Sociodemographic and illness characteristics of the non-participants when compared with the study participants.

Abbreviations: 95%CI 95% confidence interval; BMI Body mass index; FM Fibromyalgia; IBS Irritable bowel syndrome; M Median; PwME/CFS People with Myalgic Encephalomyelitis/Chronic Fatigue Syndrome; Q1–Q3 Quartile 1 to quartile 3. Bolded values indicate that at least one post-hoc test returned significance (p a Analysed with independent samples one-way ANOVA test. b Analysed with the Fisher-Freeman-Halton test. c Sum of percentages is greater than 100.0% for each cohort, as some participants identified more than one ethnicity. d Analysed with the Chi-square test. e Analysed with the Kruskal-Wallis H test. f Presence of frequent concurrent diagnoses with ME/CFS captured within the CCC and ICC reported at least once across the three time points for the n = 32 pwME/CFS, at T0 for the n = 19 participants lost to follow-up or at screening for the n = 18 ineligible respondents. g Includes participants self-reporting a diagnosis of acid reflux or gastro-oesophageal reflux disease. * Ineligible respondents (XLSX)