Inhaled vitamin D: why the delivery route may matter

Inhaled vitamin D raises a simple but important question: if oral supplementation has produced little consistent benefit in chronic lung disease, could the problem be partly one of delivery? Researchers at the University of North Carolina argue that vitamin D may deserve another look when delivered directly to the airways. Their paper, however, is a Perspective rather than a clinical trial of a new treatment.

The argument begins with a long-standing paradox. Low blood levels of vitamin D have repeatedly been associated with worse outcomes in asthma, chronic obstructive pulmonary disease, and cystic fibrosis. Yet randomized trials of oral supplementation have generally failed to reproduce the benefits suggested by those observational associations.

Across studies, supplementation has not consistently improved lung function, quality of life, hospitalization rates, or exacerbations. Some analyses have suggested possible benefits in people with very severe vitamin D deficiency, but the overall clinical results have been much less impressive than the epidemiological associations initially implied.

A possible delivery problem

Vitamin D taken by mouth is absorbed, metabolized, circulated through the body, and ultimately has to reach its target tissue. The authors focus particular attention on CYP24A1, an enzyme that inactivates biologically active vitamin D metabolites. CYP24A1 is highly expressed in pulmonary vascular endothelial cells.

This leads to a plausible hypothesis: systemically delivered vitamin D may be diluted or partly inactivated before enough active compound reaches the airway surface to produce a substantial local effect. Importantly, this has not been demonstrated as the explanation for the negative clinical trials. It remains a mechanistic proposal that needs direct testing.

The airway epithelium itself is well equipped to respond to vitamin D. Airway cells express the vitamin D receptor as well as enzymes involved in local production of calcitriol, the hormonally active form of vitamin D. Vitamin D signaling can influence antimicrobial peptides, epithelial barrier integrity, inflammatory pathways, and antioxidant defenses.

The proposed shift is therefore from treating vitamin D primarily as a systemic nutritional supplement to investigating it as a locally delivered respiratory agent.

What the preclinical studies show

There is experimental support for this approach. In one mouse study, nebulized calcitriol reduced inflammatory-cell infiltration and helped preserve epithelial barrier function after exposure to bacterial lipopolysaccharide, without substantially changing circulating vitamin D levels.

Other studies have tested inhaled vitamin D metabolites in mouse models of hypersensitivity pneumonitis, pulmonary fibrosis, and chronic lung injury. Some reported reductions in inflammatory or fibrotic changes, while pulmonary administration of active vitamin D improved lung function and promoted alveolar regeneration in a mouse model relevant to COPD.

Evidence also exists in human cells. A 2025 study using primary human bronchial epithelial cells found that aerosolized vitamin D attenuated several inflammatory and transcriptional responses triggered by ozone exposure. Such experiments are relevant because the compound is applied directly to the epithelial surface that would be targeted by an inhaled therapy.

There is an important qualification, however. These studies have not all tested the same substance. Some used calcitriol, the active form of vitamin D, while others used 25-hydroxyvitamin D or other preparations. The concept therefore cannot be translated into simply putting an ordinary vitamin D supplement into a nebulizer.

Human evidence is still missing

According to the authors, pulmonary vitamin D delivery has not yet been tested in clinical trials involving people with chronic lung diseases such as COPD. Safety, effective dosing, formulation, pharmacokinetics, and the consequences of repeated long-term exposure all remain unresolved.

Formulation itself is a significant challenge. Vitamin D is fat-soluble and poorly soluble in water, complicating the development of an inhaled preparation that can deliver a predictable dose to the appropriate regions of the lung. Researchers would also need to determine how much remains locally, how much enters the circulation, and whether a respiratory effect can be achieved without clinically important systemic exposure.

The most interesting message is therefore not that vitamin D has been shown to treat damaged lungs. It has not. The more defensible idea is that failure of an oral supplement does not necessarily rule out a local pharmacological effect. For some molecules, the route by which they reach a tissue may be as important as the molecule itself.