Maitake mushroom supplementation modulates upper respiratory symptoms and immune responses in healthy adults: a randomized, double-blind, placebo-controlled trial.

Upper respiratory tract infections (URTIs) are common viral illnesses whose burden increases with age-related declines in innate immunity. Maitake mushrooms (Grifola frondosa) have immunomodulatory properties, but evidence in healthy humans, particularly regarding age-related innate immune responses, remains limited. In this randomized, double-blind, placebo-controlled trial, healthy young adults (18-20 years) and middle-aged/older adults (40-85 years) consumed bread containing one of two maitake strains (Y10M or C5304) or a placebo daily for 11 weeks (young adults) or 18 weeks (middle-aged/older adults). While the overall incidence of URTI episodes did not differ between groups, symptom-specific analyses suggested strain-dependent differences. Y10M supplementation was associated with fewer cumulative days of fever and runny nose in both age groups and with sore throat in young adults, whereas C5304 showed limited effects. Notably, Y10M was associated with changes in innate immune parameters in middle-aged/older adults, including increased plasmacytoid dendritic cell frequency, activation of conventional dendritic cells, and upregulation of type I interferon-related gene expression and IL12A following ex vivo stimulation. These findings suggest that dietary maitake supplementation, particularly with the Y10M strain, may reduce the cumulative burden of selected URTI-related symptoms and modulate antiviral innate immune responses in healthy adults, with effects that may differ by strain and age cohort.