An increased intestinal permeability (IP), also known as leaky gut, could be responsible for activation of the immune system and inflammation, a common mechanism in many chronic age-related diseases. 

The MaPLE project tested the hypothesis that an increased intake of polyphenol-rich foods can reduce IP and the quantity of inflammogenic bacterial factors in the bloodstream, promoting a protective metabolic phenotype. A dietary protocol was developed to deliver three portions per day of specific polyphenol-rich foods/beverages (berries and derived products, blood oranges and juice, pomegranate juice, Renetta apple and purée, green tea and dark chocolate products) to older participants living in a controlled setting (i.e. nursing home and residencies).

The polyphenol-enriched diet caused reductions in IP (reduced serum zonulin) and the effect was greatest in participants with higher baseline IP and metabolic dysregulation. Moreover, a reduction of diastolic blood pressure was found.Food metabolome profiles were characterized by an increase of flavanols (present in cocoa), the flavanones hesperetin and naringenin (present in oranges) and specific products of microbial metabolism, such as coumaric acids and urolithins (derived from blueberries and pomegranate). After polyphenol-rich diet intervention, the reductions in IP (serum zonulin) were associated with increases in theobromine and two methylxanthine metabolites.  We were unable to detect significant changes in the overall faecal metabolites. However, it is noteworthy that, following the polyphenol-rich dietary pattern, we found a significant increase of microbial-diversity suggesting an improvement in the gut microbial ecosystem, a reduction of specific pro-inflammatory bacteria and an associated increase of protective microbial derived metabolites.

Mechanisms were investigated using cultured intestinal cell and animal models. Using very old mice, it was shown that supplementation with a mixture of polyphenols that were similar to those incorporated into the older human participant diets caused a significant reduction in IP. No decreases in IP were observed in the younger mice, suggesting that the polyphenol-mediated effects are restricted to the very old animals. The polyphenol-supplemented diet did not change the gut microbiota structure or faecal metabolite profiles and the gut immune response to a flagellin challenge. Using cultured intestinal cells, it was shown that specific flavonoids caused rapid and sustained increases in barrier function. In addition, specific polyphenols caused significant changes in the secretion of inflammatory cytokines in response to flagellin, a bacteria-derived inflammatory challenge. 

The MaPLE project will generate new data for the exploitation in terms of future dietary guidelines that can be easily applied in different contexts such as nursing homes or for the support in the management of different clinical conditions associated with increased IP.

Aging is a societal challenge that need to be faced through multi target strategies including the important contribution of an adequate diet to healthy aging. Experimental evidence suggests that an alteration in human intestinal barrier properties can occur during aging. In this regard, the Maple project has produced several important findings and possibility of exploitation in different contexts as it is reported below.

- Intestinal permeability appeared to be increased in aged people and should be considered as a potential target for screening (e.g. of increased risk of inflammation, metabolic disorders and associated clinical conditions)

- Serum zonulin, a protein involved in the regulation of intercellular junctions, was correlated with circulating microbial DNA suggesting these as potential future evaluation as candidate biomarkers for age-related clinical conditions

- A polyphenol-rich diet had a positive impact on IP in a group of older subjects, and these findings were supported by similar findings in an animal model of the ageing gut

- We have generated evidence to support a plausible (but as yet unproven) mechanism of action, i.e. that the beneficial effects of the PR diet on IP were the result of decreases in a group of pro-inflammatory and IP-associated bacteria, and these changes were also associated with changes in important bacterial metabolites (SCFAs) associated with a healthy gut.

- In vitro, specific polyphenols were shown to (i) cause rapid and sustained increases in intestinal barrier function (flavonoid-induced increases in TEER) and (ii) reduce the expression of inflammatory cytokines such as IL-8 in flagellin-challenged CaCo2 cells (quercetin and genistein, but not epigallocatechin gallate).

- An increased polyphenol intake caused also a reduction in diastolic blood pressure and the effect was stronger in women.

- We generated data that boost the study of polyphenol metabolites derived from bacterial activity, in order to reflect the microbiota's footprint on nutritional biomarkers.

-The inhibition of the intestinal enzyme PARP1 by specific methylxanthine metabolites and microbial derived polyphenol metabolites could explain, at least in part, and at molecular scale, the positive effects of the MaPLE diet on the intestinal permeability.

-The development of an in-house polyphenol database will support estimations of the intakes of different classes of and individual polyphenols, and their food sources, in future cohort and RCT studies. It will also underpin efforts to develop new targeted approaches for dietary recommendations in older adults.

-Our study demonstrated that older subjects are available to include food products in their diet if this does not change dramatically their dietary habits. The results will impact on future dietary guidelines, the development of new food products and translational research including efforts to increase awareness of the general public