Dietary β-1,3/1,6-Glucan from Baker’s Yeast Supports Upper Respiratory Mucosal Immune Health in Healthy Adults: Evidence from a Randomized, Double-Blind, Placebo-Controlled Trial

<b>Background/Objective</b>: Dietary polysaccharides are increasingly recognized as functional nutritional components that support human health through modulation of immune function. However, clinical evidence linking their intake to site-specific upper respiratory mucosal immune health in humans remains limited. This study investigated whether dietary β-1,3/1,6-glucan (SC-BG) from baker's yeast may support upper respiratory mucosal immune health in healthy adults. <b>Methods</b>: Following in vitro assays on human dendritic cells (DCs), a randomized, double-blind, placebo-controlled parallel-group trial was performed in healthy adults (18-69 years) who consumed either SC-BG or placebo capsules for 12 weeks in Japan. The primary outcome was circulating DC activation. Secondary outcomes were mucosal immune markers including secretory immunoglobulin A (s-IgA) and lysozymes from site-specific mucosal swabs (posterior oropharyngeal wall/nasopharynx) and salivar, and self-perceived upper respiratory tract symptoms. <b>Results</b>: SC-BG increased CD80 expression in DCs in vitro. In the clinical trial, 40 participants were randomized (n = 20 per group), and 39 (SC-BG: n = 19, placebo: n = 20) were analyzed. At week 8, the SC-BG group showed higher cDC1 CD80 expression than placebo. SC-BG intake also attenuated declines in s-IgA levels in the posterior oropharyngeal wall and nasopharynx and increased salivary lysozyme concentrations. Participants receiving SC-BG reported fewer cumulative days with upper respiratory tract-related local and systemic symptoms. No test food-related adverse events were observed. <b>Conclusions</b>: These findings provide preliminary clinical and mechanistic observations suggesting that dietary SC-BG may enhance circulating cDC1 activation and may help support upper respiratory local mucosal immune health in healthy adults, highlighting the potential of dietary polysaccharides as functional nutritional strategies for maintaining immune resilience.