Increasing evidence links intestinal permeability (IP), a feature of the intestinal barrier, to several pathological or dysfunctional conditions. Several host and environmental factors, including dietary factors, can affect the maintenance of normal IP. In this regard, food bioactives, such as polyphenols, have been proposed as potential IP modulators, even if the mechanisms involved are not yet fully elucidated. The aim of the present paper is to provide a short overview of the main evidence from in vitro and in vivo studies supporting the role of polyphenols in modulating IP and briefly discuss future perspectives in this research area.

Background: During aging, alterations of the intestinal microbial ecosystem can occur contributing to immunosenescence, inflamm-aging and impairment of intestinal barrier function (increased intestinal permeability; IP). In the context of a diet-microbiota-IP axis in older subjects, food bioactives such as polyphenols may play a beneficial modulatory role. Methods: MaPLE is a project centered on a randomized, controlled cross-over dietary intervention trial [polyphenol-rich diet (PR-diet) versus control diet (C-diet)] targeted to older people (≥ 60 y) living in a wellcontrolled setting (i.e. nursing home). The 8-week interventions are separated by an 8-week wash-out period. Three small portions per day of selected polyphenol-rich foods are consumed during intervention in substitution of other comparable products within the C-diet. Biological samples are collected before and after each treatment period to evaluate markers related to IP, inflammation, vascular function, oxidative stress, gut and blood microbiomics, metabolomics. A sample size of 50 subjects was defined based on IP as primary outcome. Discussion: Evidence that increasing the consumption of polyphenol-rich food products can positively affect intestinal microbial ecosystem resulting in reduced IP and decreased translocation of inflammogenic bacterial factors into the bloodstream will be provided. The integration of data from gut and blood microbiomics, metabolomics and other IP-related markers will improve the understanding of the beneficial effect of the intervention in the context of polyphenols−microbiota−IP interactions. Finally, findings obtained will provide a proof of concept of the reliability of the dietary intervention, also contributing to future implementations of dietary guidelines directed to IP management in the older and other at risk subjects. Trial registration: The trial is registered at (ISRCTN10214981); April 28, 2017.

No abstract

HIGHLIGHTS• Improvements in dietary assessment will deal with weaknesses between diet and health.• Multi-metabolite biomarker panels offer a better estimation than single biomarkers.• Untargeted metabolomics enables the proposal of new multi-metabolite biomarker panels.• A series of challenges should be addressed before panels can reach their full potential.• The combined use of biomarker panels with questionnaires will enable increasing accuracy and precision in dietary assessment. ABSTRACTBackground: Accurate measurement of food intake is the cornerstone of understanding the links between diet and optimal health status or risk of disease. The utilization of metabolomics approaches is revolutionizing the field of dietary assessment by associating metabolic profiles with intake of specific foods or dietary patterns and/or investigating human health status in nutritional trials. Combining dietary biomarkers with conventional dietary assessment methods is considered a potential strategy for tackling the complexity of dietary exposure fingerprinting. Scope and approach:We discuss existing approaches among dietary assessment methods and dietary biomarkers. A combined approach taking into consideration data from dietary questionnaires with measurements of dietary biomarkers is emphasized. Key findings and conclusions:Trends in novel strategies for improving dietary exposure assessment will be influenced by the discovery and validation of dietary exposure biomarkers.Among different strategies, multi-metabolite biomarker panels enable more reliable estimation of dietary exposure than does the traditional single-biomarker approach. Therefore, a combined approach using data from dietary questionnaires along with measurements of dietary biomarkers is considered an excellent strategy for improving dietary exposure assessment.

The gastrointestinal tract hosts the largest microbial population of the human body, which works in symbiosis with the host to provide several important functions and contributes to the maintenance of host health. The diet is one of the factors that can most affect the gut microbiota, with subsequent consequences on host health. One consequence of changes in microbiota is changes in intestinal permeability (IP); disruption of this latter is related to the development of several diseases and is a frequent condition in older people. Nevertheless, the molecular pathways regulating these effects are still unclear, and a comprehensive understanding of the dietary components that can affect IP is lacking. Metabolomics, that has been widely used to study the transformation of nutrients by intestinal microbiota, could be a suitable approach for this purpose. However, up to now, the research has focused mainly on dietary fibers and tryptophan, while the activity of dietary polyphenols remains almost completely unexplored. Hence, the aim here was to review the most recent literature concerning the application of metabolomics in the study of the correlation between diet-induced alterations of gut microbiota and the effects on intestinal permeability, with a particular focus on the discovery of the molecular pathways involved. An additional aim was to give a perspective on the future research involving dietary polyphenols, given that despite their potential implication for the prevention and treatment of several diseases related to increased intestinal permeability, few studies have been reported to date.

Background &aim: The MaPLE study was a randomized, controlled, crossover trial involving adults !60 y.o. (n ¼ 51) living in a residential care facility during an 8-week polyphenol-rich (PR)-diet. Results from the MaPLE trial showed that the PR-diet reduced the intestinal permeability (IP) in older adults by inducing changes to gut microbiota (GM). The present work aimed at studying the changes in serum metabolome in the MaPLE trial, as a further necessary step to depict the complex crosstalk between dietary polyphenols, GM, and intestinal barrier. Methods: Serum metabolome was monitored using a semi-targeted UHPLC-MS/MS analysis. Metataxonomic analysis (16S rRNA gene profiling) of GM was performed on faecal samples. Clinical characteristics and serum levels of the IP marker zonulin were linked to GM and metabolomics data in a multi-omics network. Results: Compared to the control diet, the PR-diet increased serum metabolites related to polyphenols and methylxanthine intake. Theobromine and methylxanthines, derived from cocoa and/or green tea, were positively correlated with butyrate-producing bacteria (the order Clostridiales and the genera Roseburia, Butyricicoccus and Faecalibacterium) and inversely with zonulin. A direct correlation between polyphenol metabolites hydroxyphenylpropionic acid-sulfate, 2-methylpyrogallol-sulfate and catecholsulfate with Butyricicoccus was also observed, while hydroxyphenylpropionic acid-sulfate and 2methylpyrogallol-sulfate negatively correlated with Methanobrevibacter. The multi-omics network indicated that participant's age, baseline zonulin levels, and changes in Porphyromonadaceae abundance were the main factors driving the effects of a PR-diet on zonulin. Conclusion: Overall, these results reveal the complex relationships among polyphenols consumption, intestinal permeability, and GM composition in older adults, and they may be important when setting personalized dietary interventions for older adults. Trial registration number: ISRCTN10214981.

Polyphenols have great potential in regulating intestinal health and ameliorating pathological conditions related to increased intestinal permeability (IP).However, the efficacy of dietary interventions with these phytochemicals may significantly be influenced by inter-individual variability factors affecting their bioavailability and consequent biological activity. In the present study, urine samples collected from older subjects undergoing a crossover intervention trial with polyphenol-rich foods were subjected to metabolomics analysis for investigating the impact of increased IP on the bioavailability of polyphenols.Interestingly, urinary levels of phase II and microbiota-derived metabolites were significantly different between subjects with healthier intestinal barrier integrity and those with increased IP disruption. Our results support that this IPdependent impaired bioavailability of polyphenols could be attributed to disturbances in the gut microbial metabolism and phase II methylation processes. Furthermore, we also observed that microbiota-derived metabolites could be largely responsible for the biological activity elicited by dietary polyphenols against age-related disrupted IP.

Scope Dietary polyphenols can alter the gut microbiota (GM) and promote the production of bioactive metabolites. Several indoles result of GM metabolism of dietary tryptophan have been associated with intestinal barrier integrity. Our aim is to study the changes in GM‐derived indoles during a polyphenol‐rich (PR) diet intervention in older adults. Methods and Results Randomized, controlled, crossover trial in adults ≥ 60 years living in a residential care facility during an 8‐week PR versus control diet (n = 51). Seven GM‐tryptophan metabolites are measured in serum, and metataxonomic analysis of GM is performed on fecal samples. Exploratory subgroup analyses are performed based on renal function (RF). The PR‐diet significantly increases serum indole 3‐propionic acid (IPA) in subjects with normal RF, but not in subjects with impaired RF. Other GM‐tryptophan metabolites are not affected. Comparison of baseline GM composition shows shifts in Bacteroidales order members as well as higher abundance of Clostridiales in participants with normal RF. During the trial, variations of IPA are associated with changes in C‐reactive protein (β = 0.32, p = 0.010) and GM, particularly with the Clostridiales (r = 0.35, p < 0.001) and Enterobacteriales (r = −0.15, p < 0.05) orders. Conclusion A PR diet increases the serum concentration of IPA in older adults with normal RF. Our findings may be important when defining appropriate dietary interventions for older adults. Trial registration number: ISRCTN10214981 (https://doi.org/10.1186/ISRCTN10214981).