By directly measuring vitamin C inside human skin, researchers show that diet can boost skin vitamin C content and influence skin structure, while also revealing clear limits to its effects on collagen formation and UV protection.
Letter to the Editor: Improved Human Skin Vitamin C Levels and Skin Function after Dietary Intake of Kiwifruit: A High-Vitamin-C Food. Image Credit: Meomeow / Shutterstock
Vitamin C (ascorbic acid) is widely promoted for supporting collagen production and skin health, yet little is known about how oral intake changes dermal or epidermal ascorbate concentrations. A new study in the Journal of Investigative Dermatology directly examines how skin compartments respond to dietary ascorbate and whether these changes translate into measurable improvements in skin function.
Distinct Roles of Dermal and Epidermal Vitamin C
Vitamin C is present in both skin layers: the collagen-rich dermis and the highly cellular epidermis. As an antioxidant, ascorbate neutralizes ultraviolet-induced free radicals, protects against oxidative stress, and stimulates both fibroblast collagen synthesis and keratinocyte proliferation, processes central to skin thickness and anti-aging benefits.
Challenges of Topical Delivery and Dependence on Active Transport
Topical products must stabilize dissolved ascorbate and deliver it across the stratum corneum, an effective barrier that complicates absorption. Systemically, the skin relies on sodium-dependent vitamin C cotransporters (SVCT1/SVCT2) to actively import circulating ascorbate. Prior research has offered almost no data on dermal vs. epidermal ascorbate content or on functional consequences of dietary supplementation.
Study Goal: Mapping Skin Ascorbate Across Compartments
Researchers quantified ascorbate concentrations in dermis, epidermis, and whole skin from healthy adults. They also conducted a pilot dietary intervention using kiwifruit, providing ~250 mg/day of vitamin C, to test whether increasing plasma ascorbate levels elevate skin ascorbate content and alter skin function outcomes.
Dermal Cells Contain Far More Ascorbate Than Epidermal Cells
Using per-cell measurements of DNA content, investigators found that the epidermis had 11-fold more DNA than the dermis, enabling cellular-level concentration estimates. Dermal fibroblasts contained ~6.4 mM ascorbate, whereas epidermal keratinocytes contained ~0.9 mM, a seven-fold difference. High dermal ascorbate parallels levels in adrenal and brain tissue, where ascorbate acts as an enzymatic cofactor, likely supporting robust collagen synthesis in fibroblasts.
Plasma Vitamin C Strongly Predicts Skin Compartment Levels
Whole-skin, dermal, and epidermal ascorbate concentrations rose proportionally with blood ascorbate. In the kiwifruit supplementation cohort, participants with below-average baseline levels achieved plasma saturation (>60 μM), accompanied by higher dermal ascorbate in biopsy samples. At a second study site, suction-blister sampling showed that increases in plasma ascorbate were mirrored in blister fluid and epidermal blister-roof tissue, confirming active epidermal uptake via SVCT transporters.
Vitamin C Intake Improves Skin Density and Epidermal Proliferation
Kiwifruit supplementation increased skin density from ~0.15 to ~0.23 scanner units, an indicator of greater dermal structural protein content. Epidermal cell proliferation also increased. However, skin elasticity declined by a small (~7%) amount, and UVA-induced oxidative stress protection did not improve. Procollagen type I peptides in blister fluid also did not increase, suggesting that changes in collagen synthesis may be subtle or not captured by this biomarker, despite rising skin density.
Implications for Skin Health and Dietary Supplementation
Findings suggest that dietary vitamin C effectively elevates ascorbate levels across all skin compartments through active transport mechanisms. Improvements in skin density and epidermal renewal may reflect enhanced collagen support or TET-mediated transcriptional regulation, as observed in prior in vitro work. The authors conclude: “Increasing dietary ascorbate intake will result in effective uptake into all skin compartments and will benefit skin function.”