and L-ascorbic acid (2%). Another study showed that sodium ascorbyl phosphate maintained its stability at nearly 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 odourless
Method of production	Phosphorylation reaction of ascorbic acid with sodium and phosphate compounds
Molar mass	322.05 g/mol
Solubility	Soluble in water and water-based solvents (glycerine, etc.), insoluble in ethanol
Stability	Good stability, superior to that of conventional vitamin C

As a precursor, however, it still needs to be converted into free ascorbic acid in order 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). Indeed, the phosphate group and the salt with which it is associated act in synergy with skin enzymes to split 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 minimises the risk of irritation.

KRISTL J. & 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 numerous beneficial effects on the skin.

Protect the skin against the harmful effects of free radicals.
Just like its counterpart ascorbic acid, sodium ascorbyl phosphate would be expected to exert 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 lesions in mice in vitro is due to the maintenance of normal ascorbic acid levels through the conversion of SAP into ascorbic acid within skin tissues. However, topically applied sodium ascorbyl phosphate has been shown to be less effective than topically applied ascorbic acid in reducing oxidative stress in human skin in vivo, which highlights the importance of enzymatic conversion efficiency when assessing its actual efficacy.
Helps maintain 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 preparation 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.
Further studies, involving a larger number of participants and additional facial areas, will be required to confirm the relevance of sodium ascorbyl phosphate for visible signs of skin ageing. Thus, these initial results indicate that sodium ascorbyl phosphate appears to strengthen the dermal matrix, and to maintain the structure and function of the skin, although the underlying biological mechanism remains to be elucidated.
Soothing skin rashes.
Studies in humans have reported the efficacy 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 oxidises 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 acne development (sebaceous gland hyperplasia, keratinocyte proliferation, inflammatory response).
In a 2005 in vivo study (n = 20), SCHEHLMANN V. and colleagues showed that an O/W formulation of sodium ascorbyl phosphate reduced squalene peroxide formation by 30 to 40%, preventing UVA‑induced oxidation of sebum. Consequently, the treatment of acne with ascorbyl phosphate could help to prevent comedogenesis. Furthermore, some studies also report that sodium ascorbyl phosphate may exert strong antimicrobial activity and inhibit the growth of Propionibacterium acnes, the principal bacterium involved in the development of acne. However, this view is contradicted by other studies. Further research will therefore be useful 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 emulsion (W/O/W) reduced facial sebum secretion after 90 days of application, suggesting a potential role of these vitamin C derivatives in the control of seborrhoea.

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 an in-depth assessment of the available scientific literature, concluded that the ingredient is safe as used in cosmetic formulations. However, some individuals have reported dryness, itching, flaking and/or mild erythema. A slight, transient redness was reported in an animal skin irritation study after semi-occlusive application, which resolved spontaneously in under 3 days. It nevertheless remains less likely to trigger skin reactions than ascorbic acid. As with any active ingredient, it is therefore advisable tocarry out a patch test before incorporating it into a skincare routine.

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

The CIR Expert Panel has not issued any specific warnings, even though no dedicated study has to date assessed 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?

Although sodium ascorbyl phosphate and L-ascorbic acid share the same biological precursor, their respective profiles — in terms of 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 – Formulate at a pH < 3.5
Solubility	Water-soluble (hydrosoluble) – low cutaneous penetration	Soluble in water (hydrosoluble) – Low cutaneous penetration, optimised by a low pH (< 3.5) to reduce its charge density and favour the use of the non-ionised form
Dosage	Between 0.5 and 15%	Between 8 and 20% (a concentration above 20% leads to a decrease in tissue ascorbic acid levels, for reasons that remain unknown)
Tissue elimination half-life	Undefined	Approximately 4 days
Antioxidant protection	Less potent than L-ascorbic acid – No effect on lipid peroxidation	Recognised powerful antioxidant – Exhibits low pro-oxidant activity following the electron transfer reaction – Regenerates oxidised vitamin E molecules
Lightening 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
Regenerating properties	Unproven – An inconclusive study on eye-contour wrinkles	Essential cofactor for two enzymes (lysyl hydroxylase and prolyl hydroxylase) that enable the stabilisation and maturation of collagen fibres; 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 against Propionibacterium acnes, pending confirmation	Demonstrated, but mechanism of action unresolved – no study on a direct effect on Propionibacterium acnes
Anti-inflammatory properties	Unproven	Limits the activation of pro-inflammatory signalling pathways (indirect effect)
Moisturising properties	Unproven	Promotes the synthesis and organisation of barrier lipids in the skin and increases cornified envelope formation during differentiation
Skin tolerance	Softer on the skin – Suitable for dry and sensitive skin	May 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 its excellent cutaneous tolerance.