Vitamin D has long been known to increase calcium absorption for bone health, but recently has been associated with several other health benefits including cancer prevention, autoimmune disease and cardiovascular disease [1]. Vitamin D₃ is synthesized in the skin and lasts twice as long as vitamin D ingested from the diet [2]. 7-Dehydrocholesterol is converted to cholecalciferol (vitamin D₃) in the skin when exposed to UVB radiation and then hydroxyalated to 25 hydroxyvitamin D₃ (25(OH)D) and then further hydroxyalated to 1,25 dihyroxyvitamin D₃ (calcitriol). Calcitriol is known to modulate proliferation and differentiation of keratinocytes [3] and analogs of vitamin D₃ have been used to normalize hyperproliferation present in psoriatic skin. Research has been conducted on serum vitamin D₃ (25(OH)D) levels, but little is known about skin levels of vitamin D₃ and even less is known about its topical benefits in normal skin. There has been research showing that vitamin D serum deficiency could be related to severity of atopic dermatitis, as skin barrier dysfunction [4]. This investigation was conducted to examine the association between serum 25-hydroxyvitamin D (25(OH)D) levels and stratum corneum hydration levels based on conductance measurements. We further explored the effect of topical application of cholecalciferol (vitamin D₃) on dry skin.

Serum 25(OH)D levels ranged from 4.8 to 48.8 ng/mL with 19 subjects classified as deficient (<10 ng/mL), 53 as insufficient (10–30 ng/mL) and 11 subjects as sufficient (>30 ng/mL) based on published literature [5]. A non-invasive measurement was used to assess stratum corneum water content. This instrument measures the high frequency conductance of skin which correlates with skin surface water content [6]. Figure 1 shows that as serum 25(OH)D levels increased, skin conductance also increased (p = 0.02, ANOVA). Mean ± SEM values were 7.9 ± 0.69 μS, 10.7 ± 0.72 μS and 13.0 ± 1.57 μS for deficient, insufficient and sufficient subjects respectively.

In the subset of subjects with non-sufficient vitamin D₃ levels, (n = 61 including those deficient and insufficient), the addition of vitamin D₃ to the moisturizing formulation significantly improved (p < 0.05 Wilcoxon Signed Rank Test) dryness both in clinical grading and by self assessment over the vehicle at 3 weeks (Figure 2b,d). The look of flakes was also statistically significant (p < 0.05 Wilcoxon Signed Rank Test) by self assessment (Figure 2c). Vitamin D₃ addition to a topical moisturizer increased skin conductance over the vehicle, but did not reach statistical significance in this study utilizing ANOVA (Figure 2a). There was a significant increase in conductance when comparing before and after treatment in both groups. This is probably due to the high emolliency of the vehicle used.

The number of patients with non-sufficient vitamin D₃ levels may be rising in the population [7]. This could be due to the low consumption of vitamin D fortified foods and the increased awareness of skin cancer through UVB exposure [1]. There are a few demographic groups that are more at risk for being vitamin D₃ (25(OH)D) insufficient including females, African Americans, and those with higher body mass index [8]. It is estimated that 3 out of every 4 Americans [9] and 1 billion people worldwide are insufficient [1].

The current study demonstrates a relationship between a subject’s vitamin D₃ (25(OH)D) serum levels and their innate skin hydration state. These results in Figure 1 demonstrate that as vitamin D₃ serum level decreases, skin conductance measurements also decreased indicating that the subject’s skin was drier. Furthermore, topical application of vitamin D₃ was shown to enhance skin conductance reflected in an increase in moisture levels in the skin for subjects with non-sufficient levels of vitamin D₃. Interestingly, there is a strong seasonal variation in 25-hydroxyvitamin D levels with significantly lower values in the winter which coincides with wintertime dry skin [10]. The onset of winter xerosis results in a transient but acute perturbation of filaggrin proteolysis [11] and vitamin D analogs have been shown to increase filaggrin expression [12]. Vitamin D₃ (cholecalciferol) may have potential role in increasing filaggrin and NMF. These results suggest that diminished 25-hydroxyvitamin D levels may be a factor contributing to xerotic skin. Other research supports the hypothesis that vitamin D₃ serum deficiency may be related to dry skin states including atopic dermatitis [4].

Overall these findings suggest a relationship between serum vitamin D₃ (25(OH)D) levels and hydration of the stratum corneum and further demonstrate the skin moisture benefit from topical application of vitamin D₃. Future studies could explore optimal levels of skin vitamin D and the relationship with serum levels, as well as mechanistic studies to understand the influence of vitamin D₃ on xerotic skin.