Creative Touch Permanent Makeup and Aesthetics

Vitamin D is unusual among vitamins in that the skin does not merely use it but manufactures it in full. The complete UVB-induced pathway from 7-dehydrocholesterol to calcitriol occurs in keratinocytes without requiring hepatic or renal conversion, meaning the skin has its own local vitamin D signalling system operating independently of systemic status. Calcitriol acts through VDR/RXRα heterodimers, sharing the RXR partner that retinoids require, to drive the differentiation programme that produces a competent stratum corneum, to upregulate cathelicidin (LL-37) for pathogen defence, and to regulate TLR2 expression in wounded skin for innate immune protection. Deficiency impairs all three simultaneously, and the consequences are observable in the characteristic presentations of both atopic dermatitis and impaired wound recovery.

Vitamin D is a fat-soluble secosteroid – structurally a steroid hormone rather than a conventional vitamin – that exists in multiple forms across the synthesis cascade from skin precursor to biologically active metabolite. Vitamin D₃ (cholecalciferol) is the form synthesised in skin and found in animal-source foods including grass-fed tallow, eggs, and oily fish. Vitamin D₂ (ergocalciferol) is the plant and fungal-source form. Both require two hydroxylation steps to become biologically active: first in the liver to 25-hydroxyvitamin D (calcifediol, the circulating storage form measured in blood tests), then in the kidney (and critically, also in the skin itself) to 1,25-dihydroxyvitamin D₃ (calcitriol), the hormonal form that binds the vitamin D receptor (VDR). ^[2]

The Skin as Both Producer and Target

The skin’s relationship with vitamin D is unique among human tissues. The complete UVB-induced biosynthetic pathway – from 7-dehydrocholesterol (7-DHC) through previtamin D₃, vitamin D₃, 25(OH)D, and finally to calcitriol – occurs entirely within keratinocytes under physiological conditions. ^[6] No other tissue completes this full cascade autonomously. UVB radiation at wavelengths 290–315 nm (peak conversion at 295–297 nm) photoisomerises 7-DHC in the stratum spinosum and stratum basale to previtamin D₃, which then thermally isomerises to vitamin D₃ within hours. carta.anthropogeny.org The keratinocyte then performs the CYP27B1 (1α-hydroxylase) conversion to calcitriol locally, creating an autocrine and paracrine signalling loop within the epidermis that operates independently of circulating 25(OH)D levels.

This local synthesis loop matters clinically. A client with adequate systemic vitamin D status may still have compromised cutaneous vitamin D signalling if their UV exposure history is minimal; the skin’s local calcitriol production is driven by UVB availability, not systemic repletion. Conversely, regular modest UV exposure maintains local VDR signalling in keratinocytes even when systemic 25(OH)D hovers at the lower end of the reference range.

VDR-Mediated Keratinocyte Differentiation

Calcitriol acts through the vitamin D receptor (VDR), a ligand-activated nuclear receptor expressed in keratinocytes, dermal fibroblasts, sebocytes, and skin-resident immune cells including dendritic cells and macrophages. VDR functions as a heterodimer with retinoid X receptor alpha (RXRα), binding to vitamin D response elements (VDREs) in target gene promoters to drive transcription of differentiation, antimicrobial, and immune-regulatory programmes. ^[1] The shared RXRα partner with retinoid signalling is significant: RAR/RXR and VDR/RXR compete for the available RXRα pool in keratinocytes, meaning vitamin D and retinoid signalling interact at the heterodimerisation step rather than operating as fully independent pathways. [pmc.ncbi.nlm.nih.gov]

VDR is also a signalling node in the gut–skin axis. Alongside aryl hydrocarbon receptor (AHR) signalling and immune activation pathways, VDR signalling creates a channel through which gut microbiota composition influences skin barrier function and innate immune tone – meaning that systemic vitamin D status is not simply a skin-local variable but part of the broader microbiome–host communication architecture. This connection is developed further in the Vitamin D, LL-37, and the Skin Microbiome section below.

In keratinocytes, VDR activation drives the differentiation programme that builds a competent stratum corneum. Calcitriol at subnanomolar concentrations upregulates involucrin, loricrin, transglutaminase-1, filaggrin, and cornified envelope proteins – the structural components of the cornified envelope that the terminal differentiation programme assembles. ^[1] It simultaneously suppresses basal keratinocyte proliferation at concentrations above 1 nM, creating the differentiation-favouring balance that produces an organised, well-stratified epidermis rather than a hyperproliferative one.

Loss of VDR function, whether through genetic deletion in animal models or through severe vitamin D deficiency, disrupts this differentiation programme measurably: basal layer hyperproliferation occurs, upper layer differentiation is disorganised, filaggrin expression is reduced, and barrier permeability increases. ^[1] The downstream effects on barrier competence are mechanistically equivalent to the filaggrin-deficiency pathway established in the Filaggrin entity: reduced NMF production, impaired stratum corneum acidification, and a compromised barrier that cannot exclude allergens or maintain hydration.

Cathelicidin, Innate Immunity, and Wound Defence

Calcitriol is one of the primary inducers of cathelicidin antimicrobial peptide (CAMP/LL-37) in keratinocytes, operating through VDR/RXRα binding to the CAMP gene promoter. ^[10] Cathelicidin LL-37 is a broad-spectrum antimicrobial peptide that directly kills bacteria, fungi, and enveloped viruses at the skin surface; it also functions as an immunomodulatory signal that bridges innate and adaptive immune responses.

The wound healing context is particularly precise. A 2007 Journal of Clinical Investigation study demonstrated that wounding upregulates CYP27B1 (the 1α-hydroxylase) in keratinocytes, increasing local calcitriol production specifically at the wound site. jci.org This locally elevated calcitriol then induces TLR2 (Toll-like receptor 2) expression in keratinocytes – enabling them to recognise bacterial components and respond with cathelicidin production – and directly upregulates CAMP transcription. The result is a vitamin D-dependent innate immune activation specifically at sites of barrier disruption: the skin amplifies its antimicrobial defence precisely where and when it is most needed.

Clients with low vitamin D status have a measurably attenuated version of this wound-site immune activation. Their keratinocytes at the wound margin produce less cathelicidin in response to the same bacterial challenge; a direct consequence of insufficient calcitriol to drive VDR/RXRα-mediated CAMP transcription. The clinical implication is not theoretical: impaired wound healing and increased infection susceptibility in vitamin D deficiency are mechanistically explained by this pathway.

Vitamin D, LL-37, and the Skin Microbiome

LL-37 production in keratinocytes is regulated by two independent upstream signals that converge on the same output: the VDR/calcitriol pathway described above, and TLR2 activation by commensal Staphylococcus epidermidis. In healthy skin with adequate vitamin D status and an intact commensal community, these two signals reinforce each other – LL-37 levels are sustained by both the endocrine pathway and the microbial one simultaneously. When either fails, the remaining pathway provides partial compensation. When both fail together, LL-37 depletion becomes clinically significant.

Vitamin D deficiency removes the VDR/CAMP pathway contribution to LL-37 at baseline – before any microbial disruption occurs. If the same client also has dysbiosis with reduced S. epidermidis populations, the TLR2-driven pathway for LL-37 is simultaneously attenuated. The deficit is compounded from two independent directions, neither of which is visible in the other’s data. A serum 25(OH)D measurement tells you nothing about S. epidermidis colonisation status; a microbiome assessment tells you nothing about calcitriol availability. Clinically, they need to be considered together when LL-37-dependent protection is the concern.

S. aureus adds a third layer to this dynamic. Its virulence factors – α-toxin and δ-toxin – actively degrade LL-37 that has already been produced, directly eliminating the innate defence that would otherwise limit its colonisation. In a client with low vitamin D (reduced LL-37 synthesis) and early dysbiosis (reduced S. epidermidis TLR2 stimulation), the LL-37 baseline is already depressed before S. aureus begins degrading it. The combination creates the conditions for S. aureus to establish rapidly and persistently – explaining why the clinical presentations of vitamin D deficiency and dysbiosis overlap so substantially in atopic and barrier-compromised skin.

The gut–skin axis adds a further dimension. VDR signalling is one of three established channels – alongside AHR signalling and immune activation – through which gut microbiota composition influences skin barrier function and innate immune tone. Gut microbial diversity affects systemic vitamin D metabolism, and VDR expression in intestinal and immune cells is modulated by gut microbiota composition. This bidirectional relationship means that optimising skin vitamin D status and supporting gut microbiome health are not separate interventions operating on separate systems – they interact through the VDR signalling axis that both depend on.

Vitamin D and Atopic Dermatitis

AD patients have measurably lower VDR expression and vitamin D binding protein (VDBP) levels in skin biopsies compared to non-atopic controls, alongside reduced cathelicidin, HBD-2, and HBD-3 expression in lesional skin. ^[7] The vitamin D/AD relationship operates through the same VDR-differentiation pathway: reduced VDR signalling → impaired filaggrin expression → barrier disruption → allergen sensitisation and S. aureus colonisation – the dysbiosis loop established in the Skin Microbiome entity, with vitamin D deficiency as an upstream contributor to the conditions that enable it.

The supplementation evidence in AD is suggestive but not yet definitive. A 2013 double-blind RCT found oral vitamin D supplementation in AD subjects increased cathelicidin expression in lesional skin, with a weak negative correlation between vitamin D change and IL-13 expression consistent with the mechanistic prediction but not achieving clinical severity score improvement in this sample size. ^[5] The mechanistic evidence is strong; the clinical outcome evidence in supplementation trials is mixed, likely reflecting the heterogeneity of AD aetiology and the ceiling effect of supplementation in non-severely deficient subjects.

Deficiency Risk and the UK Context

The UK’s latitude (50–59°N) means that UVB at the wavelengths required for vitamin D synthesis reaches the skin surface only between approximately April and September, and only during midday hours. From October through March, cutaneous synthesis is negligible regardless of time spent outdoors. ^[1] The UK’s current position in the northern latitude makes vitamin D deficiency structurally common. Dietary vitamin D from food alone is difficult to achieve at adequate levels for most adults.

Public Health England recommends 10 µg (400 IU) daily supplementation for all UK adults during autumn and winter – a dose established to prevent rickets and maintain bone mineral density, but increasingly recognised as insufficient for optimal immune, barrier, and cellular function. A 2024 review in Nutrients argued for 2000 IU (50 µg) daily as an efficient and safe approach to prevent and treat vitamin D deficiency in the general adult population, supported by the Endocrine Society clinical practice guideline. ^[9] A 2025 evidence review went further, identifying 2000–4000 IU daily as the range needed to achieve serum 25(OH)D levels associated with protection against a broader range of adverse health outcomes, with 4000–6000 IU considered for higher-risk groups. ^[4] The 400 IU recommendation reflects a minimum threshold, not an optimal one. In the UK’s northern latitude context, the gap between the two is clinically meaningful.

The at-risk groups for more significant deficiency relevant to skin outcomes include: older adults (reduced 7-DHC in the epidermis with age, reduced conversion efficiency), darker skin tones (melanin competes with 7-DHC for UVB photons, requiring longer exposure for equivalent synthesis), and clients with predominantly indoor occupations or consistent high-SPF use year-round. The clinical relevance is that these are not rare presentations but describe a substantial proportion of the typical aesthetic clinic demographic.

Vitamin D in Tallow and Animal-Source Foods

Grass-fed tallow contains vitamin D₃ and its precursors from the animal’s own cutaneous synthesis and dietary intake. Unlike vitamin B12, vitamin D₃ is fat-soluble and survives the rendering process at meaningful concentrations – it is not degraded by heat in the way water-soluble vitamins are. Tallow therefore genuinely contributes a small but chemically stable vitamin D₃ dose as part of its fat-soluble vitamin profile, alongside vitamins E and K. The concentration is modest and variable depending on the animal’s sun exposure and diet, but the presence is real and chemically defensible – a distinction from the B12 situation worth noting explicitly.