Vitamin D3 (cholecalciferol) is increasingly studied for its role in supporting the respiratory system, particularly in immune modulation and the prevention of respiratory infections. Several observational studies have identified a correlation between low vitamin D levels and increased susceptibility to respiratory tract infections, including pneumonia and acute viral infections. Randomized controlled trials (RCTs) and meta-analyses, such as the 2017 BMJ meta-analysis by Martineau et al., suggest that vitamin D supplementation may reduce the risk of acute respiratory infections, especially in individuals with significant deficiency. The proposed mechanism involves vitamin D's ability to enhance innate immunity by stimulating the production of antimicrobial peptides in the respiratory epithelium, as well as its role in modulating inflammatory responses.

However, the evidence is not uniformly strong. Some RCTs have failed to demonstrate a significant benefit in the general population, and the effect size appears modest and primarily relevant for those with baseline deficiency. Regarding chronic lung conditions like asthma or COPD, data are less conclusive, with some studies showing mild improvements in exacerbation rates, while others show no effect.

In summary, there is scientific evidence to support the use of vitamin D3 for respiratory (lung) health, particularly in reducing the risk of respiratory infections in deficient individuals. However, the strength of evidence is moderate, with greatest benefit observed in those with low baseline vitamin D levels.