Incorporating Euglena gracilis into the habitual diet modulates immune function and reduces common cold-like symptoms: a randomized controlled trial

BACKGROUND: Dietary approaches to support immune function have emphasized the importance of dietary fiber, including non-digestible polysaccharides, although intake remains below recommended levels in many populations. In this context, microalgae have attracted attention as sustainable sources of these components. Euglena gracilis, a microalga used in foods, contains paramylon, a β-1,3-glucan with reported immunomodulatory properties; however, its effects on plasmacytoid dendritic cell (pDC) activation in humans within a dietary context remain incompletely understood. METHODS: This randomized, double-blind, placebo-controlled trial investigated the effects of 8-week E. gracilis intake (1,000 mg/day) on immune function and common cold symptoms in healthy Japanese adults. A total of 200 participants were randomized, of whom 181 (91 in the Euglena group and 90 in the placebo group) were included in the per-protocol analysis. The primary endpoint was the change in the log-transformed median fluorescence intensity (MFI) of CD86 on pDCs at Week 8. RESULTS: Analysis using a linear mixed model revealed that the mean ± standard deviation of the log-transformed CD86 MFI at Week 8 was higher in the Euglena group than in the placebo group (placebo: 7.0 ± 2.1; Euglena: 7.7 ± 0.4; group difference 0.7; 95% confidence interval 0.2-1.1; P = 0.004). As exploratory secondary outcomes, the circulating monocyte percentage was higher and the incidence of common cold symptoms-including chills, fatigue, sneezing, rhinorrhea, nasal congestion, sore throat, cough, and headache-was lower in the Euglena group than in the placebo group. CONCLUSIONS: These findings suggest that daily consumption of E. gracilis may support immune function and be associated with a reduced incidence of common cold-like symptoms. The intervention was well tolerated, with no adverse events reported and no clinically meaningful changes in hematological or urinalysis parameters. These findings support the potential of E. gracilis consumption as a viable dietary strategy. TRIAL REGISTRATION: UMIN Clinical Trials Registry, UMIN000056125.