Blueberries are a rich source of polyphenols, widely studied for the prevention or attenuation of metabolic diseases. However, the health contribution and mechanisms of action of polyphenols depend on their type and structure. Here, we evaluated the effects of a wild blueberry polyphenolic extract (WBe) (Vaccinium angustifolium Aiton) on cardiometabolic parameters, gut microbiota composition and gut epithelium histology of high-fat high-sucrose (HfHS) diet-induced obese mice and determined which constitutive polyphenolic fractions (BPF) was responsible for the observed effects. To do so, the whole extract was separated in three fractions, F1) Anthocyanins and phenolic acids, F2) oligomeric proanthocyanidins (PACs), phenolic acids and flavonols (PACs degree of polymerization DP < 4), and F3) PACs polymers (PACs DP > 4) and supplied at their respective concentration in the whole extract. After 8 weeks, WBE reduced OGTT AUC by 18.3% compared to the HFHS treated rodents and the F3 fraction contributed the most to this effect. The anthocyanin rich F1 fraction did not reproduce this response. WBE and the BPF restored the colonic mucus layer. Particularly, the polymeric PACs-rich F3 fraction increased the mucin-secreting goblet cells number. WBE caused a significant 2-fold higher proportion of Adlercreutzia equolifaciens whereas oligomeric PACs-rich F2 fraction increased by 2.5-fold the proportion of Akkermansia muciniphila. This study reveals the key role of WBE PACs in modulating the gut microbiota and restoring colonic epithelial mucus layer, providing a suitable ecological niche for mucosa-associated symbiotic bacteria, which may be crucial in triggering health effects of blueberry polyphenols.Wild blueberries (Vaccinium angustifolium Aiton) are recognized as a rich source of bioactive phenolic compounds, the consumption of which has been associated with the attenuation of metabolic disorders through its beneficial action on glucose homeostasis and the reduction the oxidative stress and intestinal inflammation 1,2 . Until now, these positive effects have predominantly been attributed to flavonoids and in particular to anthocyanins 3,4 . However, wild blueberry has a complex profile of bioactive polyphenols; in addition to anthocyanins, it contains chlorogenic acid, and oligomers and polymers of flavan-3-ols, such as proanthocyanidins (PACs) 5 . Interestingly, there are increased evidences showing that PACs can attenuate the progression of metabolic syndrome, including type 2 diabetes and obesity 6 . Pre-clinical studies have suggested different mechanisms by Figure 3. Wild blueberry polyphenolic fractions restore the mucus thickness and polymeric PACs-rich fraction significantly stimulated mucin-secreting goblet cells number in HFHS-fed mouse colon. The effect of HFHS-diet and the supplementation with WBE and BPF: anthocyanin and phenolic acids (F1), oligomeric PACs, phenolic acids and flavonols (F2) and polymeric PACs (F3) were studied in mouse colon morphology. A combination of Alcian Blue and periodic acid-Schiff...