Vitamin d3

Mechanism of action

1,25(OH)2D binds to the VDR, which heterodimerizes with RXR and modulates the transcription of genes involved in calcium/phosphate homeostasis and immunity. In the gut–immunity axis: (1) it supports the integrity of the epithelial barrier by regulating tight junction proteins (e.g. claudins/occludin in experimental models) and the repair response; (2) it induces antimicrobial peptides (e.g. cathelicidin/LL-37 and β-defensins), contributing to innate defense; (3) it modulates antigen presentation and the maturation of dendritic cells, promoting a more tolerogenic profile; (4) it influences the T helper/Treg balance, with a tendency to reduce pro-inflammatory signals (e.g. some Th1/Th17 cytokines) and support immune tolerance; (5) it may interact indirectly with the microbiota through effects on the barrier, mucosal immunity, and substrate availability, although the direction and magnitude of the effects on the human microbiota are still under investigation.

Supported benefits

• Correction/prevention of vitamin D deficiency and maintenance of adequate 25(OH)D levels (when intake or sun exposure is insufficient) (strong)
• Support for immune function: observational associations between low 25(OH)D levels and greater susceptibility to respiratory infections; some RCTs show a modest reduction in the risk of acute respiratory infections especially in subjects with low baseline levels and with regular administration (moderate)
• Possible contribution to the maintenance of the intestinal barrier and the modulation of mucosal inflammation (mechanistic and clinical evidence is heterogeneous; results vary in gastrointestinal/inflammatory conditions) (limited)
• Reduced risk of asthma exacerbations in some studies, with a more evident effect in deficient subjects (moderate)

Safety & side effects

• Generally well tolerated at usual dosages; adverse effects are mainly related to excess (hypervitaminosis D)
• Hypercalcemia/hypercalciuria: nausea, vomiting, constipation, polyuria, polydipsia, weakness, confusion
• Risk of nephrolithiasis (kidney stones) in predisposed subjects, especially with high calcium intake
• Tissue calcifications and kidney damage in cases of prolonged excess

FAQ

Vitamin D3 and D2: what is the difference?

D3 (cholecalciferol) comes from animal sources or skin synthesis; D2 (ergocalciferol) comes from fungi/yeasts. Both increase 25(OH)D, but in many studies D3 is more effective at maintaining/increasing 25(OH)D at the same dose.

Why is vitamin D linked to the gut?

The VDR is expressed in the intestinal epithelium and in immune cells of the mucosa. Vitamin D can influence the intestinal barrier, the production of antimicrobial peptides, and inflammatory tone, with possible indirect effects on the microbiota.

Is taking it with food important?

Yes. Being fat-soluble, absorption tends to improve when taken with a meal containing fats; conditions of fat malabsorption can reduce absorption.

What is the best marker for assessing vitamin D status?

Serum 25(OH)D is the most commonly used marker because it reflects overall exposure (diet, sun, supplementation) and has a half-life of weeks. 1,25(OH)2D is tightly regulated and does not represent stores well.

Are high “bolus” doses equivalent to a daily dose?

Not necessarily. At the same total dose, pharmacokinetic profiles differ and some clinical outcomes (e.g. risk of falls in specific high-dose protocols) may vary. The literature is heterogeneous and depends on the population and regimen.