Abstract:
Asthma is the 14th most important disorder in the world in terms of the extent and the duration of disease. The New Zealand Health Surveys have reported an asthma prevalence in adults of 11.5% in 2006-07 and 11.1% in 2011-13. In New Zealand, one in four adults (27.1%) has a 25-hydroxyvitamin D (25OHD) level below the recommended level [50 nmol/L]. These surveys also showed that vitamin D status, as measured by 25OHD, differed by ethnicity, socioeconomic status and BMI. The epidemiology of vitamin D deficiency has a similar pattern to that seen with the asthma epidemic. Aim This study aims to investigate in participants aged 50-84 years in the Vitamin D Assessment (ViDA) study: 1) whether serum 25OHD concentrations are associated with the presence or absence of asthma; 2) in participants with asthma whether 25OHD concentrations are associated with asthma medication usage, asthma severity, asthma control, urgent health facility usage and hospital admission (asthma exacerbations); and 3) whether serum 25OHD concentrations are associated with lung function. Method A cross-sectional analysis was carried out on baseline data collected from 5,088 participants in the ViDA study, which is a large randomised controlled trial, to determine whether vitamin D supplementation prevents cardiovascular disease. Participants were mainly recruited from general practice registers in the Auckland region during 2011-2012. Baseline interviews were carried out by staff from the School of Population Health at the Tamaki campus of the University of Auckland. As part of the baseline interview, 680 participants reported that they had been told by a doctor that they had asthma, and provided further information on their asthma severity and exacerbations. Serum 25OHD was measured by liquid chromatography-tandem mass spectrometry, and converted to a deseasonalised value for each participant. Lung function was measured with a KoKo Trek spirometer, according to American Thoracic Society guidelines, except only three expirations were carried out due to time constraints, and to avoid exhaustion of elderly participants. Results Mean (SD) levels of deseasonalised 25OHD were not different between those with and without asthma [65.5 (25.0) nmol/L vs 66.2(22.3) nmol/L, P =0.5]; and neither were they related to having severe asthma , having an asthmatic attack in the last 12 months , having to use asthma medication , and having uncontrolled asthma . In contrast, asthma participants who needed emergency care at a clinic, hospital or who had a hospital admission in the last 12 months had significantly lower mean deseasonalised 25OHD levels than those who did not, adjusting for age, sex, ethnicity, BMI, smoking, sun exposure, physical activity and skin reaction to sun [Mean (SD): 60.7(2.8) nmol/L, 53.1(3.6) nmol/L , P = 0.009 and 60.2(2.8) nmol/L, 44.8(7.0) nmol/L, P = 0.03]. Lung function measures (Forced Expiratory Volume in One Second, Forced Vital Capacity) were positively associated with 25OHD categories of deficient, insufficient, sufficient and high, independent of age, sex, ethnicity, BMI, smoking, physical activity and sun exposure and skin reaction to sun (p<0.05). Conclusion Vitamin D status was not related to the presence of asthma in older New Zealanders. However, asthma participants who needed emergency care for their asthma in the last 12 months had lower vitamin D levels, while lung function was reduced in those with low vitamin D status. These results support the need for further clinical trials of vitamin D supplementation in asthma patients to determine if asthma exacerbations can be prevented and lung function can be improved.