By directly measuring vitamin C in human skin, the researchers show that diet can enhance 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 food high in vitamin C. Image credit: Meomeow / Shutterstock
Vitamin C (ascorbic acid) is widely promoted to support collagen production and skin health, yet little is known about how oral intake alters dermal or epidermal concentrations of ascorbate. A new study in Journal of Investigative Dermatology directly examines how skin compartments respond to dietary ascorbic 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 layers of the skin: the collagen-rich dermis and the highly cellular epidermis. As an antioxidant, ascorbate neutralizes UV-induced free radicals, protects against oxidative stress, and stimulates fibroblast collagen synthesis and keratinocyte proliferation, processes central to skin thickness and antiaging benefits.
Challenges of local delivery and reliance on active transport
Topical products must stabilize dissolved ascorbate and deliver it to the stratum corneum, an effective barrier that complicates absorption. Systemically, the skin relies on sodium-dependent vitamin C cotransporters (SVCT1/SVCT2) to actively introduce ascorbate into circulation. Previous research has provided almost no evidence for dermal versus epidermal ascorbate content or for the functional consequences of dietary supplements.
Aim of the study: Mapping of ascorbic skin in all compartments
The researchers quantified ascorbate concentrations in the 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 increases skin ascorbate content and alters skin function outcomes.
Dermal cells contain much more ascorbate than epidermal cells
Using cell-by-cell measurements of DNA content, the researchers found that the epidermis had 11 times more DNA than the dermis, allowing estimates of concentration at the cellular level. Dermal fibroblasts contained ∼6.4 mM ascorbate, whereas epidermal keratinocytes contained ∼0.9 mM, a sevenfold difference. High skin ascorbate parallels levels in adrenal and brain tissue, where ascorbate acts as an enzymatic cofactor, possibly supporting robust collagen synthesis in fibroblasts.
Plasma vitamin C strongly predicts skin compartment levels
Whole skin, skin, and epidermis ascorbate concentrations increased in proportion to blood ascorbate. In the kiwifruit supplementation group, participants with below-average baseline levels achieved plasma saturation (>60 μM), accompanied by higher skin ascorbate in biopsy specimens. At a second study site, suction-cell sampling showed that increases in plasma ascorbate were reflected in cell fluid and cell-roof epidermal 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 was also increased. However, skin elasticity was reduced by a small percentage (~7%) and protection against UVA-induced oxidative stress was not improved. Type I procollagen peptides in the cell fluid also did not increase, suggesting that changes in collagen synthesis may be subtle or not captured by this biomarker, despite increased skin density.
Implications for skin health and dietary supplements
The findings suggest that dietary vitamin C effectively increases ascorbate levels in 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 previous in vitro work. The authors conclude: “Increasing dietary intake of ascorbate will result in efficient uptake in all skin compartments and benefit skin function.”
