Flavonoids are a diverse group of phytonutrients found in many fruits, vegetables, and herbs. Their use in supporting the prevention of colds is justified by a modest body of scientific evidence. Several clinical studies and meta-analyses have investigated the effects of flavonoids, particularly subclasses such as flavonols (e.g., quercetin), on immune function and the incidence of upper respiratory tract infections (URTIs), including the common cold.

A 2016 meta-analysis published in Nutrients reviewed randomized controlled trials involving flavonoid supplementation and found a statistically significant reduction in URTI incidence among individuals supplementing with flavonoids compared to placebo. The proposed mechanisms include modulation of immune cell activity, anti-inflammatory effects, and antiviral properties.

Despite these findings, the evidence is considered moderate (rated 2 out of 5) due to limitations in study design, sample size, and heterogeneity of flavonoid types and dosages used in trials. While the results are promising, more large-scale, well-designed clinical trials are required to confirm the preventive effects of flavonoids against colds and to establish optimal types and dosages.

In conclusion, there is some scientific evidence to support the use of flavonoids for the prevention of colds, but the strength and consistency of the evidence are not strong enough to warrant a high level of recommendation.