Advances in Vitamin D encapsulation: from lipid formulations to polymeric nanoparticles

1 Introduction

Vitamins are essential micronutrients required for the regulation and integration of physiological and biochemical processes, playing a crucial role in maintaining homeostasis and coordinating various bodily functions (Papadopoulou et al., 2025). Vitamins are categorised as fat-soluble and water-soluble. Fat-soluble Vitamins play critical roles in human physiology, including heart health, immune function, vision, and coagulation (Hussain et al., 2024). In this regard, Vitamin D, which is classified as a fat-soluble Vitamin, is involved in the function of vital organs, such as the kidneys, intestinal mucosa, and bones, to regulate calcium and phosphate metabolism (Awuchi et al., 2020). Vitamin D is an active ingredient (Papadopoulou et al., 2025). More specifically, studies are showing the protective role of Vitamin D against cancer (Seraphin et al., 2023), cardiovascular diseases (Thompson et al., 2023), obesity (Krajewska et al., 2022), autoimmune diseases (Wimalawansa, 2023; Adorini et al., 2008), influenza (Grant et al., 2020), and type-2 diabetes (Pittas et al., 2019). Over the last decade, more than 300,000 papers have been published on Vitamins (Mishra, 2023) and over 1/3 of these publications specifically mention Vitamin D. In addition, the global Vitamin D supplements market is estimated to be a $1.90 billion industry and is expected to grow to $2.40 billion by 2030 (Gupta, 2022; Spherical Insights, 2024). Vitamin D is a popular supplement, with data suggesting it accounts for a significant portion of the Vitamin supplement market, with some studies indicating it is the most popular single Vitamin supplement, with 38% of Vitamin supplement users taking Vitamin D (MINTEL, 2020).

In general, exposure to sunlight is the primary way the body generates Vitamin D (Razzaque, 2018). Lack of sun exposure in countries like those in northern Europe and Arctic regions contributes to Vitamin D deficiency (Mosekilde and Holick, 2010). However, even in countries with abundant sunlight, such as Australia, deficiency in this key Vitamin can be experienced due to poor dietary intake and also through extensive use of suncreams (Mohammad Alayed Albarri et al., 2022). These creams protect the skin against UV-associated damage and cancer, and consequently they limit UV-mediated pre-Vitamin D3 photolysation from 7-dehydrocholesterol (Holick, 2014).

Although Vitamin D occupies a prominent position in the biomedical literature, with over 100,000 publications addressing its physiological and therapeutic roles, it remains comparatively underexploited from a pharmaceutical formulation standpoint, with few marketed products beyond conventional oral preparations despite the well-documented health consequences of its deficiency. The majority of existing reviews on Vitamin D continue to centre on its biological and clinical significance, including its skeletal and extra-skeletal effects, immunomodulatory properties, and the ongoing debate surrounding updated clinical practice guidelines. While a number of reviews have addressed individual aspects of Vitamin D delivery, such as nanoencapsulation for food fortification or stability in beverage matrices (Hussain et al., 2024; Vieira and Souza, 2022), a structured and critical comparison of the principal encapsulation platforms in relation to their formulation performance and suitability for specific administration routes is lacking. This review addresses this gap by providing a formulation-focused critical analysis of the major encapsulation strategies employed for Vitamin D delivery, namely, emulsions, micelles, lipid nanoparticles, and polymeric nanoparticles, evaluating each in terms of the physicochemical challenges outlined above. In doing so, this review further examines how the choice of encapsulation strategy determines the suitability of a given formulation for oral, dermal, or transdermal administration, with the aim of identifying the most promising platforms for future development and clinical translation of Vitamin D delivery systems.

1.1 Biosynthesis of vitamin D

Vitamin D has two main biologically relevant forms: Vitamin D₂ (ergocalciferol) and Vitamin D₃ (cholecalciferol), Figure 1 (Chen et al., 2021). Vitamin D₃ is synthesised in the human skin from 7- dehydrocholesterol (proVitamin D₃) after exposure to UV-B. Vitamin D₂ is not naturally produced in humans but can be manufactured synthetically from ergosterol in yeast or fungi and is less common in commercial preparations (Mohammad Alayed Albarri et al., 2022; Yasuda et al., 2017). Unlike Vitamin D₃, which is primarily animal derived, Vitamin D₂ is plant-derived, making it a suitable choice for those adhering to plant-based diets (Benedik, 2022). In addition, through a simple internet search, Vitamin D₂ is approximately 18.5% cheaper than Vitamin D₃.

The main structural difference between the two forms lies within the side chains (Figure 1). Vitamin D₃ is more active than Vitamin D₂ (Tripkovic et al., 2012; Seamans and Cashman, 2009) due to its structure leading to stronger interactions with the Vitamin D binding protein (DBP) (Seamans and Cashman, 2009). DBP interacts with the majority (approximately 85%–88%) of circulating Vitamin D₃ in the bloodstream, making it the primary transporter of Vitamin D metabolites (Chun et al., 2019). Owing to this more robust interaction, Vitamin D3 demonstrates better stability and a longer half-life than Vitamin D₂. Vitamin D₃ can also be absorbed more effectively than Vitamin D₂ (Houghton and Vieth, 2006) due to a more efficient conversion to its active form in the liver (Tripkovic et al., 2012). Studies have consistently shown that Vitamin D3 supplements lead to higher levels and more sustained circulating concentrations of 25-hydroxyvitamin D in the blood compared to Vitamin D2 supplementation at the same dosage (van den Heuvel et al., 2024). Additionally, Vitamin D3 tends to maintain elevated Vitamin D levels for a longer period compared to Vitamin D2, meaning less frequent supplementation may be needed (van den Heuvel et al., 2024).

When administered orally or topically, Vitamin D can be metabolised by the liver, skin (DeLuca, 2004) and other tissues (Bikle, 2014). These tissues, including the pancreas and immune cells containing the VDR, produce Vitamin D3 from 7-dehydrocholesterol when exposed to ultraviolet light (Kitson and Roberts, 2012). The first enzymatic hydroxylation is carried out in the liver, where a mitochondrial hydroxylase enzyme introduces a hydroxyl group at the C-25 carbon position, to produce 25-hydroxy Vitamin D [25(OH)D], the inactive storage form (Figure 2) (Bikle, 2014). Following that, 25-hydroxy Vitamin D is transported to the kidney for a second hydroxylation at the 1 carbon position, converting it to 1,25-dihydroxy cholecalciferol [1,25(OH)₂D], the bioactive form of Vitamin D (Jones et al., 2011). This conversion is predominantly facilitated by the enzyme CYP27B1, which is expressed in the kidney and other tissues, including epithelial cells, immune cells, and the parathyroid gland (Jones et al., 2011).

The renal synthesis of 1,25-dihydroxyVitamin D [1,25(OH)₂D] is tightly regulated: it is stimulated by parathyroid hormone (PTH) and inhibited by calcium, phosphate, and fibroblast growth factor 23 (FGF23) (Bikle, 2014). In keratinocytes and immune macrophages, this conversion process is regulated differently (He et al., 2026). Specifically, cytokines are crucial in managing local activation, enabling fine-tuned Vitamin D production based on the inflammatory state (Lopez et al., 2021; Bishop et al., 2021).

Regardless of their structural differences, both forms bind to the Vitamin D Receptor (VDR) to initiate downstream signalling (Chen et al., 2021). The VDR is a member of the nuclear receptor family of transcription factors (Takeyama et al., 1999). VDR mediates the absorption of calcium and phosphate from the intestines, regulates genes involved in cell proliferation, and is involved in modulating immunity against infections and autoimmune diseases (Usategui-Martín et al., 2022). The VDR, alongside vitamin D-metabolising enzymes, is present in immune cells such as antigen-presenting-cells, T cells, B cells, and monocytes (Daryabor et al., 2023). The receptor binds to the active forms of Vitamin D2 and Vitamin D3 with high affinity, within a specific ligand binding domain. This binding induces VDR to form a heterodimer with the retinoid X receptor (RXR) and bind to Vitamin D Response Elements (VDREs) on DNA, thereby regulating target gene expression (Haussler et al., 2011).

1.2 Importance of Vitamin D

While rickets and osteomalacia were identified as early as the 1600s the nutritional factor responsible, Vitamin D, was not discovered until the early 20th century (Jones, 2018). Initial epidemiological observations in the 1800s linked higher rickets incidence to urban, sun-deprived environments, leading to the historical use of cod liver oil as an empirical treatment (Mohr, 2009). Edward Mellanby demonstrated that cod liver oil cured the disease, leading to the identification of Vitamin D as a crucial nutrient (Holick, 2006). Subsequent elucidation of its chemical structure in the 1930s classified Vitamin D as a secosteroid synthesised photochemically in the skin (Mishra, 2023). During the 1940s and 1960s, research significantly advanced our understanding of Vitamin D’s role in calcium absorption and bone health, revealing that Vitamin D deficiency could lead to conditions beyond rickets, including osteoporosis and other health issues (Holick, 2014). Since the 1970s, studies have shown that Vitamin D plays a role in immune function, cardiovascular health, and even mental health. Additionally, the importance of maintaining adequate Vitamin D levels for overall health is now widely recognised (Jones, 2022).

Extensive genetic, molecular and cellular research indicates that Vitamin D signalling exerts diverse effects beyond skeletal health (Bouillon et al., 2019; Giustina et al., 2023). While skeletal benefits of Vitamin D are well-established, particularly the role in calcium absorption and bone health, extraskeletal effects are primarily supported by observational studies (Bouillon et al., 2019; Giustina et al., 2023). Furthermore, interventional studies consistently show that Vitamin D supplementation primarily benefits individuals with deficient baseline levels (Bouillon et al., 2022). Conversely, those who are already Vitamin D replete experience little to no additional clinical improvement (Singh et al., 2025).

This wide-ranging biological activity stems from the interaction between Vitamin D and the immune system, and axis that has been recognised for over three decades (Lagishetty et al., 2011). Beyond classical bone maintenance (Laird et al., 2010; Rishi Ravindra Naik and Selomulya, 2024), Vitamin D signalling demonstrates significant anti-inflammatory (Fenercioglu, 2024), antimicrobial (White, 2022), and antioxidant (Wiseman, 1993) properties. Consequently, its therapeutic potential is being actively explored across diverse metabolic and chronic pathologies, including cardiovascular diseases (Thompson et al., 2023), diabetes (Pittas et al., 2019) and dementia (Ghahremani et al., 2023). Given the presence of the VDR in epithelial and cutaneous tissues, particular attention has focused on its role in dermatological homeostasis, where it modulates pathways involved in wound healing (Reins, 2016; Bikle, 2023; Wu et al., 2024; Ding et al., 2016), psoriasis (Barrea et al., 2017; Soleymani et al., 2015), atopic dermatitis (Hidayati et al., 2022; Park et al., 2023; Sanmartin et al., 2020; van der Schaft et al., 2016), vitiligo (Karagüzel et al., 2016), acne (Ruikchuchit and Juntongjin, 2024; Alhetheli et al., 2020; Lim et al., 2016; Iqbal et al., 2023), and rosacea (Algarin et al., 2024; Mao et al., 2023).

1.2.1 Role of Vitamin D in bone health

Vitamin D plays a critical role in maintaining bone health by regulating calcium and phosphate homeostasis, which are essential for bone formation, mineralisation, and remodelling (Laird et al., 2010). The active form of Vitamin D, calcitriol (1,25-dihydroxyVitamin D3), facilitates intestinal absorption of these minerals while reducing their renal excretion and directly modulating osteoblast and osteoclast function during bone remodelling (Christakos et al., 2011; Verlinden and Carmeliet, 2021). Low levels of 1,25(OH)₂D₃ compromise this homeostatic loop, precipitating systemic mineral deficits that manifest clinically as rickets in children, osteomalacia in adults, and accelerated osteoporosis with elevated fracture risk (Wang H. et al., 2017). Concomitant skeletal muscle weakness frequently accompanies these mineral deficiencies due to the role of calcium signalling in myofibrillar function (Dawson-Hughes, 2017).

1.2.2 Role of Vitamin D on the immune system

Overall, Vitamin D provides beneficial effects on immune function, particularly in relation to autoimmune conditions, by mitigating inflammation and promoting immune tolerance (Prietl et al., 2013). This systemic regulation occurs due to the diverse expression of VDR across leukocyte populations, including B and T lymphocytes, monocytes, dendritic cells, and macrophages (Kamen and Tangpricha, 2010; Martineau et al., 2007). Consequently, Vitamin D modulates both innate and adaptive arms of the immune system, presenting a viable candidate for host-directed therapy (HDT) paradigms that reinforce host defences rather than targeting pathogens directly (Aranow, 2011).

Within the innate immune response, in vivo models and clinical data demonstrate that Vitamin D supplementation upregulates type I interferon signalling (Reid et al., 2023), promotes T-cell proliferation and interleukin-2 production during immunosuppression (Wang Z. et al., 2017), and correlates with elevated cathelicidin concentrations that protect against acute respiratory infections (Laaksi et al., 2024). Simultaneously, Vitamin D drives adaptive immunity toward a tolerogenic phenotype. In vitro mechanisms reveal that 1,25-dihydroxyVitamin D3 primes dendritic cells into a tolerogenic state, which subsequently alters T-cell differentiation by suppressing pro-inflammatory Th1 and Th17 lineages while expanding regulatory T cells (Tregs) (Ferreira et al., 2014). This transition, alongside the stimulation of local mast cells, upregulates anti-inflammatory cytokines like interleukin-10 (IL-10) to control localized inflammation and promote tissue homeostasis (Bishop et al., 2021).

1.2.3 Antimicrobial role of Vitamin D

Regarding antimicrobial properties, substantial evidence indicates that Vitamin D can inhibit the growth of various pathogenic bacteria, including Staphylococcus aureus, Streptococcus pyogenes, Klebsiella pneumoniae, and Escherichia coli (Youssef et al., 2011). Vitamin D functions as an adjunctive therapy in bacterial infections, particularly in tuberculosis, sepsis, and respiratory infections (Martineau et al., 2007; Liu et al., 2007). One of its primary mechanisms involves the induction of antimicrobial peptides (AMPs), such as cathelicidin (LL-37) and beta-defensins, which disrupt microbial membranes to induce lysis and modulate downstream immune signaling for enhanced pathogen clearance (White, 2022; Max et al., 2023). Additionally, Vitamin D influences autophagy and phagocytosis, which are key processes for intracellular pathogen degredation (Verway et al., 2013). Studies have demonstrated that Vitamin D enhances the antimicrobial functions of macrophages and neutrophils, promoting the clearance of intracellular bacteria such as Mycobacterium tuberculosis (Eletreby et al., 2024).

1.2.4 Anti-inflammatory role of Vitamin D

Vitamin D exerts potent anti-inflammatory actions that mitigate the risk of pathologies driven by chronic systemic inflammation (Krajewska et al., 2022). It modulates the inflammatory response by reducing excessive pro-inflammatory cytokine production while maintaining an effective immune defence (Prietl et al., 2013). One of its key mechanisms is the suppression of pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNF-α), interleukin-6 (IL-6), and interleukin-1 beta (IL-1β), which are major drivers of chronic inflammation (Yin and Agrawal, 2014). Vitamin D achieves this by binding to the VDR expressed on immune cells, leading to the inhibition of nuclear factor-kappa B (NF-κB), a central transcription factor that regulates inflammatory gene expression (Zhang et al., 2013). Additionally, Vitamin D promotes the production of anti-inflammatory cytokines, such as interleukin-10 (IL-10), transforming growth factor-beta (TGF-β), and interleukin-4 (IL-4), helping to balance immune responses and prevent excessive inflammation (Barragan et al., 2015).

2 Formulation challenges

2.1 Poor aqueous solubility

Vitamin D3 is a highly lipophilic molecule with a logP value of approximately 9, rendering it insoluble in aqueous media. This characteristic severely limits its dissolution in gastrointestinal fluids and impairs its transport across structural biological barriers such as the blood-brain barrier (Ramezanli et al., 2017; Zhao et al., 2025). This physicochemical property presents a fundamental challenge for formulation design, as poor aqueous solubility directly limits the concentration of Vitamin D that can be achieved at the site of absorption, while restricting its incorporation into conventional dosage forms such as aqueous solutions or hydrophilic matrices (Ramezanli et al., 2017; Zhao et al., 2025). Due to its high lipophilicity, Vitamin D3 cannot readily partition out of hydrophobic environments, meaning that effective systemic absorption is largely contingent on encapsulation within lipid-based delivery systems whose breakdown during digestion facilitates release into the aqueous intestinal environment (Dałek et al., 2022; Stielow et al., 2023). This dependence on lipid vehicles introduces additional formulation complexity, as the nature and composition of the carrier directly influence the extent of solubilisation and subsequent bioavailability.

2.2 Oxidative instability

Beyond solubility, Vitamin D is highly susceptible to chemical degradation, which poses significant challenges for both formulation development and long-term storage stability (Tsai et al., 2017; Mahmoodani et al., 2017). Vitamin D3 undergoes degradation through multiple pathways, including photolysis upon exposure to UV light, oxidation in the presence of oxygen, thermal decomposition at elevated temperatures, and hydrolysis under acidic conditions (Tsai et al., 2017; Mahmoodani et al., 2017). These degradation pathways are particularly problematic during processing, storage, and gastrointestinal transit, where the molecule may be exposed to light, heat, and varying pH environments simultaneously (Tsai et al., 2017; Mahmoodani et al., 2017).

The consequences of oxidative and photolytic degradation extend beyond potency loss, as degradation products may exhibit altered or reduced biological activity. Protecting Vitamin D from these degradation routes therefore represents a key design criterion for any delivery system, and has been a primary motivation for the development of encapsulation strategies including solid lipid nanoparticles, which shield the compound against oxygen and light (Kumar et al., 2014; Zazuli et al., 2025), as well as mixed micelles engineered to preserve Vitamin D3 during UV-C irradiation, thermal processing, and extended storage (Mulrooney et al., 2021a).

2.3 Low intestinal absorption and first-pass effect

Oral bioavailability of Vitamin D is inherently limited by several physiological barriers encountered along the gastrointestinal tract, representing one of the most significant challenges in its therapeutic delivery (Kojecký et al., 2025). Following ingestion, Vitamin D must be incorporated into mixed micelles formed during lipid digestion before it can be absorbed across the intestinal epithelium (Mulrooney et al., 2022). However, this micellarisation process is highly dependent on co-administered dietary lipids, bile salts, and pancreatic lipase activity (Deepika et al., 2025; Dawson-Hughes et al., 2015). In the absence of sufficient dietary fat, micellarisation is impaired, and absorption is markedly reduced. This introduces considerable variability in therapeutic outcomes across patients with different dietary habits, gastrointestinal conditions, or fat malabsorption disorders (Deepika et al., 2025; Dawson-Hughes et al., 2015).

Once absorbed via the intestinal lymphatic pathway, Vitamin D is further subject to hepatic first-pass metabolism, where it undergoes hydroxylation to 25(OH)D before renal activation. While physiologically necessary, this initial metabolism can reduce the immediate systemic availability of the un-metabolised parent molecule to peripheral target tissues (Jones et al., 2011). These compounding barriers frequently result in inconsistent therapeutic serum levels following conventional oral supplementation, a challenge amplified in populations with inflammatory bowel disease, where mucosal absorption is inherently compromised (Deepika et al., 2025; Dawson-Hughes et al., 2015).

The extent of intestinal absorption is also influenced by formulation-specific factors. Studies demonstrate that the lipid droplet size of emulsion-based systems significantly affects the rate of digestion and subsequent bioaccessibility of Vitamin D in vitro and in vivo (Salvia-Trujillo et al., 2017). These observations collectively underscore that overcoming gastrointestinal absorption barriers requires both strategic lipid vehicle selection and precise architectural optimisation of the vehicle to survive gastrointestinal transit (Hsu et al., 2019). In this regard, transdermal and dermal delivery systems present a promising alternative, circumventing both fat-dependent intestinal absorption and hepatic first-pass metabolism (Jefferson and Borges, 2022). Similarly, buccal nanoemulsion formulations have demonstrated improved serum 25(OH)D outcomes compared to conventional oral supplementation in patients with impaired intestinal absorption, highlighting the critical role of delivery route and formulation design in overcoming these physiological barriers (Jefferson and Borges, 2022; Sawarkar et al., 2020).

2.4 Dose variability

A further challenge in Vitamin D delivery is the considerable interindividual variability in absorption and bioavailability, which complicates the establishment of effective and consistent dosing regimens (Liu et al., 2026). Total serum 25-hydroxyvitamin D [25(OH)D], which encompasses both the 25(OH)D₂ and 25(OH)D₃ metabolites, serves as the clinical surrogate marker and definitive indicator of overall Vitamin D status (Schuster, 2011; Zeng et al., 2021). The National Academy of Medicine (NAM) recommends a minimum serum level of 20 ng/mL for skeletal health (Dawson-Hughes, 2025), though optimal homeostasis for basal extraskeletal functions is often argued to require 30 ng/mL (Holick et al., 2011). Conversely, the established upper threshold to avoid potential toxicity and subsequent hypercalcemia is 50 ng/mL (Vieth et al., 2001).

Factors contributing to this variability include age, body mass index, baseline Vitamin D status, genetic polymorphisms in Vitamin D-binding protein (DBP) and metabolising enzymes, dietary fat intake at the time of administration, and differences in gastrointestinal physiology (Tripkovic et al., 2012; van den Heuvel et al., 2024). For transdermal and dermal delivery, additional sources of variability arise from differences in skin thickness, hydration, lipid composition of the stratum corneum, and the presence of skin conditions that alter barrier function (Alsaqr et al., 2015). Achieving these target ranges consistently across diverse patient populations remains challenging with current formulation approaches, highlighting the need for delivery systems capable of providing controlled and reproducible Vitamin D release independent of individual physiological variation.

2.5 Formulation-dependent bioaccessibility

Closely related to the above challenges is the well-documented dependence of Vitamin D bioaccessibility on the specific formulation strategy employed. Bioaccessibility, which is defined as the fraction of an ingested compound that is released from the formulation matrix and becomes available for intestinal absorption, varies considerably across different delivery systems and even across formulations of the same type (Pais et al., 2024). This principle applies broadly across fat-soluble Vitamins: systematic reviews have demonstrated that nano-delivery systems can offer 75%–88% bioaccessibility for Vitamin D and enhance its cellular transport up to five-fold compared to conventional formulations (Erem et al., 2025). The choice of encapsulation platform, carrier composition, and particle size all contribute independently to the bioaccessible fraction, meaning that two formulations of identical Vitamin D dose can yield substantially different therapeutic outcomes (Glo et al., 2019).

Moreover, encapsulating Vitamin D within emulsions, lipid carriers, or microencapsulated systems significantly enhances in vivo absorption and bioavailability by improving solubility and protecting the molecule from degradation in the gastrointestinal tract, effectively mimicking natural digestive processes (Żurek et al., 2023; Ezcurra-Hualde et al., 2025).

This variability is further compounded by the food matrix effect: the presence or absence of dietary fat at the time of administration has been shown to influence Vitamin D absorption, particularly for formulations where the Vitamin has not been pre-solubilised within a lipid vehicle (Lavelli et al., 2021). More recently, buccal nanoemulsion delivery has been explored as a strategy to circumvent gastrointestinal bioaccessibility barriers entirely, with a 2025 randomised controlled trial in inflammatory bowel disease patients demonstrating that reformulation of the delivery vehicle can meaningfully alter serum 25(OH)D outcomes in populations with impaired intestinal absorption (Kojecký et al., 2025). Taken together, these observations underscore that formulation strategy is a primary determinant of Vitamin D therapeutic efficacy, and that the comparative evaluation of delivery platforms is essential for rationalising delivery system selection in clinical and nutraceutical contexts.

3 Formulation strategies

These challenges highlighted in the previous section underscore the importance of formulation design in determining the fate of Vitamin D after ingestion. Over the past decades, researchers have investigated a broad spectrum of delivery systems, ranging from lipid-based carriers to advanced nano- and micro-encapsulation platforms, to overcome solubility limitations and degradation. The subsequent sections present a structured overview of these formulation strategies, highlighting their mechanisms, advantages, and limitations.

3.1 Emulsions formation

Emulsions, either microemulsions containing particles 1–400 μm (µm) in diameter or nanoemulsions with particles in sizes of 1–500 nm (nm), are isotropic dispersions of two non-miscible liquids (oil and water) that act as lipid-based pharmaceutical systems with a high potential to increase the permeation of drugs across biological barriers like the intestinal lining or the skin (Souto et al., 2022). There are three primary types of emulsions: oil-in-water (O/W), in which oil droplets are dispersed within a continuous aqueous phase; water-in-oil (W/O), where water droplets are dispersed within a continuous oil phase; and bicontinuous emulsions, characterised by an interconnected, sponge-like structure composed of continuous water and oil nanochannels (Nafisi et al., 2017), Figure 3. While microemulsions are thermodynamically stable, o/w emulsions and nanoemulsions (NEs) are thermodynamically unstable (Souto et al., 2022). The particles’ size can be controlled by adjusting the stir rate, type and amount of dispersing agent, viscosity of organic and aqueous phases, and temperature (Pinto Reis et al., 2006). Microemulsions have been approved to increase the absorbance of active ingredients which have low absorption ability due to the barrier behavior of the stratum corneum (Kogan and Garti, 2006; Hathout et al., 2010).

Due to Vitamin D’s lipophilic nature, emulsions are a beneficial and preferred delivery system since it is relatively easy to incorporate the fat-soluble Vitamin into the oil phase (Mulrooney et al., 2022). Owing to its crystalline form at ambient conditions, Vitamin D₃ requires to be dissolved within a suitable carrier oil before it can be incorporated into NEs-based delivery systems. Compositional parameters such as emulsifier and carrier oil type critically influence lipid digestion and the resulting bioavailability of these lipophilic targets (Yang and McClements, 2013). For instance, in vitro simulated gastrointestinal models reveal that Vitamin D3 bioaccessibility is heavily dependent on core oil chemistry, decreasing in the order of corn oil ≈ fish oil > orange oil > mineral oil > medium-chain triglycerides (Ozturk et al., 2015). Furthermore, droplet dimensions introduce distinct kinetic variables; while smaller lipid droplets are digested more rapidly than larger ones in vitro due to increased surface area, in vivo models can demonstrate the opposite trend, underscoring the complexity of physiological absorption (Salvia-Trujillo et al., 2017).

The choice of fabrication pathway also dictates final nanoemulsion behaviour. Guttoff et al. have developed a NE system using spontaneous emulsification method for the oral bioavailability enhancement of Vitamin D₃. The spontaneous emulsification method mainly depends on the formation of tiny oil droplets during the titration of oil/surfactant into an aqueous solution. The smallest particle size droplets were obtained in the NEs using Tween 80 as a surfactant as compared to Tween 20, 40, 60, and 85, at a stirring speed of 800 rpm. The NEs were also considerably stable to the droplet growth at ambient temperature as indicated by less than 10% increase in diameter after 1 month of storage but highly unstable to heating above 80 °C (Guttoff et al., 2015). Following that, Zhang et al. formulated Vitamin D₃-based NEs using Tween 80 and Span 80 as surfactants. It was demonstrated that the NEs were spherical with sizes in the range of 391.47 ± 12.89 nm. The encapsulation efficiency was 99.9%, and the encapsulated NEs displayed higher stability under gastric digestion and stronger thermal stability than bare Vitamin D₃ (Zhang et al., 2022). Jan et al. used a low-temperature-aided sonication method to develop NEs containing Vitamin D_3. The optimum NE illustrated a particle size of 169 nm, zeta potential of −22.6 mV, turbidity of 1.78, and encapsulation efficiency of 91%. Moreover, no chemical interaction between Vitamin D₃ and any of the surfactants, including polyethylene glycol, Tween 20, Tween 80, Span 20, Span 80, Labrasol, glycerol, Poloxamer 188, ethylene glycol, and KRH-40, occurred, and the NEs were found to be stable over time. A release study revealed that while in simulated gastric solution 26.30% of Vitamin D₃ was released, in intestinal simulated solution the corresponding number was 78.15%. However, among the drawbacks lay the irregular shapes of NEs and a slight decrease in Vitamin D₃ concentration over 3 months of storage (Jan et al., 2022). Asfour et al. investigated the potential of Vitamin D₃-loaded NE formulation to improve Vitamin D₃ oral bioavailability for management of Vitamin D₃ inadequacy in autistic children. The Vitamin D₃ NEs were formulated using a high-speed homogenization technique, with Span 20 as the emulsifier. The drug content was a minimum of 98%, and their size ranged from 61.15 to 129.8 nm, with a narrow size distribution and zeta potential values between −9.83 and −19.22 mV. Moreover, these NEs were biocompatible and demonstrated increased shelf-stability. Also, apart from proving oral bioavailability, Figure 4, serum calcium, oxidative stress, and apoptosis biomarkers remained within normal levels, indicating the safety of the optimum formulation (Asfour et al., 2023).

Following these findings, a recent study by Goo et al. has demonstrated that mung bean protein/low-methoxyl pectin emulsion gels can effectively encapsulate and protect Vitamin D₃. The addition of cinnamaldehyde improved gel stability and increased encapsulation efficiency, while enabling sustained Vitamin D₃ release under simulated intestinal conditions. These systems show promise as functional carriers for Vitamin D₃ delivery in dysphagia-oriented foods (Goo et al., 2025). In addition, recent work by Capar et al. generated Vitamin D₃-loaded nanoemulsions that have been encapsulated within pea protein–alginate hydrogel beads to improve Vitamin stability and delivery. The hydrogel matrix protected Vitamin D₃ during simulated gastrointestinal digestion and enhanced its bioaccessibility, with optimised formulations achieving approximately 86% bioaccessibility. These findings highlight the potential of plant-based hydrogel–emulsion systems for improving oral Vitamin D delivery (Dursun et al., 2026).

Laçej et al. generated cholesterol-functionalised polyacrylic acid (PAAbCH)-based oil-in-water emulsions that have been investigated as a multifunctional delivery system for Vitamin D₃. Acting simultaneously as an emulsifier and rheology modifier, PAAbCH enabled the formation of physically stable emulsions exhibiting shear-thinning behaviour and stable droplet size distributions. The optimised formulations also improved protection of Vitamin D₃ against oxidative degradation, highlighting their potential for topical Vitamin D delivery applications (Laçej et al., 2026). Li et al. created a novel co-assembly system based on egg white protein and konjac glucomannan was recently used to stabilize high-internal-phase Pickering emulsions for the co-delivery of Vitamin D₃ and retinyl palmitate. The resulting emulsions exhibited excellent storage stability, strong resistance to gastrointestinal conditions, and significantly enhanced Vitamin D₃ bioaccessibility, reaching 87.3% following in vitro digestion. These findings demonstrate the potential of protein–polysaccharide assemblies as biocompatible carriers for multifunctional nutrient delivery (Li et al., 2026). These observations further support nanoemulsification as an effective strategy to overcome the poor aqueous solubility and limited intestinal absorption of Vitamin D₃, particularly for food-fortification and nutraceutical applications, and they highlight the importance of emulsion interface design in maximizing Vitamin D bioavailability.

3.2 Micelles formation

Micelles are considered nanoparticles with a liquid core, similar to nanoemulsions. Nevertheless, micelles are formed by self-assembly or the assistance of surfactants, creating spherical structures with hydrophobic cores (Chevalier et al., 2019; Balamurugan, 2018), Figure 5.

Mulrooney et al., following previous work, sought to demonstrate a correlation between the bioaccessibility/bioavailability of Vitamin D₃ and the formation of mixed micelles (MMs), including the fatty acid (FA) type, the phospholipid concentration, and the salt concentration. MMs were formed containing bile salts, phospholipids and fatty acids (FA), both C8 and C18, at NaCl and KCl concentrations ranging from 20 to 100 mM. While MM hydrodynamic size increased with increasing l-α-phosphatidylcholine concentration and FA chain length, Vitamin D₃ incorporation was not affected by the FA used (Mulrooney et al., 2021b). Another study of the same group demonstrated enhanced properties of Vitamin D₃ when loaded in MMs. MMs showed potential to protect Vitamin D₃ following exposure to UV-C light, heat treatment, and storage conditions of over 4 weeks at refrigeration or freezer conditions (Mulrooney et al., 2021a). Finally, they also compared the ability of MM-based delivery systems and o/w emulsions to examine whether mixed micelles can enhance Vitamin D₃ bioaccessibility more effectively than o/w emulsion systems. The Vitamin D3-loaded o/w emulsions were created using either coconut oil or olive oil, along with added l-alpha-phosphatidylcholine (a phospholipid). All o/w emulsions demonstrated similar particle sizes, whereas MMs had significantly larger particles present. However, MM proved to enhance Vitamin D₃ bioaccessibility more than the o/w emulsions, Figure 6. Among prepared o/w emulsions, bioaccessibility was greater with added phospholipid for o/w emulsions, Figure 6. High levels of free FA were released, suggesting that the systems were all digested successfully. Overall, MMs demonstrated good potential as a delivery system for Vitamin D in vitro (Mulrooney et al., 2022).

Recent advances in self-assembled colloidal systems have further expanded the potential of mixed micellar formulations for Vitamin D₃ delivery. Haider et al. developed an inclusion complex of Vitamin D3 with Captisol® using a green microwave-assisted method, which, when incorporated into mouth-dissolving films, demonstrated rapid disintegration within 36 s, complete drug release within 5 min, and content uniformity of 98.8%. This approach is particularly noteworthy for its capacity to bypass gastrointestinal absorption barriers through buccal delivery, circumventing both the hepatic first-pass effect and the fat-dependent intestinal uptake that constrains conventional oral Vitamin D formulations (Haider et al., 2025). Peixoto et al. developed enzyme-responsive Vitamin D₃-based micelles by conjugating Vitamin D₃ with polyethylene glycol (PEG), producing self-assembled micellar systems with diameters below 100 nm and high encapsulation efficiencies. The micelles exhibited excellent colloidal stability, minimal batch-to-batch variation, and enzyme-triggered disassembly in the presence of esterases, enabling controlled drug release. Notably, Vitamin D₃ served both as the hydrophobic core-forming component and as a bioactive molecule, demonstrating the potential of Vitamin D-based micelles as multifunctional delivery systems (Peixoto et al., 2025). These findings reinforce the role of micellar systems as simple yet effective carriers for hydrophobic Vitamins.

3.3 Lipid carriers

Owing to its poor water-solubility and low oral bioavailability, Vitamin D₃ is preferably encapsulated within lipid-based delivery systems that can improve its bioaccessibility (Ozturk et al., 2015). Lipid nanoparticles (LNPs) can be generally bigger and more complex structures than micelles, requiring specialized techniques to create the various oil droplets (Singh et al., 2016), Figure 7.

LNPs are categorised into distinct architectural classes based on their component structure, including liposomes, lipid-core nanoparticles (LCNPs), and lipid–polymer hybrid nanoparticles (LPHNPs) (Mehta et al., 2023; Graván et al., 2023), Figure 8. Although liposomes represent the classical first generation of lipid vehicles, LCNPs are considered the most widely investigated type for hydrophobic cargo (Graván et al., 2023) while LPHNPs function as core-shell nanostructures derived from both liposomes and polymeric nanoparticles, where a polymer core remains enveloped by a lipid layer (Gajbhiye et al., 2023). LCNPs are spherical vesicles consisting of ionizable lipids that are neutral at physiological pH. They are classified based on the physical state of their lipid core component into three main categories: Liquid Lipid nanoparticles (LLNs), which include Lipid nanoemulsions (LNEs) and Lipid nanocapsules (LNCs), Solid Lipid Nanoparticles (SLNs), and Nanostructured Lipid Carriers (NLCs) (Graván et al., 2023), Figure 8. SLNs are composed of a structured solid lipid matrix, while NLCs contain both solid and liquid lipids, creating a less structured core (López et al., 2023).

Dałek et al. compared the efficiency of liposomes and oily formulations to encapsulate Vitamin D₃. The results obtained show that in order to achieve efficient absorption of Vitamin D₃, liposomes are the preferred delivery system. It was shown that while the liposomal Vitamin D₃ formulation delivered orally increased the calcidiol concentration in the serum significantly, no such effect was observed when the oily solution was used (Dałek et al., 2022).

SLNs are generally considered a combination of lipid emulsion and polymer (Mukherjee et al., 2009). The solid lipid core matrix of SLNs can entrap lipophilic molecules and aid in their solubilisation within the surfactant-stabilised aqueous solution (Gupta et al., 2018). It can also aid in the controlled release of a drug, drug targeting and increased drug bioavailability (Mishra et al., 2018).

San Martin-Gonzalez and Patel formulated polysorbate 20-stabilised tripalmitin solid lipid nanoparticles of Vitamin D₂ by hot homogenization technique. It was shown that loading ergocalciferol within the lipid crystal not only enhanced its stability, protecting it from oxygen and light, but also increased the encapsulated amount of ergocalciferol, Figure 9. These Vitamin-loaded SLNs are beneficial for the formation of clear fortified solutions (Patel and San Martin-Gonzalez, 2012). Following this study, Kumar et al. developed a sensitive ultra-performance liquid chromatography (UPLC) method to estimate the total drug content and entrapment efficiency of the retinoic acid and Vitamin D₃-loaded SLNs and confirmed the simultaneous administration of these two Vitamins and their improved and prolonged bioavailability (Kumar et al., 2014).

In addition, Demirbilek et al. further proved the efficiency of SLNs for the delivery of Vitamin D₃ by using a beeswax-stearic acid-based formulation. These formulations created particles with sizes in the range of 30–60 nm range and enhanced degradation rate, encapsulation efficiency, and release rate, which also increased when beeswax was increased. The Vitamin D₃-loaded SLNs were observed to be immunocompatible and non-cytotoxic to keratinocytes (HaCaT), endothelial (HUVEC), and fibroblast (L929) cell lines (Demirbilek et al., 2017). SJ Park et al. focused the research on the oral bioavailability enhancement of Vitamin D₃ by formulating NLCs using a high-pressure homogenization technique. In this work, optimum Vitamin D₃-NLCs contained entrapped retinoic acid/Vitamin D₃ and were obtained with a small diameter (132.9 nm), a high zeta potential (41.90 mV), and a high encapsulation efficiency (85.6%). In vitro digestion in simulated gastrointestinal fluids demonstrated their capability for controlled release because the NLCs were able to remain stable, as demonstrated by particle size measurements, and protect the Vitamin D₃ in simulated gastric fluid but released more than 90% of the Vitamin D₃ in simulated intestinal fluid, Figure 10. Therefore, the developed NLCs are promising carriers for increasing the oral bioavailability of Vitamin D₃ (Park et al., 2017).

Kaur et al. presented an invention of preparing Solid Lipid Nanoparticles (SLNs) for delivery of Vitamin D₃ and retinoic acid (RA), two of the most fat-soluble Vitamins (Kaur and Verma Kumar, 2023). Using the microemulsion technique, the solid lipid core containing entrapped retinoic acid/Vitamin D₃ was suspended in the solution of surfactant and co-surfactant. The optimised formulations comprised emulsifiers, such as polysorbate 80 (45.45%) and soy lecithin (0.58%), and a lipid composition of 7.27%. The characterisation of particles suggested 100% release of the drug after 7 days and proved the utility in the treatment of diseases like tuberculosis. Following that, Kumar et al. proved the synergetic effect of Vitamin D₃ and all-trans retinoic acid (ATRA) by showing that concurrently orally administering Vitamin D₃-entrapped SLNs and ATRA-entrapped SLNs was beneficial for tuberculosis control with improved and prolonged bioavailability. Vitamin D₃-and ATRA-loaded SLNs were formulated separately but administered simultaneously. The pharmacokinetic profile demonstrated the improved and prolonged bioavailability of ATRA and Vitamin D₃ upon incorporation into SLNs (Kumar et al., 2014).

3.4 Polymeric nanoparticles

The term polymeric nanoparticles (NPs) refers to particles within the size range from 1 to 1,000 nm (Zielińska et al., 2020). Nanoparticles can be loaded with active pharmaceutical ingredients (APIs) which are entrapped onto the polymeric core, overcoming the limitations of low bioavailability and storage degradation (Feczkó, 2021). This approach possesses the potential to increase drug solubility, enhance release behaviour, and improve cellular uptake (Patra et al., 2018; Pateiro et al., 2021). A study by Steven P. Schwendeman et al. showed that biodegradability and biocompatibility are two essential parameters for the selection of polymers for encapsulating hydrophobic drugs into polymeric cores. Depending on the morphological structure, nanoparticles can be grouped into nanocapsules and nanospheres (Zielińska et al., 2020), Figure 11. The main difference between these two types is the way the APIs are encapsulated by the polymeric nanoparticles. In nanospheres, they are integrated into a polymeric matrix, whereas in nanocapsules, the compounds are covered by polymers in the nanostructure core (Zielińska et al., 2020; Deng et al., 2020; Vasile and Vasile, 2019).

Ahmad Wsoo et al. developed a novel implantable delivery system for encapsulating Vitamin D₃ containing an electrospun cellulose acetate (CA) and ε-polycaprolactone (PCL) nanofibrous membrane. Vitamin D₃ is encapsulated in the core consisting of CA nanofiber, which is surrounded by a PCL membrane. Electrospinning was used to load Vitamin D₃ at the concentrations of 6, 12, and 20% (w/w) _. The fibers illustrated diameters in the range of 324–428 nm and increased with an increase in Vitamin D₃ content. The controlled drug release profile followed the zero-order model. Nevertheless, the outer PCL membrane affects the long-termed control drug release. The system illustrated superior mechanical properties compared to the cellulose/Vitamin D₃. In addition, the system not only proved its non-toxicity, but also illustrated high cell viability and proliferation of HSF cells, Figure 12 (Wsoo et al., 2021).

Following this work, Colturato et al. developed a crystalline nanocellulose membrane for the release of Vitamin D_3. The cellulose membranes were obtained using the “casting” technique. While FT-IR demonstrated the presence of all the active components in the membrane sample, without structural alterations or the formation of new bonds, FEG-SEM images proved the presence of Vitamin D₃ crystals on the surface and in the interior of the membrane. The release of Vitamin D₃ occurred in a sustained manner, Figure 13 (Colturato and Goveia, 2022).

Regarding the encapsulation of Vitamin D₃, an interesting study by Ramezanli et al. investigated the potential application of polymeric nanospheres (known as TyroSpheres) as a formulation carrier for topical delivery of Vitamin D₃ with the goal of improving the solubility, efficacy, and stability of Vitamin D₃. When Vitamin D₃ is loaded in the nanoparticles, high drug loading and binding efficiencies are observed, while the solubility of Vitamin D₃ is enhanced. In addition, an ex vivo skin distribution study showed that the NP formulations delivered 3–10 mg of Vitamin D₃ into the epidermis, significantly higher than that delivered from control vehicles. No alterations of drug activity and non-toxicity of the NP encapsulated drug were confirmed by an in vitro cytotoxicity assay using keratinocytes, Figure 14 (Ramezanli et al., 2017).

More recent studies have continued to demonstrate the versatility of polymeric nanoparticles for Vitamin D₃ delivery. Chang et al. developed human serum albumin/polystyrene sulfonate nanoparticles co-loaded with raloxifene and Vitamin D₃, using a lyophilisation-hydration method, demonstrating successful encapsulation, good biocompatibility and hemocompatibility, prolonged circulation time, and the potential to reduce treatment dosage for postmenopausal osteoporosis (Chang et al., 2025). Similarly, Mukherjee et al. generated keratin-based nanoparticles that have been employed to encapsulate Vitamins D₂, D₃, and K₂, providing controlled release, enhanced delivery of poorly water-soluble Vitamins, and promising anticancer activity in breast cancer cells (Mukherjee et al., 2026). These findings highlight the versatility of polymeric nanoparticles as multifunctional carriers capable of improving both the stability and therapeutic performance of Vitamin D formulations.

3.4.1 Polymeric microparticles

Microparticles refer to spherical particles with diameters ranging the micron range from 1 μm to 1,000 μm formed by interfacial tension of immiscible liquid phases (Campos et al., 2013; Bhujel et al., 2021). Polymeric microparticles typically consist of a polymer matrix that can immobilize a smaller quantity of an active compound (Campos et al., 2013). Based on their internal structure, these systems are categorized as either microspheres or microcapsules (Campos et al., 2013; Bhujel et al., 2021). While microspheres feature drugs uniformly distributed within the polymeric matrix, either in a dissolved or suspended state, microcapsules are heterogeneous systems where drugs are encapsulated within a membranous shell (Bhujel et al., 2021). The polymeric matrix offers protection to the encapsulated drugs from environmental factors such as humidity, temperature, pH, oxygen, and enzymes (Bhujel et al., 2021). Polymeric microparticles (MPs) can be synthesised through various methods, including dispersion polymerisation and precipitation polymerisation (Wang et al., 2014). However, these methods often result in polydisperse sizes and a limited microsize range (Wang et al., 2014). Therefore, techniques such as copolymer self-assembly, phase-separated polymer precipitation, and techniques involving sacrificed solid-core templates like layer-by-layer deposition offer limited control over particle size and are confined to specific materials (Wang et al., 2014).

Nguyen et al. demonstrated that Vitamin D₃ can inhibit the proliferation of cancer cells, including colorectal and hepatocellular cells, which are mainly responsible for liver cancer, by developing a controlled release system of the active form of Vitamin D₃ by formulating cross-linked microspheres of poly (vinyl neodecanoate-crosslinked-ethyleneglycol dimethacrylate) prepared by suspension polymerisation. Microspheres demonstrated sizes in the range of 35 μm and showed 1% release of the drug up to 40 days, Figure 15. The prepared microspheres were confirmed to be non-toxic by MTT, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide, colorimetric assay and direct contact cytotoxicity assay (Nguyen et al., 2007).

4 Route of administration

As demonstrated, oral is the most studied route for administering Vitamin D₃. In the market, Vitamin D₃ is encapsulated in different dosage forms, including capsules, tablets, and powders, where Vitamin D₃ can be found alone or in combination with other ingredients such as Vitamin C, K2, and magnesium (Rejinold et al., 2019; Maurya et al., 2020; Maurya et al., 2023). Moreover, in vivo absorption of Vitamin D is markedly affected not only by the specific formulation strategy, but also by the presence of dietary fat, as thoroughly documented (Deepika et al., 2025; Dawson-Hughes et al., 2015; Dawson-Hughes et al., 2013; Cavalier et al., 2016). Oral administration of Vitamin D with a fat-containing meal typically enhances absorption compared to fasting or fat-free conditions (Deepika et al., 2025; Dawson-Hughes et al., 2015; Dawson-Hughes et al., 2013; Cavalier et al., 2016).

Nevertheless, due to the occurrence of many physiological barriers and various changes in conditions along the gastrointestinal tract, oral absorption can demonstrate low efficiency; therefore, more recent studies investigate alternative potential routes for administering Vitamin D₃ (Kim et al., 2018; Ha et al., 2022).

Compared to the most common route, the oral route, skin delivery, including dermal and transdermal delivery, offers a convenient way to obtain a sustained supplement of Vitamin D3 by avoiding side effects on the gastrointestinal tract, providing sustained release over long periods, and being easy to apply or remove (Alsaqr et al., 2015; Kim et al., 2018). Topical, or dermal, delivery refers to the process of transporting active ingredients directly to the skin, whereas transdermal delivery refers to the transportation of active ingredients through the skin and into the bloodstream (Biana et al., 2012). Siegfried and Mads studied the biological properties of Vitamin D₃ connected to immunomodulation, including inhibiting epidermal dysregulation, epidermal hyperproliferation, and inducing keratinocyte differentiation. The clinical advantages of a fixed-combination product containing calcipotriol, as the Vitamin D₃ analogue, and a corticosteroid, betamethasone dipropionate, enhanced efficacy and tolerability compared to other monotherapies. These active ingredients appear to act in synergy when administered topically to reduce inflammation (Segaert and Ropke, 2013). Following that, Simioni et al. also explored the topical administration of Vitamin D₃ by formulating hydrogel-NPs, Figure 16. The nanostructured archaeolipid carriers loaded with Vitamin D₃ and a carotenoid were added into a Carbopol-based hydrogel and showed pseudoplastic non-Newtonian behaviour, stability over time, good occlusion capacity and spreadability and demonstrated their anti-inflammatory and antioxidant activity (Simioni et al., 2024). Rusell from Johnson and Johnson Skin Research Centre investigated the correlation between topical application of Vitamin D₃ and hydration of the stratum corneum and further proved that Vitamin D₃ enhances skin moisture (Russell, 2012).

However, the transdermal route can overcome the barrier properties of the skin that limit the use of topical applications of Vitamin D₃ (Alsaqr et al., 2015). Among the transdermal delivery formulations, gels, ointments, microemulsions, nanocarriers, and vesicular delivery systems, such as liposomes, niosomes, transfersomes, and ethosomes, can be found (Kaur and Muskan, 2024; Valdivia-Olivares et al., 2021; Garg and Jain, 2022; Kim et al., 2020). By being a potent lipophilic drug with a short plasma elimination half-life that is required in a small daily dose, Vitamin D₃ can be considered as a good candidate for transdermal delivery (Alsaqr et al., 2015). Alsaqr et al. investigated the transdermal delivery of Vitamin D₃ by creating ointment formulations using different chemical penetration enhancers. The optimal formulation contained dodecylamine and ethanol as penetration enhancers, which showed enhanced permeability using porcine skin (Alsaqr et al., 2015). Vora et al. formulated cholecalciferol-loaded poly (lactic-co-glycolic acid) microspheres by an o/w emulsion-solvent evaporation technique, also using Tocopherol Succinate as the stabiliser. Investigating different formulations and process parameters, including concentration of emulsifier, concentration of stabiliser, and drug: polymer mass ratios, it has been demonstrated that the optimal formulation was found to have a mean particle size of 28.62 ± 0.26 μm, Encapsulation Efficiency of 94.4% ± 5.4% and drug loading of 5.19% ± 0.29%, Figure 17. In-vitro release showed that Vitamin D₃ release followed near zero-order drug release kinetics over nearly 1 month. The biodegradable polymeric system efficiently protected Vitamin D₃ and allowed its controlled release (Vora et al., 2017).

While transdermal Vitamin D formulations offer a bypass of oral absorption issues, it can lead to several risks. These include skin irritation, potential for allergic reactions, and the possibility of excessive Vitamin D levels leading to hypercalcemia (Jefferson and Borges, 2022; Rejinold et al., 2019). Furthermore, the efficacy and safety of these formulations are still under investigation, particularly regarding long-term use and individual variations in skin absorption (Jefferson and Borges, 2022; Rejinold et al., 2019).

5 Comparison of formulation techniques

The four principal encapsulation strategies reviewed; emulsions, micelles, lipid nanoparticles, and polymeric nanoparticles/microparticles, each present distinct advantages and limitations depending on the intended route of administration and therapeutic goal, Table 1. Among these, nanoemulsions have been extensively studied for oral Vitamin D delivery, offering high encapsulation efficiencies (91%–99.9%) and well-established manufacturing processes that favour industrial scale-up, though their thermodynamic instability and susceptibility to thermal degradation above 80 °C remain notable limitations (Ozturk et al., 2015; Guttoff et al., 2015; Zhang et al., 2022; Jan et al., 2022; Asfour et al., 2023). Mixed micelles demonstrate competitive bioaccessibility enhancement in vitro, outperforming oil-in-water emulsions in some comparative studies and offer reasonable protection against UV and thermal stress (Mulrooney et al., 2021a; Mulrooney et al., 2022; Mulrooney et al., 2021b). However, their application to dermal and transdermal delivery remains largely unexplored. Lipid nanoparticles, particularly SLNs and NLCs, offer a favourable balance across multiple parameters: high encapsulation efficiency, solid-matrix protection from oxidative degradation, and the capacity for controlled intestinal release while resisting gastric breakdown (Patel and San Martin-Gonzalez, 2012; Demirbilek et al., 2017; Park et al., 2017). Crucially, lipid-based systems have also demonstrated applicability across both oral and skin-based routes, with liposomal formulations showing superior serum calcidiol elevation compared to oily solutions ¹²⁴, and nanocarriers proving effective for dermal Vitamin D delivery (Demirbilek et al., 2017; Simioni et al., 2024). Polymeric nanoparticles and microparticles distinguish themselves through their capacity for prolonged and tunable release, with some systems sustaining Vitamin D release for up to 40 days 183, and their versatility across oral, dermal, and transdermal routes (Ramezanli et al., 2017; Wsoo et al., 2021; Nguyen et al., 2007; Vora et al., 2017). Nevertheless, the manufacturing complexity associated with techniques such as electrospinning and suspension polymerisation presents a barrier to large-scale production compared to emulsion-based systems. Overall, no single formulation strategy is universally superior; rather, the optimal choice is dictated by the clinical application, with lipid nanoparticles best suited for oral bioavailability enhancement and polymeric carriers offering the greatest potential for controlled and targeted skin-based delivery of Vitamin D.

TABLE 1. Comparative overview of encapsulation strategies for Vitamin D delivery across key formulation parameters. Encapsulation efficiency, physicochemical stability, release characteristics, bioavailability enhancement, and route of administration are evaluated based on studies specifically reporting Vitamin D delivery. Scale-up feasibility is assessed based on general manufacturing considerations reported in the broader nanoparticle and microparticle literature.

6 Conclusion and outlook

Given the critical role of Vitamin D in maintaining various physiological functions and the widespread prevalence of its deficiency, it is essential to address the limitations of current delivery systems and their formulation strategies. As highlighted in this review, despite the availability of multiple routes of Vitamin D administration, several limitations persist, including the lack of sustained, prolonged release, the hepatic first-pass effect, and gastrointestinal side effects, all of which can hinder its therapeutic efficacy. The comparative analysis presented in Section 5 underscores that no single encapsulation strategy is universally superior. Rather, the optimal formulation choice is dictated by the intended route of administration and clinical goal. Nanoemulsions offer high encapsulation efficiencies and industrially scalable production, making them well-suited for oral delivery, yet their thermodynamic instability remains a persistent challenge. Mixed micelles demonstrate competitive bioaccessibility enhancement in vitro, though their potential for skin-based delivery remains largely unexplored. Lipid nanoparticles, particularly SLNs and NLCs, provide a favourable balance of encapsulation efficiency, oxidative stability, and controlled intestinal release, and have demonstrated applicability across both oral and dermal routes. Polymeric nanoparticles and microparticles distinguish themselves through prolonged and tunable release profiles, with some systems sustaining Vitamin D delivery for up to 40 days, and represent the most versatile platform across oral, dermal, and transdermal administration.

Given these findings, the exploration of alternative delivery methods for Vitamin D is highly beneficial.

• In particular, skin-based delivery systems, including transdermal and dermal routes, present a promising strategy for achieving prolonged and controlled Vitamin D release. Although preliminary studies have investigated the incorporation of Vitamin D into microneedles and micropatches, research in this area remains insufficient.

Each delivery system has its unique benefits depending on the specific application and desired outcomes. For instance, lipid-based nanoparticles are excellent for enhancing bioavailability, while polymeric nanoparticles are ideal for controlled release. Regarding topical delivery of Vitamin D₃, lipid carriers and emulsions have been studied primarily for their ability to deliver Vitamin D₃.

• However, polymeric carriers, with their ability to enhance bioavailability, provide controlled release, and ensure targeted delivery, represent a transformative approach for the topical delivery of Vitamin D₃, warranting further scientific investigation to unlock their full potential in dermatological applications

Vitamin D has been shown to exhibit synergistic effects when combined with other bioactive compounds, particularly in processes such as cell differentiation and antiproliferative activity. However, its broader biological properties, including antibacterial and anti-inflammatory effects, remain relatively underexplored.

• The combination of transdermal or dermal Vitamin D delivery with these biological functions opens new avenues for the development of novel therapeutic systems targeting bacterial infections and inflammatory conditions.
• Further investigation is warranted to explore whether the efficacy of Vitamin D₂ can be enhanced through the presence of other pharmacological agents. This could position Vitamin D₂ as a cost-effective and naturally derived alternative to Vitamin D3, particularly given its plant-based origin and potential for sustainable production
• Current studies have explored the synergistic effects of retinoic acid in combination with Vitamin D (Kumar et al., 2014), particularly in relation to antiproliferative activity. Given that retinoic acid is the most biologically active form of Vitamin A, while retinol is a less potent derivative commonly used in dermatological applications, the combined use of Vitamin D₂, Vitamin D₃, and retinol in transdermal formulations may hold promise for cosmetic and dermatological applications.

To summarise, Vitamin D has demonstrated significant potential in various physiological and therapeutic applications. Therefore, it is crucial to investigate and develop innovative formulation strategies that optimise its bioavailability and therapeutic efficacy. As evidenced by the comparative analysis in this review, the rational selection of an encapsulation platform, whether lipid-based for oral bioavailability enhancement or polymeric for prolonged skin-based delivery, is as important as the route of administration itself. Advancing these strategies could contribute to improved clinical outcomes and broaden the scope of Vitamin D-related treatments in healthcare and dermatology.

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Footnotes

Edited by: Mohamed BRAHMI, Université Claude Bernard Lyon 1, France

Reviewed by: Snežana Papović, University of Novi Sad, Serbia

Chandan Nayak, Berhampur University, India