Recruitment and sampling strategy:
Participants will be recruited from the University of Chester (staff and students) and surrounding areas, United Kingdom (UK) only. Emails, posters and social media will be used to advertise the study to the target group (adults with asthma). A total of 28 adults with asthma aged 18-65 years old will be included in the study based on criteria described below. Eligibility will be based upon previous diagnosis of asthma by a general practitioner (GP) and the number of times asthma medication is required at screening before the start of the study. Participants will be required to complete a screening questionnaire which will provide information about participant's health status, drugs/medication/supplement usage and sun exposure. Those that have had asthma diagnosed by a GP and require their medication more than twice per week will be included. All eligible participants will be briefed on the study protocol before the beginning of the study, and the eligible participants will be given a participant information sheet (PIS) and asked to provide written informed consent.
A 12 week, double blind, randomised controlled trial will be carried out in the clinical laboratory at The University of Chester, UK. Most vitamin D and asthma studies have used supplement concentrations up to 120, 000 international units (IU) with no adverse effects. The European Food Standards Agency (EFSA) set a no-observed-adverse-effect-level (NOAEL) of 10,000IU/day (COT, 2015).
Participants will be randomly allocated to 2 groups:
1. vitamin D group
2. placebo group
Participants assigned to the vitamin D group will consume one capsule containing 5000 IU vitamin D every day. Participants in the placebo group will take an identical capsule containing inert filler. Participants will be asked to maintain their dietary habits and physical activity. The remaining supplements will be returned at the final clinic (week 12) and compliance will be estimated.
Participants will be required to attend three clinics in total over a period of 12 weeks, and they will be assessed at baseline (Day 0), interim (Week 6) and post-intervention (Week 12). At each clinic, a 30ml blood sample will be taken and both height and weight will be measured. Before the start and end of the study, participants will be required to keep a 3-day food diary which includes two weekdays and one weekend day to represent the habitual dietary intake of the participants. At each clinic, participants will complete spirometry to measure forced expiratory volume in one second (FEV1) and forced vital capacity (FVC) to assess lung function and complete the Asthma Control Test™ (ACT) to assess asthma symptoms. Blood samples will be analysed on collection for a total blood cell count. Blood samples will be centrifuged to remove plasma. Plasma samples will be batch analysed at the end of the intervention to assess biomarkers of vitamin D metabolism and inflammation.
Sample size and justification:
Sample size was estimated using vitamin D supplementation data from a study in which FEV1 post intervention at 6 months (mean ± standard deviation (S.D)) of 51.6 ± 9.4 L (vitamin D treatment group) and 31.9 ± 7.6 L (placebo group) was reported in chronic obstructive pulmonary disease patients receiving 100, 000 IU vitamin D each month. With an effect size of 1.8493555, the total sample size required for each group in the proposed study is 9. Taking into account a 20% drop out rate, and this study lasting only 12 weeks, the total sample size required is 14 (power = 0.9). Therefore, the total sample size needed for the whole study is 28. Sample size was estimated using G Power Software.
Statistical Analysis:
All statistical analyses will be conducted with the International Business Machines (IBM) Statistical Package for the Social Sciences (SPSS) Statistic Data Editor Software (version 25). Assuming the data is normally distributed after performing a Shapiro Wilks normality test, a mixed model analysis of variance (ANOVA) with t-test post-hoc analysis will be conducted to compare the mean differences for all measured parameters between the groups at the different time points. A non-parametric approach will be adopted if the data is not normally distributed for all analyses. Differences will be considered significant with a p value ≤ 0.05.
