Previous studies have suggested that vitamin D may affect respiratory health via mechanisms like immune modulation and effects on lung structure, but due to a number of confounding factors, the association between vitamin D and respiratory health is still up for debate.
Researchers at Sir Charles Gairdner Hospital and the University of Western Australia in Perth used a cross-sectional analysis of a large community-based sample to examine the link between vitamin D status and respiratory disease.
A breathy affair
They recruited 5,011 adults aged between 45 and 69, collecting information on their serum vitamin D levels and respiratory health history via a questionnaire. They also measured the participants' spirometry (volume and speed of air they could inhale and exhale).
They used three different adjustment models: A, B and C. In Model A, they adjusted for sex, age, smoking status, and season. In Model B, they adjusted for BMI and physical activity level. Finally, in Model C, they adjusted for history of chronic diseases.
Among the participants, 45% were male and 55% female, 10% were current smokers, and 12% were taking vitamin D supplements.
The researchers then found that in all three models, vitamin D was "significantly associated" with asthma, wheeze, and chest tightness.
In addition, across all three models, those with vitamin D levels of higher than 100nmol/L had higher forced vital capacity — how much air one's lungs can forcibly exhale after maximum inhalation — than those under Model C, who had 50 nmol/L to 100 nmol/L.
The study stated that after adjusting for potential confounders, low levels of serum vitamin D were found to be "independently associated" with asthma, bronchitis, wheeze and chest tightness. Conversely, higher levels of vitamin D were linked to better lung function.
In conclusion, the researchers wrote: "Respiratory illness and symptoms are associated with lower vitamin D levels in a relatively large representative adult Caucasian population.
"Lower vitamin D levels confer an increased risk of reported asthma, bronchitis and symptoms of wheeze and chest tightness after adjustment for potential confounders that include BMI, physical activity and chronic disease. Higher vitamin D serum levels correlate with higher levels of lung function.
"The findings in this study, coupled with recent studies investigating asthma outcomes and use of vitamin D supplements, strengthen the proposed mechanistic relationship between vitamin D deficiency and respiratory disease."
"Vitamin D and respiratory health in the Busselton Healthy Ageing Study"
Authors: Siobhain Mulrennan, et al.