Serum vitamin D levels had a significant positive correlation with pulmonary function, most prominently in patients with a history of tuberculosis (TB), data from a large cross-sectional study showed.

Patients with the highest serum levels of 25-hydroxyvitamin D (25-OHD) had significantly higher forced expiratory volume at 1 second (FEV1) and forced vital capacity (FVC) as compared with patients who had the lowest levels of 25-OHD (P<0.001 and P<0.005, respectively), reported Churl-Min Kim, MD, PhD, of the Catholic University of Korea in Seoul, and colleagues.

In the subgroup of patients with a history of pulmonary tuberculosis, the absolute difference in FEV1 by 25-OHD level was four times greater than the difference in the overall population, they wrote in the Journal of Clinical Endocrinology & Metabolism.

“We found a robust positive association between serum 25(OH)D level and lung function in Korean adults. This association was independent of age, sex, body mass index (BMI), lifestyle (smoking and regular exercise), occupation, residence, season, and some respiratory diseases,” they explained.

With regard to the findings in the TB subpopulation, the authors speculated that the results suggest “that the susceptibility of pulmonary TB might be related to vitamin D deficiency and also that vitamin D therapy may be beneficial for lung function in this population.”

“The precise mechanism for this phenomenon remains unknown, but it has been suggested that vitamin D accelerates recovery from infection by enhancing innate immunity via upregulation of antimicrobial peptides,” they added.

Observational studies of vitamin D and respiratory function have yielded mixed results. Clinical trials of vitamin D supplements as prophylaxis against respiratory disease also failed to demonstrate a definitive association, the authors said.

Current recommendations for 25-OHD intake are based on maintenance of bone health, not optimization of immunologic and other nonskeletal outcomes.

Previous studies of 25-OHD and pulmonary function focused primarily on Western populations so Kim’s group sought to examine the association in a large Asian population.

Data for the analysis came from the third and fourth Korean National Health and Nutrition Examination Surveys (2008 to 2010). Pulmonary function and serum 25-OHD levels were assessed during the surveys, and the final analysis included 10,096 participants who had complete data for both outcomes.

Serum 25-OHD measurements showed that 636 participants had values <10 ng/mL, 5,384 had values of 10 to 20 ng/mL, 3,274 had values of 20 to 30 ng/mL, and 802 had values ≥30 ng/mL. The median values were 20.6 ng/mL for men and 17.5 ng/mL for women. Overall, 59% of the participants had 25-OHD levels <20 ng/mL.

The participants were grouped into quartiles of 25-OHD levels, ranging from <15.3 ng/mL to ≥24.9 ng/mL for men and from <12.8 to ≥21.4 ng/mL for women.

After controlling for age, sex, height, and season (of 25-OHD measurement), the authors found significant associations between 25-OHD level and pulmonary function. Comparing the highest and lowest quartiles of 25-OHD, Kim and colleagues found differences of 51 mL for FEV1 and 58 mL for FVC. The trend remained after adjustment for smoking and exercise, BMI, occupation, region, and history of asthma, pulmonary TB, and obstructive lung disease.

Serum levels of 25-OHD did not vary significantly in patients with a history of asthma, but participants with obstructive lung disease or pulmonary TB had significantly lower levels compared with participants who did not have either condition (P<0.05).

Comparing the top and bottom quartiles of 25-OHD, the authors found a difference of 229 mL for FEV1 in the subgroup of participants with a history of pulmonary TB (P<0.01).

The study had some limitations, namely that it was cross-sectional so reverse causality could not be ruled out. Also, the overall low 25-OHD levels in the participants limited the authors’ ability to adequately estimate optimal vitamin D levels for lung function. Finally, data on sun exposure and dietary or supplementary vitamin D intake were not available.

The authors reported no conflicts of interest.

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