Gut health has become a global concern due to its significant impact on human well-being; it is often called “the second brain” [96]. The human body houses a substantial number of bacteria, estimated to be 3.9 × 10¹³ in the entire body of a 70 kg “reference man” [97]. The gut microbiota includes Bacteroides, Clostridium, Prevotella, Eubacterium, Ruminococcus, Fusobacterium, Peptococcus, and Bifidobacterium, with a smaller presence of Escherichia and Lactobacillus [98,99]. This microbiota plays a vital role in the innate and adaptive immune system, which ensures a symbiotic relationship with the microbiota [100]. The gut microbiota plays a significant role in metabolizing ACNs into phenolic acids and aldehyde metabolites, primarily through specific bacteria in the large intestine. These metabolites can influence various physiological processes [101,102,103]. Conversely, ACNs contribute to the enhancement of gut health by modulating the function of the gut barrier and promoting the colonization of beneficial bacterial species. This dynamic interplay leads to heightened protection against pathogens, optimized nutrient metabolism, and an overall reinforcement of the immune response [57].

A significant portion of dietary ACNs remains unabsorbed, arriving at the colon without absorption during digestion, then, interacting with gut microbiota and undergoing degradation, potentially increasing their bioavailability through bacterial or chemical processes. Notably, certain bacteria in the colon, such as Bacteroides spp., Enterococcus casseliflavus, Eubacterium spp., and Clostridium spp., play a key role in this process by producing enzymes, such as β-glucosidase, α-galactosidase, and α-mannosidase, that break down ACNs [104,105]. In the colon, unabsorbed ACNs are hydrolyzed by microbial enzymes capable of cleaving sugar linkages and releasing ACN aglycones [106,107], which are then further metabolized into phloroglucinol aldehyde (e.g., 2,4,6-trohydroxybenzaldehyde, 3,4-dihydroxy benzaldehyde) and phenolic acids (PAs) derived from phloroglucinol (A ring) and benzoic acids (B ring) [106]. Research findings have identified distinct metabolic pathways associated with the fermentation of various anthocyanidins (aglycones), leading to diverse phenolic compounds. For example, the fermentation of cyanidin-3-O-rutinoside and C3G yields PCA, p-coumaric acid, and phloroglucinol aldehyde [108]. Similarly, delphinidin-3-O-rutinoside undergoes elaboration, resulting in gallic acid, syringic acid, and phloroglucinol aldehyde [108]. M3G is converted to syringic acid [109] while pelargonidin-3-O-glucoside (Pel3G) is metabolized into 4-hydroxybenzoic acid [110]. The initial conversion of petunidin yields 3-O-methyl gallic acid; subsequent O-demethylation leads to the production of gallic acid [110]. Furthermore, peonidin undergoes sequential transformations, first into vanillic acid (3-methoxy-4-hydroxybenzoic acid) and then into PCA [109]. In red radish, acylated ACNs, such as pelargonidin-3-sophorosid-5-glucoside acylated with ferulic acid and malonic acid, undergo degradation, resulting in the formation of 4-hydroxybenzoic acid, p-coumaric acid, ferulic acid, and caffeic acid [111].

A high-fat, high-sucrose diet induced metabolic and inflammatory changes in mice, resulting in decreased levels of beneficial gut bacteria (Bacteroidetes and Muribaculaceae) and propionate. However, supplementation with Saskatoon berry powder, C3G, or PCA mitigated these effects and increased fecal Muribaculaceae and propionate [112]. Another study using an Apc^Min/+ mouse model compared the effectiveness of the dietary administration of PCA and black raspberries (BRB) in preventing colorectal cancer. Both 5% BRBs and 500 ppm PCA-supplemented diets significantly reduced the development of adenoma in the small intestine and colon. They promoted a shift to anti-inflammatory bacterial profiles, suggesting potential benefits for colorectal cancer patients. Interestingly, 500 ppm PCA increased IFN-γ and SMAD4 (mothers against decapentaplegic homolog 4) in primary cultured human natural killer cells. In comparison, 1000 ppm PCA did not show the same effects [113]. Furthermore, a study investigated the long-lasting cardioprotective effect of dietary ACNs by examining gene expression, histology, and resistance to ischemia/reperfusion in mice hearts after a month following the cessation of a C3G-enriched diet. The results indicate that the cardioprotective benefits persisted, independent of immune responses, and were attributed to changes in the gut microbiota induced by the C3G-enriched diet [69].

C3G also alleviated polystyrene (PS)-induced toxicities in Caco-2 cells and Caenorhabditis elegans by promoting autophagy and discharge. In C57BL/6 mice, C3G supplementation reduced tissue accumulation, promoted fecal PS discharge, alleviated oxidative stress and inflammation caused by PS, and modulated PS-associated gut microbiome perturbations, indicating its potential protective role against PS-induced toxicity and gut dysbiosis [114]. In a study investigating the potential of purple sweet potato ACN extract in modulating the gut microbiota in a dextran sulfate sodium (DSS)-induced chronic colitis mouse model, treatment with the extract prevented the loss of beneficial bacteria (Bifidobacterium and Lactobacillus), inhibited increases in harmful bacteria (Gammaproteobacteria and Helicobacter), and maintained colonic tight junction protein expression and architecture, resulting in attenuated intestinal inflammation. These findings suggest that the extract could be an effective and safe treatment for chronic colitis [115]. Combining bilberry ACN with chitosan and low molecular citrus pectin was proposed to enhance digestive stability and modulate the microbiome for programmed cell death-ligand 1 (PD-L1) blockade treatment. This combo enriches subdominant species, increases butyrate concentration, enhances intra-tumoral CD8+ T cell infiltration, and restores gut microbiome diversity, improving control over tumor growth. These findings indicate potential opportunities for probiotic combos to enhance the therapeutic efficiency of immune checkpoint inhibitors through gut microbiome manipulation [116].

Bilberry ACN supplementation attenuated western diet-induced serum aspartate aminotransferase, alanine transaminase, low-density lipoprotein cholesterol levels, liver fat content, thiobarbituric acid reactive substances, and alpha-smooth muscle actin. Additionally, it modified the gut microbiota by reducing the Firmicutes/Bacteroidetes ratio and increasing the relative abundance of Akkermansia spp. and Parabacteroides spp., which suggests it may have the potential to ameliorate nonalcoholic fatty liver disease by addressing dyslipidemia and gut microbiome dysbiosis [117]. M3G ingestion improved histopathological scores, enhanced IL-10 expression, promoted beneficial microbial interactions, and reduced the abundance of pathogenic bacteria and inflammatory mediators, demonstrating its unique mechanism of action on the gut microbiome compared to a whole blueberry in the DSS-induced colitis mouse model [118]. C3G has shown the potential to support gut integrity and overall health by producing phenolic compounds that combat gut inflammation and oxidative stress. These metabolites activate nuclear factor erythroid 2-related factor 2 (Nrf2) and influence TGFβ-activated kinase 1 (TAK1)-mediated mitogen-activated protein kinase (MAKP) and sphingosine kinase (SphK)/sphingosine-1-phosphate(S1P)-mediated NF-κB pathways, collectively reducing inflammation and creating an optimal environment for gut metabolism [119].

ACNs have gained considerable attention in the nutraceutical industry, primarily for their role as natural colorants and antioxidants (Figure 3). ACN chalcones and quinoidal bases have been reported to be efficient antioxidants, with the glycosylated B-ring structure of ACNs enhancing their antioxidant activity, especially in terms of ortho-hydroxylation and methoxylation [33,48,123]. However, anthocyanidin, the aglycone form of ACNs, exhibits higher ORAC values due to its greater stability and reactivity [124]. The acylation of ACNs with phenolic acids significantly increases their antioxidant activity; diacylation enhances their activity further while 5-glycosylation reduces said activity [124,125]. Studies have shown that various ACNs extracted from plants possess antioxidative properties. C3G, D3G, and Peo3G from black bean seed coats exhibit strong antioxidative activity and reduce malondialdehyde formation by ultraviolet B (UV-B) irradiation [126]. Delphinidin and D3G show the highest inhibitory effect on lipid peroxidation and O^2•− scavenging activity. On the other hand, cyanidin and C3G demonstrate the highest inhibitory effect on copper (II)-induced low-density lipoprotein (LDL) oxidation, while delphinidin has intermediate efficacy [127]. Moreover, ACNs activate Nrf2 and antioxidant enzymes, such as superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx), enhancing their enzymatic activity [128,129]. In addition to their antioxidant effects, ACNs demonstrate potent activity against inflammation and associated disease conditions [50]. The imbalance between proinflammatory molecules and anti-inflammatory mediators leads to chronic inflammation, which is linked to various chronic diseases, such as neurodegenerative disease, arthritis, allergies, liver disease, diabetes, obesity, cardiovascular disease, and cancer [130]. Several studies showed that the ACNs suppress the activation of NF-κB [131] and MAPK signaling cascades [64], inhibiting the production of proinflammatory cytokines and enzymes, such as COX-2 [132]. ACNs directly inhibit COX-1 and COX-2 enzymes, thus reducing the production of prostaglandin E₂ (PGE₂) [132]. Moreover, ACNs also suppress NLRP3 inflammasomes, a complex involved in inflammatory responses [133,134].

Furthermore, ACNs exhibit promising potential as oral hepatoprotective agents against chemically-induced liver damage. They effectively combat liver lipid accumulation, inhibit lipogenesis, promote lipolysis, and counteract oxidative stress. They decrease liver enzyme levels, enhance antioxidant enzymes, and suppress the expression of COX-2 and iNOS, suggesting their potential therapeutic use in treating oxidative stress in the liver and its associated diseases [135]. C3G–lauric acid ester displayed enhanced antioxidant activity compared to its precursor C3G; safeguarding cells through the regulation of oxidative stress and PI3K/Akt-mediated Nrf2–heme oxygenase 1 (HO-1)/nicotinamide adenine dinucleotide (phosphate) reduced quinone oxidoreductase (NQO1) pathway activation, highlighting the potential of acylation for improved antioxidative effects and stability [136]. Myrciaria jaboticaba (Vell.) O.Berg and Syzygium cumini (L.) Skeels peel powders exhibited protective effects against liver fibrosis and hepatocarcinogenesis while Syzygium malaccense (L.) Merr. and L.M.Perry showed antioxidative benefits, emphasizing the anthocyanin profile effect [137]. Lonicera caerulea L. anthocyanin extract demonstrated hepatoprotective potential against alcoholic steatohepatitis through AMP-activated protein kinase (AMPK)-mediated anti-inflammatory and lipid-reducing pathways [138]. Bilberry fruit anthocyanin extract showed hepatoprotective effects against acute liver failure, reducing damage markers, oxidative stress, inflammation, and necrosis; these effects are attributed to antioxidative and anti-inflammatory properties [139].

Due to a high lipid content and energy demand, the central nervous system (CNS), particularly the brain, is highly vulnerable to excessive ROS [140]. ROS can be generated internally during cellular respiration and externally through environmental factors, such as pollution, smoking, UV-B radiation, and infections [141]. An excessive inflammatory response in the CNS can contribute to neuronal apoptosis and the progression of neurodegenerative diseases [142]. In this context, ACNs found in various fruits and vegetables have shown neuroprotective effects in preclinical models of neurodegenerative diseases and cerebral ischemia [143]. ACNs exert their neuroprotective effects through multiple mechanisms. They act as direct scavengers of ROS [144], enhance antioxidant response pathways [145], and inhibit neuroinflammation by modulating specific signaling pathways [146,147,148]. Studies in animal models have demonstrated that ACNs can prevent cognitive decline, delay the onset of neurodegenerative diseases, such as Alzheimer’s [149] and Parkinson’s [150], and reduce brain damage in cerebral ischemia. In a randomized controlled trial involving twelve participants with mild cognitive impairment, older adults demonstrated enhanced learning and memory capacity after 12 weeks of grape juice supplementation, compared to the placebo group [151]. Similarly, in another study, a 12-week supplementation of blueberry juice in the same group improved memory function [152,153]. These findings suggest that ACNs could be a potential co-adjuvant therapy to pharmacological treatments or a preventive strategy to reduce drug dosage and adverse effects in managing neurodegenerative diseases, cerebral ischemia, and cognition.

Dietary ACNs have demonstrated beneficial effects in protecting against cardiovascular diseases (CVDs) in animal and human studies [153]. Animal studies have shown that ACNs can prevent hypertriglyceridemia, hypercholesterolemia, and platelet hyperactivity induced by high-fat or high-fructose diets, while also protecting cardiac tissue against oxidative stress in ischemia/reperfusion conditions [154]. Human intervention studies have revealed that berry ACN intake can increase high-density lipoprotein (HDL)-cholesterol and reduce LDL-cholesterol, triglycerides, blood pressure, and inflammatory markers [155,156]. Furthermore, higher ACN intake has been associated with a reduced risk of myocardial infarction and decreased incidence of coronary heart disease and CVD-related mortality [157,158]. The cardioprotective mechanisms of ACNs include increasing plasma antioxidant capacity and nitric oxide levels, reducing LDL oxidation and platelet aggregation, and enhancing glutathione and omega-3 fatty acid synthesis. Cardiovascular diseases, including myocardial infarction, ischemic heart disease, and stroke, are major causes of mortality globally. Atherosclerosis, driven by oxidized low-density lipoproteins (ox-LDL), plays a key role in these diseases. ACNs have been found to protect against atherosclerosis in animal models by reducing the formation of atherosclerotic plaques and improving dyslipidemia. ACNs also prevent LDL-cholesterol oxidation [159], reduce inflammation [160], and decrease the expression of adhesion molecules in the aorta [160,161], leading to decreased leukocyte infiltration and proinflammatory cytokines. Additionally, ACNs have shown cardioprotective effects in animal models of myocardial infarction and chemotherapeutic drug-induced cardiotoxicity [162].

Obesity is a significant global health issue associated with numerous metabolic disorders. Studies have shown that dietary ACNs can have beneficial effects in combating obesity. One mechanism through which ACNs act as anti-obesity agents is by increasing energy expenditure [163]. Berries rich in ACNs, such as petunidin and malvidin, were found to effectively reduce the metabolic damage caused by a high-fat diet by boosting energy expenditure and reducing mitochondrial dysfunction in adipose tissue [164]. Moreover, ACNs have been found to modulate AMPK, a key regulator of energy balance [165,166]. By activating AMPK, ACNs can promote mitochondrial biogenesis, reduce lipid metabolism, increase fatty acid oxidation, and improve glucose metabolism. This activation of AMPK has been associated with improved glucose uptake in skeletal muscle, decreased glucose production in the liver, and decreased liver lipid content and serum lipoproteins. Furthermore, dietary ACNs have been shown to affect lipid metabolism by downregulating key molecules, such as fatty acid synthase and sterol-regulatory element-binding proteins. ACNs from different sources were found to reduce hyperglycemia and inhibit hepatic lipogenesis, potentially suppressing inflammation in obese individuals [167,168].

In addition to their effects on energy expenditure and lipid metabolism, ACNs have been linked to the modulation of the gut microbiota [169]. Changes in the gut microbiota in obese individuals can contribute to metabolic disorders and ACNs have been studied for their potential role in regulating the gut microbiota to combat obesity-related conditions. Moreover, ACNs have demonstrated promising results in protecting pancreatic β cells, improving insulin resistance, and reducing cholesterol levels in diabetic subjects [170]. ACN-rich beverages have also positively affected the postprandial glycemic response and insulin excretion [171]. Numerous in vitro and in vivo studies have consistently supported the potential anti-obesity effects of ACNs, demonstrating their ability to inhibit fat accumulation [172], improve lipid profiles [168], and reduce adipocyte hypertrophy [173]. Furthermore, ACNs have shown anti-inflammatory properties by suppressing NF-κB signaling and promoting an anti-inflammatory phenotype in adipose tissue macrophages [174]. Thus, ACNs present a promising natural approach to combat obesity and related metabolic disorders. The mechanisms through which ACNs exert their anti-obesity effects involve increased energy expenditure, modulation of AMPK, lipid metabolism, gut microbiota, and anti-inflammatory properties.

ACNs have emerged as potential antitumoral agents, attracting significant interest in recent decades [175,176]. Cancer is characterized by the uncontrolled cell proliferation, resistance to apoptosis and cell migration, often involving the dysregulation of pathways, such as Notch, Wnt/β-catenin, NF-κB, and MAPK [177]. Dietary ACNs extracted from berries have been found to modulate the levels of proteins involved in these pathways, leading to cell growth inhibition. Notably, ACN mixtures have shown enhanced effects compared to single purified ACNs, indicating overlapping pathways in cell growth inhibition [178]. ACNs exhibit diverse mechanisms for the prevention and inhibition of cancer, targeting pathways related to cell survival, proliferation, apoptosis, inflammation, and angiogenesis. They have been shown to suppress NF-κB [179], PI3K/Akt [180,181], and Akt/mTOR [182] signaling pathways. ACNs can also inhibit tumor growth, promote apoptosis and autophagy in cancer cells, and inhibit angiogenesis and metastatic migration [183]. In vitro and in vivo studies have demonstrated the anticancer potential of ACNs from various sources, including berry extracts [78,184] and pure ACNs, such as delphinidin [185] and cyanidin 3-rutinoside [181]. These findings suggest that ACNs hold promise regarding potential anticancer properties, making them a subject of considerable research and exploration in cancer therapy and prevention.

ACN pigments found in various berries are beneficial for maintaining good vision and have been associated with improved night vision [48,186]. Berries rich in ACNs have long been known for their positive effects on eye health. For instance, administering bilberry extract, containing about 39% ACNs, to mice has been shown to protect photoreceptor cell function during retinal inflammation [187]. In a study with 132 patients suffering from normal-tension glaucoma, daily supplementation with two capsules containing 60.0 mg ACNs from bilberry resulted in improved visual function [188]. Additionally, other berries, like blackcurrant [189] and purple corn seed [190], have shown a protective effect on eyesight, with blackcurrant ACN supplementation increasing ocular blood flow in patients with open-angle glaucoma and purple corn seed extract reducing lens opacity and malonaldehyde levels. Furthermore, ACNs from black soybean seed coats have effectively prevented retinal degeneration and suppressed human lens epithelial cell death under oxidative stress [191].

ACNs exhibit potent antimicrobial properties against a wide range of microorganisms, particularly in inhibiting the growth of food-borne pathogens. Their antimicrobial effects are attributed to various mechanisms, including cell wall, membrane, and intercellular matrix damage [175]. For instance, maqui berry extracts have shown antibacterial activity, being highly effective in working against Aeromonas hydrophilia and Listeria innocua [192], commonly associated with refrigerated foods and used as indicators of pathogenic or spoilage microorganisms [193]. Cranberry extract also demonstrates antibacterial activity against certain resistant strains of Enterococcus faecium, Pseudomonas aeruginosa, Staphylococcus aureus, and Escherichia coli, with its effect not solely due to low pH but rather to specific bioactive components, such as ACNs and flavonols [194]. ACN-rich extracts from various fruits, like blueberry, raspberry, blackcurrant, and strawberry, display inhibitory effects on gram-negative bacteria but not on gram-positive bacteria [195], likely due to differences in cell wall structures. These antimicrobial actions are likely a result of synergistic effects from various phytochemicals in the extracts, including ACNs, weak organic acids, phenolic acids, and other chemical forms [196]. Hence, the complex nature of these compounds necessitates further comprehensive analysis of their antimicrobial potential.