**3.2 Celiac disease**

Celiac Disease (CD) is an immune-mediated enteropathy triggered by the ingestion of gluten proteins by genetically susceptible individuals [53]. Although there is, so far, no clear explanation for the burst of gluten-related disorders in recent years, it has been speculated that this may have occurred due to an increase in the consumption of gluten-containing foods, some based on novel wheat stains produced for technological rather than nutritional reasons, and the humans' over usage of antibiotics and pesticides, both of which rendered people more sensitive to allergenic plant proteins [54].

Mechanistically, CD is characterized by an aberrant T-cell response towards gluten peptides that, because of their high proline content, are left undigested at the gut lumen [55]. According to recent findings, some of these gluten peptides have the ability to bind to the chemokine receptor CXCR3 on the surface of intestinal epithelial cells, leading to a PKC-α dependent tight junctions disassembly and increased intestinal permeability [56]. In the lamina propria, gluten peptides are selectively deamidated by the enzyme tissue transglutaminase 2 (TG2) and later presented to gluten-specific HLA-DQ2 or HLA-DQ8-restricted CD4+ T cells by antigen-presenting cells [57]. Once activated, and in response to tissue signals provided by stressed epithelial cells, gluten-specific CD4+ T cells support the activation and differentiation of both autoreactive TG2 and gluten-specific B cells into IgA and IgG-producing plasma cells while increasing the cytolytic properties of intraepithelial cytotoxic T lymphocytes which kill distressed intestinal epithelial cells based on the recognition of stress-induced ligands e.g. MHC-class I polypeptide-related (MICA/B) and HLA-E molecules [58].

The rapid increase in the global incidence of CD, together with the growing concern of CD patients regarding their quality of life when on a gluten-excluding diet, led researchers scrambling for alternative (or complementary) ways to tackle the celiac gut's response to gluten and potentially restore tolerance [59]. Among the candidates, recent evidence brough PC into the spotlight as promising agents to be used in CD management due to their wide range of beneficial properties and positive impact on human health. Nevertheless, and despite the advances made in the past few years, there are still many unresolved questions in this area, due to the multitude of action mechanisms underlying the response to PCs intake and large interindividual variability [60].

In a CD context, PC could act at several levels: they could impair gluten digestion and peptide availability at the intestinal lumen, reduce inflammation, enhance intestinal barrier integrity and function and have a prebiotic effect through inhibition of certain pathogenic groups and stimulation of beneficial bacterial growth [61]. As shown by Dias et al. green tea PC and grape seed procyanidins can readily interact with one of the most immunoreactive gluten peptides - the 33/32-mer - primarily through an unspecific, entropy-driven, hydrophobic effect [62]. In general, these interactions were found to be similar to the interactions between polyphenols and proline-rich salivary proteins in that they are the result of cooperative binding mechanisms involving both enthalpic and entropic effects. Staggeringly, the primarily PC-binding sites within the 32-mer peptide sequence were also unveiled: they correspond to leucine, tyrosine and phenylalanine-containing domains, located in four well-defined and almost indistinguishable hydrophobic clusters, equally spaced by non-polar proline residues [63]. Subsequent transepithelial transport studies on Caco-2 cell monolayers highlighted the ability of dietary doses of

EGCG to scavenge and reduce the apical-to-basolateral translocation of the 32-mer peptide in vitro to nearly undetectable levels [62]. Still, it remains unclear whether this attenuation will have any implication in the activation and triggering of a gluten-specific T-cell mediated immune response, though the structural changes induced on the peptide upon binding provides foundational support for functional changes in its immunostimulatory action [64].

On another recent breakthrough, green tea catechins were found to prevent gluten digestion through physical interactions with gluten proteins and prevention of hydrolysis by digestive enzymes [65]. According to this study, the presence of green tea catechins resulted in a decreased formation of low molecular weight gluten peptides, decreased intestinal permeability and reduced inflammation [66]. A similar finding was also made available by Kramer and coworkers which shown a significant inhibition of IFN-γ- or gluten peptide p31–43-induced increases in CD inflammatory biomarkers including TG2, COX-2, IL-15, IL-1β, IL-6, and IL-8 by procyanidin B2-rich cocoa extracts [67].

Regarding the effect produced by PC-rich dietary patterns on gut and blood microbiomics in CD patients, data are still scarce. Nevertheless, there is now several pieces of evidence suggesting that PCs may represent a relevant factor in shaping the intestinal microbial ecosystem (i.e. the microbiota and derived metabolic products) and modifying the relative abundance of specific bacterial taxa in dysbiotic CD subjects [68]. By modulating the concentration of health-affecting microbial metabolites in the gut e.g. butyrate, polyphenols are likely to regulate a plethora of biological responses at the intestinal level that control, for instance, tight junction integrity, anti-inflammatory signaling, immune cell migration, adhesion, and cellular functions such as proliferation and apoptosis [69]. Accordingly, it has been found that treatment of CD-derived organoids with microbiota-derived bioproducts, including butyrate, improved epithelial barrier functionality and reduced gliadin-induced IFN-γ and IL-15 secretion [70]. Of note, both butyrate and lactate have been shown to exert a relevant role in regulating FoxP3 isoform expression in T cells and consequent activation of a Th17-driven immune response in CD subjects [71]. But, as well-controlled intervention studies are still lacking, future studies should be focusing on providing a proof of concept of the reliability of a PC-based dietary intervention in the context of microbiota-intestinal permeability and CD health outcomes.

## **4. Use of phenolic compounds as alternative to synthetic additives in functional foods**

In the last 50 years, food systems have dramatically changed where the access to foods, with high levels of salt, sugars and saturated fats have become cheaper and more widely available than micronutrient rich foods—such as fresh fruits and vegetables. Consequently, the incidence of NCDs greatly increase such as obesity, type 2 diabetes as well as immune reactions to food, claiming for action. Food industry faces important challenges regarding the increase in these NCDs. In addition, besides the health challenge, other key points demand advances for the production of food and significant changes in food systems, namely population growth, rural development, globalization and climate changes.

Diverse personal, cultural or economic factors influence consumers' dietary behavior. Furthermore, political issues as well as the food labeling, marketing, information about food and policies could impact on price affecting consumer demand. Under this framework, strategies to protect vulnerable populations can be addressed to achieve a global access to healthy and sustainable diets. Based on

**Figure 2.** *Use of polyphenols as ingredients in functional foods.*

aforementioned, PC extracts arise as ingredients able to promote health benefits while reducing the use of synthetic additives. This section summarizes the interest in using PC as new ingredients in food formulations including the effects on technological processes (**Figure 2**).
