Soluble fibre supplementation with and without a probiotic in adults with asthma: A 7-day randomised, double blind, three way cross-over trial

Three-arm randomized, double-blind, placebo-controlled cross-over trial in 17 adults with stable asthma (9 female, 8 male; age 19–82). Participants completed 3 x 7-day interventions (inulin, inulin + probiotic, placebo) in random order, with 2-week run-in and 2-week washout periods. Most used inhaled corticosteroids; 47% had ACQ6 ≥0.75 at baseline.

Inulin 12 g/day (6 g twice daily) for 7 days; inulin 12 g/day + multi-strain probiotic (~25 billion CFU total; Lactobacillus acidophilus LA-5 7.5B CFU, L. rhamnosus GG 8.75B CFU, Bifidobacterium animalis subsp. lactis BB-12 8.75B CFU) daily; placebo: maltodextrin powder 6 g twice daily. Powder sachets mixed with water and consumed morning and night; capsules taken in the morning.

Primary outcome (plasma SCFAs) did not differ with inulin or inulin + probiotic versus placebo. Within the inulin arm, ACQ6 decreased by median −0.35, sputum eosinophils decreased by 1.0 percentage points, and sputum HDAC9 expression decreased by −0.49 (2−ΔCt). Among participants with poorly controlled asthma at baseline (ACQ6 ≥0.75; n=8), 100% improved after inulin, with 63% achieving MCID (≥0.5). Microbiome changes included increased Bifidobacterium OTUs after both arms; significant increases in B. adolescentis and B. longum after inulin + probiotic vs placebo; Anaerostipes increased after both arms; Roseburia decreased after inulin; Erysipelotrichaceae decreased after inulin. No significant changes in GPR41/43 expression. FEV1 changes correlated with increases in fecal SCFAs after inulin. Inulin + probiotic did not yield additional clinical benefits. Treatments were well tolerated with only mild GI symptoms. Soluble fiber may be a non-pharmacological option to support asthma management, especially in poorly controlled or eosinophilic asthma, but larger, longer trials are needed.

Small sample size (n=17); cross-over design with 2-week washouts; 7-day interventions; post-hoc within-group analyses; plasma SCFA sampling at 12 h fast may miss postprandial peak; not powered for microbiome changes; baseline obesity and asthma heterogeneity may limit generalizability; synbiotic combination did not show added benefit.