Daily consumption of two SunGold kiwifruit for 8 weeks increased human skin vitamin C levels and improved skin density and epidermal cell proliferation in patients with low baseline plasma ascorbate status, according to a pilot dietary intervention study published in the Journal of Investigative Dermatology.
Whole-skin ascorbate concentrations rose from 99 nmol/g tissue to 150 nmol/g tissue following the intervention, which delivered approximately 250 mg vitamin C daily. Skin density, measured via high-resolution ultrasound, increased from approximately 0.15 scanner units at baseline to approximately 0.23 following intervention. Epidermal cell proliferation, assessed by Ki-67 staining intensity, rose from 21 to 27 intensity units.
The study, led by Juliet M. Pullar, PhD, of the Mātai Hāora Centre for Redox Biology and Medicine at the University of Otago in Christchurch, New Zealand, and colleagues, represents the first comprehensive investigation establishing the relationship between plasma and skin ascorbate levels across major skin compartments. Only one prior study had measured skin ascorbate following supplementation, and none had assessed skin ascorbate content in association with skin functions.
Compartmental Distribution and Cellular Concentrations
Cross-sectional analysis of skin samples from 38 healthy patients undergoing elective surgery revealed that epidermal ascorbate levels were higher than dermal levels, with an epidermal-to-dermal ascorbate ratio of 2.0 (range, 0.8 to 7.2). Mean concentrations were 325 nmol ascorbate/g tissue in epidermis vs 194 nmol/g tissue in dermis.
When standardized against DNA content as a measure of cellularity, dermal cells—predominantly fibroblasts—contained 6.4 mM ascorbate (range, 1.1 to 11.2 mM). Epidermal cells, predominantly keratinocytes, contained approximately seven-fold less at 0.9 mM (range, 0.3 to 1.9 mM).
The researchers wrote: "To our knowledge, this study provides a direct comparison of dermal and epidermal cellular ascorbate concentrations. Higher ascorbate concentrations in fibroblasts likely support collagen synthesis. Similar concentrations are reported for brain and adrenal cells, other known sites of ascorbate enzyme cofactor activity."
Approximately 20% of ascorbate was present as dehydroascorbate, which the researchers attributed to sample processing despite fast handling time (less than 15 minutes) and optimized extraction protocols. They noted that "dehydroascorbate is typically barely detectable in cells and plasma."
Plasma-Skin Correlations
Ascorbate levels in all skin compartments were positively correlated with plasma levels. A positive linear relationship was observed between epidermal and plasma ascorbate. Changes in plasma, blister roof, and blister fluid ascorbate were strongly positively correlated, indicating active uptake into the epidermis in response to variation in plasma levels.
The researchers concluded that the epidermis "is particularly responsive to plasma ascorbate availability."
Notably, patients with saturating plasma ascorbate status at baseline did not register measurable increases, prompting a per-protocol analysis restricted to those whose plasma ascorbate concentration increased following intervention (n = 7).
Study Design and Methodology
The investigation comprised a cross-sectional analysis and a dietary intervention conducted at two sites. At site I (Christchurch, New Zealand), 12 patients with fasting plasma ascorbate of 45 micromolar or less underwent whole-skin punch biopsies at weeks 0 and 8. At site II (Hamburg, Germany), 12 patients with low baseline plasma vitamin C underwent suction blister sampling to obtain epidermal tissue and interstitial fluid, along with skin function assessments.
Ascorbate was measured using reverse-phase high-performance liquid chromatography. Epidermal and dermal skin layers were carefully separated and analyzed independently.
Skin elasticity showed a small (roughly 7%) decrease following intervention, and no changes were observed in protection against UVA irradiation or procollagen type I peptide levels in interstitial blister fluid.
Mechanistic Considerations
The researchers proposed that the observed functional changes "may reflect optimization of ascorbate-mediated support for collagen synthesis in tissue fibroblasts and for skin epidermal cell proliferation and differentiation through the promotion of TET-mediated gene transcription regulation, as recently demonstrated in vitro."
They emphasized that in vivo delivery of ascorbate to skin is tightly regulated by active transport from plasma through sodium-dependent vitamin C cotransporters SVCT1 and SVCT2, rather than passive diffusion.
"We suggest that increasing dietary ascorbate intake will result in effective uptake into all skin compartments and will benefit skin function," the researchers concluded.
The authors declared having no competing interests.