Abstract
There is a growing interest in the characterisation of polyphenols in plant-based food because of their wide applications in human health and diseases. However, the biological properties of these compounds in vivo highly depend on the extent of their biotransformation within the gut. Almost 90% of polyphenols reach the colon intact where they encounter the gut microbiota. However, studying metabolism of dietary polyphenols by human gut microbiota is inherently challenging due to the complexity of the interactions, the metabolites’ reactivity, structural diversity, and broad concentration range. This PhD thesis aimed to investigate the interaction between two health promoting plant foods, Wild strawberry (Fragaria vesca) and Mankai® (Wolffia globosa) and the human gut microbiota. A pH-controlled batch-culture fermentation system, mimicking the environmental conditions of the proximal colon, was used measure the ability of wild strawberry and Mankai® to modulate the gut microbiota and its production of a small phenolic acids and short chain fatty acids. Wild strawberry both an in vitro digestion model and samples collected from ileostomy patients post strawberry ingestion were used as inocula for faecal fermentation experiments. Further, the anti-inflammatory and anti-cancerogenic activity of microbial metabolites were tested on different cell lines. Both Mankai® and Wild strawberry significantly modified gut microbiota composition, and experienced extensive but similar intestinal digestion and microbial transformation. There were some differences between in vivo (ileostomate) and in vitro digested materials. Fermentation of both foodstuffs resulted in increased concentrations of small phenolic acids and SCFA. Finally, measuring phenolic acid concentrations from plasma samples from the DIRECT-PLUS dietary intervention showed significant increase in naringenin and 2,5 dihydroxybenzoic acid after 18 months intake of a Green Mediterranean diet containing Mankai®. Production of naringenin in particular was correlated with microbiome patterns and upon measurement of transepithelial electrical resistance on Caco2 monolayers, appeared to improve markers of intestinal integrity. This work has confirmed the important role of the gut microbiota in converting plant polyphenols into smaller, biologically available and biologically active derivatives, and correlated metabolite production with gut microbiota composition. It has also demonstrated the reciprocal ability of whole plant foods to modulate the composition of the human gut microbiota and advanced our understanding of the important two-way communication between plant polyphenols and the gut microbiota in mediating some of the health effects of whole plant foods.