Effect of Flavonoids on Upper Respiratory Tract Infections and Immune Function: A Systematic Review and Meta-Analysis.
What this study found
Flavonoid supplementation reduced upper respiratory tract infection incidence versus control, with an overall 33% reduction (95% CI: 31%, 36%), and no clear increase in adverse effects. Evidence for reduced URTI duration or severity was unclear or inconsistent, although sick-day count decreased by 40% in some analyses. Bio-immune markers showed trivial to small differences between flavonoid and control groups, and immune responses after exercise were similar with or without flavonoids. The review concluded that flavonoids may help prevent URTIs, but the optimal flavonoid type and dose remain…
- Study & population
- Systematic review and meta-analysis of randomized controlled trials in generally healthy adults, including some athletes and trained individuals, from multiple countries.
- Intervention
- Flavonoid supplementation was evaluated across multiple randomized trials using different forms, including quercetin, anthocyanins, proanthocyanidins, isoflavones, taxifolin-containing larch extract, catechin-rich products, and flavonoid-containing foods or juices.
- Key limitation
- Included trials were heterogeneous in flavonoid type, dose, duration, and participant characteristics, which limits direct comparison.
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Open PDFOriginal abstract
Previous research on animals indicates flavonoid compounds have immunomodulatory properties; however, human research remains inconclusive. The aim of this systematic review was to assess the efficacy of dietary flavonoids on upper respiratory tract infections (URTIs) and immune function in healthy adults. A created search strategy was run against Cochrane Central Register of Controlled Trials, MEDLINE, EMBASE and EMBASE classic, CINAHL, and AMED. The returned studies were initially screened, and 2 reviewers independently assessed the remaining studies for eligibility against prespecified criteria. Fourteen studies, of 387 initially identified, were included in this review, and the primary outcome measure was the effect of flavonoids on URTI incidence, duration, and severity. Of the included studies, flavonoid supplementation ranged from 0.2 to 1.2 g/d. Overall, flavonoid supplementation decreased URTI incidence by 33% (95% CI: 31%, 36%) compared with control, with no apparent adverse effects. Sick-day count was decreased by 40% with flavonoid supplementation, although unclear. Differences in bio-immune markers (e.g., interleukin-6, tumor necrosis factor-α, interferon-γ, neutrophils) were trivial between the intervention and control groups during the intervention and after exercise when a postintervention exercise bout was included. These findings suggest that flavonoids are a viable supplement to decrease URTI incidence in an otherwise healthy population.