Introduction
Hesperidin and naringin are citrus flavonoids with known anti-inflammatory effects. Evidence also indicates that they are metabolized by gut microbes and can modulate microbiota composition and activity. We investigated the effect of a citrus flavonoid-rich extract (CE) on markers for gut health in humans and in vitro.

Methods
In a randomized, double-blind, placebo-controlled study with 50 participants, fecal short chain fatty acid (SCFA) and calprotectin were measured at baseline and after 12 weeks. In the Simulator of the Human Intestinal Microbial Ecosystem (SHIME®), SCFA production and microbiota composition were measured after 3 weeks. For both studies treatment consisted of 500 mg CE (MicrobiomeX®)/day. In an in vitro co-culture model combining Caco-2 monolayers with THP-1cells, transepithelial resistance, FITC–dextran permeation, expression of barrier related genes, NF-κB activity and cytokine production were assessed after treatment with CE, hesperidin, naringin and 10 metabolites that can be formed during intestinal metabolism.

Results
In humans, CE intake resulted in a non-significant reduction (p=0.058) in calprotectin and beneficially altered the SCFA profile. SCFAs were also beneficially affected by CE treatment in vitro, showing a significant increase in butyrate and propionate levels in the colon vessels. In addition, the relative abundance of the butyrate producing E. rectale/C. coccoides group was increased. In the co-culture study, incubation with citrus flavonoid compounds did not induce changes in barrier function outcomes but showed significant anti-inflammatory effects. NF-κB activity and production of IL-8, TNF-α and IL-6 was inhibited by most compounds while levels of IL-1β were increased, suggesting that the citrus flavonoids and their metabolites are able to exert comparable effects.

Conclusion
Evidence so far suggests that citrus flavonoids have the potential to contribute to improved gastrointestinal function and health, likely due to a combined effect of the original compounds, their metabolites, and an interaction with the intestinal microbiome.