Gut aging: A wane from the normal to repercussion and gerotherapeutic strategies

4.1.1. Metformin

Metformin (biguanide with 50% bioavailability via oral route) is the most prescribed drug for the treatment of type-2 diabetes but is currently repurposed to serve as a gerotherapeutic drug [121,122]. A study reported that a higher proportion of metformin when orally administered, accumulates in the gut (primary reservoir) and can directly regulate the gut microbiota/increased levels of Akkermansia [123]. In the intestine, it impeded stem cell aging, modulated mitochondria function, inhibited cellular senescence via restoring nicotinamide adenine dinucleotide (NAD) levels, prevented macromolecular damages, recovered tight junction protein, reduced telomere attrition, ameliorated colonic pathological inflammation via activating AMP-activated protein kinase (AMPK) while inhibiting p53 pathway and improved nutrient sensing [121,124]. In addition, it has been reported to reduce the expression of chemokines (T-cell activation gene 3), pro-inflammatory cytokines (Macrophage colony-stimulating factor, TNF-α and IL-1α), and LPS-induced nuclear factor kappa B (NF-κB) on smooth muscle cells while increasing ISC numbers via the Wnt/β-catenin pathway and promoting glycolysis in the cells [125,126]. Of note, this drug prevented “leaky gut” by suppressing the Wnt pathway to promote the differentiation pattern of ISCs into the goblet cell, modulated the gut microbiome to increase beneficial gut metabolites, increased occludin, and reduced inflammation in old mice [127]. Although it remains effective as a gut-aging intervention, further studies are required to precisely understand its interaction with the Wnt pathway in promoting ISC self-renewal and differentiation. It is noteworthy that prolonged use of metformin has been associated with adverse gastrointestinal effects (nausea, vomiting, and diarrhea) in the gut of nearly 30% of patients, and its usage was discontinued in 5% of patients due to severe adverse effects [128]. Some of the reasons for the adverse effects could be the metformin-induced metabolic remodeling of Akkermansia muciniphila and Escherichia spp that produced H₂S and CO₂ gases, or the genetic polymorphism contributing factor of human organic cation transporter (OCT1) and serotonin transporter (which promotes serotonin uptake) in intestinal enterocytes [128,129]. The OCT1 is an intestinal transporter of the drug and could prevent metformin-associated increase of serotonin, which is associated with GI disorders. A recent study reported that individuals with a reduced-function allele of OCT1 and low-expressing S∗ allele of the Serotonin reuptake transporter gene (serotonin uptake) suffered metformin intolerance [130]. Hence, metformin treatment might not be ideal for individuals possessing these genotypes. Although supplementing the gut with probiotics could help offset the effects of metabolic remodeling associated with metformin treatment, an extensive pharmacogenetic assessment is needed to delineate individuals for whom this treatment would be most suitable [129].

4.1.2. Rapamycin

Rapamycin treatment as a repurposed gerotherapeutic intervention, targets the mTOR inhibition pathway and rejuvenates ISC in mice by reversing the transcription profile of aged stem cells [63]. The aging of ISCs is reported to be driven by mTORC1 via the p38 mitogen-activated protein kinase (MAPK)-p53 pathway and inhibiting it using rapamycin could be an effective strategy [95]. It is worth noting that rapamycin does not support ISC rejuvenation by Sirtuin-activating drugs such as NAD+ precursors nicotinamide riboside (Sirtuin 1 activator) which implicates mTORC1 activation in its mechanism of gut rejuvenation [94]. These findings suggest that the concomitant use of rapamycin with mTORC1 activating drugs such as NAD+ precursors nicotinamide riboside has to be avoided. Also, based on mTORC activity, rapamycin exerts opposing effects in ISCs and niche component paneth cells [131]. This finding reveals different responses between cells in the gut epithelium.

Also, rapamycin can modulate the gut microbiota, increase autophagy, improve immune function, delay immunosenescence, and exhibit potent immunosuppressive effects via modulating adaptive and innate immune responses [132,133]. Recently, it was reported in one study that it can attenuate age-related decline by increasing lysozyme and intestinal lysosomal alpha-mannosidase V (LManV) to increase the autophagy of enterocytes in Drosophila while maintaining paneth cell structure and barrier function integrity in mice [134]. This effect was sustained even after ten days and six months of drug withdrawal in Drosophila and mice, respectively. Hence, this finding suggests that rapamycin induces a memory effect in the gut cells. Aside from the memory effect on gut cells, rapamycin also prevented inflammation by blocking the aged-related increase of NF-κB in both mice and flies, repressed PGRP-LC inflammaging mediated immune receptors in Drosophila, elevated syntaxin 12/13 levels in mice, and increased lifespan via the endothelial system [135]. Hence, this suggests the role of the rapamycin effect across many species.

Furthermore, a significant increase in mTOR activity affects Ca²⁺ mobilization in aged smooth muscle cells and could be mitigated using rapamycin. However, Martín-Cano et al. [111] reported a low response of the drug in the aged smooth muscle of guinea pig's colon due to the loss of function of its mTOR complex FK506 binding protein-12 (FKBP12) in calcium release and requires further studies to understand the constitutive activation of mTOR activity in cells. Rapamycin delayed senescence in mesenchymal stromal cells, and blocked mTORC1 and mTORC2, with the latter implicated in side effects [136,137].

Some of the side effects such as immunosuppression, impaired wound healing, hyperlipidemia, and glucose intolerance serve as potential clinical barriers to the use of rapamycin in treating age-related diseases in humans [137,138]. A recent study reported that intermittent administration of rapamycin (1X/5 days) and other Food and Drug Administration (FDA) approved rapamycin analogs (temsirolimus and everolimus) have a reduced impact on glucose tolerance, beta cell function, pyruvate intolerance, and immune system to an extent [139]. Nevertheless, these reduced side effects of rapamycin analogs have been reported to differ between individuals [139]. Also, there are varying individual responses to mTOR inhibition by rapamycin and this approach would not benefit everybody [136]. Although rapamycin and their analogs have no serious adverse effect in healthy individuals, those suffering from age-related diseases were reported to be susceptible to rapamycin-associated adverse effects such as increased triglycerides, low-density lipoprotein (LDL) cholesterol levels, and an increased number of infections [140]. Similarly, bi-daily administration of rapamycin analogs for 28 days was associated with hyperglycemia, nausea, vomiting, fatigue, diarrhea, and thrombocytopenia in both young and aged adults suffering tumor diseases (Clinical trial registration, NCT01353625) [141]. Hence, the health status of the individual and a careful assessment of the benefits against the risks should be considered before drug administration.

4.1.3. Small molecules

Small molecules are molecules developed from leads obtained from rational drug design or isolated from natural origin and characterized by low molecular weight (<900Da) [142]. Due to their beneficial role in the gut and ability to traverse cells easily, small molecules such as Butyrate, Roxadustat (FG-4592), GSK343, Garcinol, Trichostatin A, and curcumin have gained much attention in disease treatment.

Butyrate is a gut microbiota metabolite synthesized from diet fibers in the body [143]. Stimulation of butyrate level or luminal administration of butyrate plays several roles in reducing gut inflammation dose-dependently and ameliorating mucosal inflammation via preventing NF-κB activation. It has also been shown to cause IκBα degradation, and regulate oxidative state while enhancing gut contraction/motility in animal models, promoting gut barrier function and preventing “leaky gut” [144,145]. In a study, butyrate was shown to mitigate aged gut microbiota-induced detrimental effects by decreasing GIT permeability, suppressing inflammation (IL-1β, IL-6, and TNF-α), and protecting the brain via the butyrate-free fatty acid receptor 2/3 (FFAR2/3) pathway [146]. Hence, these findings suggest their beneficial use in improving aged gut health. Recently, Sodium phenylbutyrate 4 (4-PBA) was approved by the FDA in the United States of America for use in patients and has been reported to be safe [147]. However, recent paradoxical reports indicate that SCFA including butyrate could promote CD4+ T-cell effector function in ulcerative colitis patients and could worsen gut barrier disruption [148].

Roxadustat (FG-4592) is a hypoxia-inducible factor prolyl hydroxylase used in the treatment of chronic anemia. Current reports show that FG-4592 promoted ISC proliferation and differentiation by upregulating hypoxia-inducible factor 2 and regulating the Tlr4 pathway in radiated mice [149,150]. The drug is also able to modulate the gut microbiota and promote the production of beneficial metabolites [151]. Hence, this suggests it could be useful in the aged gut. However, care must be taken as several studies have reported side effects including gut disorders, thrombosis, back pain, dizziness, and nasopharyngitis in patients with peritoneal dialysis and non-dialysis chronic kidney disease [152,153].

GSK343 (inhibitor of EZH2) has been reported to protect the intestine against sepsis-induced injury in-vivo by reducing the levels of inflammatory cytokines (IL-6, TNF-α and IL-1β), improving TJ protein expression, restoring paneth cell level and preventing cellular apoptosis [154,155]. Similarly, it could suppress inflammation and increase the MDSC population in the lamina propria [156]. Scuderi et al. [157] reported that GSK343 is cytotoxic, therefore its potential effect on normal cells should be considered. Although the drug has been shown to reverse conditions such as inflammaging, further studies using aging models are yet to be assessed.

Garcinol is a polyisoprenylated benzophenone extracted from the rind of the fruit of Garcinia indica [158]. It possesses anti-neoplastic potential, anticancer activity (by regulating NF-κB and Janus kinase (JAK)/STAT3), anti-inflammatory and antioxidant activity. Recent studies have indicated that it could increase intestinal symbiotic bacteria (Firmicutes, and Rominicoccus torques), improve intestinal morphology, and bolster intestinal barrier function which is beneficial in promoting gut health [[159], [160], [161]]. Khan et al. [162] reported that it promoted intestinal growth at low concentrations (<1 μM) but inhibited it at high concentrations (3.2–21.4 μM) in vitro. In immune-regulatory function, Wang et al. [163] reported that garcinol reduced proinflammatory cytokines (IL-1β, TNF-α, and IL-6), NF-κB p65 protein, and decreased intestinal permeability in weaned piglets by increasing ZO-1 and occludin protein expression. Currently, no serious side effect has been observed in vivo following administration of 40% garcinol (100 mg/kg/day) to rodents [164]. Similarly, Choudhury et al. [165] reported fewer side effects with the use of garcinol. Although this drug has no serious side effects, further studies are required to support its use for improving gut health in the elderly.

Trichostatin A, a derivative of dienohydroxamic acid is a small molecule with several roles including alleviating gut dysbiosis, inhibiting inflammatory mediators (down-regulating NF-κB, cyclooxygenase-2 (COX-2), iNOS expressions, inhibiting prostaglandin E2 (PGE2) and protein nitrosylation levels), protecting the intestine against injury and promoting structural restoration of the irradiated intestine [166,167]. Although its role in the gut has been reported, it is more suited as an anticancer agent with no significant drug-associated toxicities observed in cancer patients [168]. Furthermore, its role in the aged gut is yet to be explored. Nevertheless, the information above indicates that it could promote gut health in the elderly.

Sirtuin activators including NAD+ supplements remain an important small molecule with promising aging intervention activities [169,170]. They play a role in maintaining metabolic homeostasis, DNA damage repair, cell survival, and differentiation which are needed in mitigating intestinal aging [171]. In the gut, the mitochondria sirtuin 4 (Sirt4) was reported to shape the gut microbiota via modulating lysosomal overexpression in flies [172]. Similarly, sirtuin 3 modulated the gut microbiota, maintained expression of tight junction protein (claudin 8, claudin 15, and occludin), and possessed anti-inflammatory properties by decreasing pro-inflammatory cytokine (IL-1β and TNF-α) in mice [173]. Also, sirtuin 1 (Sirt1) activation was reported to increase ISC numbers by increasing translation via deacetylating ribosomal S6 kinase (S6K1) and promoting mTORC1-dependent phosphorylation of S6K1 [131]. Recently, Igarashi et al. [94] reported that Sirt 1 activation by NAD+ precursor nicotinamide riboside could rescue age-associated decline in ISC pool numbers, and function and repair gut damage. Although Sirt activators could prolong lifespan, few side effects have been reported [174,175]. Another study reported that Sirt activators have opposing effects on different targets and are tissue-specific [176]. Also, individuals with single nucleotide polymorphism in their Sirt1 gene could have a compromised Sirt1 activity and may not benefit from Sirtuin activators [177]. However, this information suggests that it could be used in the treatment of aging gut repercussions.

Curcumin (diferuloylmethane) is obtained from Curcuma longa (turmeric plant) and possesses immunomodulatory, antioxidative, antiapoptotic, and anti-inflammatory functions. Recently, Gan et al. [178] reported that curcumin increased intestinal antioxidative capacity and mRNA expression of tight junction proteins. Furthermore, curcumin could positively alter microbial composition by increasing beneficial microbes, improving barrier function, and suppressing inflammation [179]. The use of curcumin in healthy individuals has great benefits and is considered safe for use especially when administered at 180 mg/day [180]. However, minimal side effects such as diarrhea, yellow stool, and headache have been reported in seven individuals administered with varying doses of curcuminoid (500 -12000 mg) [181]. This finding raises concerns about its long-term usage. Also, individuals who received curcumin (0.45-3.6 g/day) for four months suffered adverse effects including an increase in lactate dehydrogenase content, diarrhea, nausea, and many more [182]. Likewise, an increase in urinary frequency and nausea has been associated with the use of curcumin [180]. Nevertheless, its anti-aging role has been thoroughly reviewed by Izadi et al. [183].

4.1.4. Senolytic drugs

Senolytic drugs (Fisetin, glycosides, A1331852, quercetin, FoxO4 peptides, B-cell lymphoma-extra large (BCL-XL) inhibitor A1155463, Navitoclax, and Dasatinib) have been reported to slow aging, and prevent aging-associated diseases including cancer by selectively removing deleterious senescent cells [6]. Recently, Ramesh et al. [184] reported that combining low doses of BCL-XL inhibitor A1155463 with FGFR4 inhibitors could eradicate even the most resistant human colorectal cancer while providing a non-toxic therapeutic opportunity.

Aside from this, Navitoclax (ABT-263) is reported to inhibit anti-apoptotic protein (BCL-XL, B-cell lymphoma-w (BCL-w), B-cell lymphoma-2 family protein resembling Boo (Bcl-b), and the notable B-cell lymphoma-2 (Bcl2)) which tends to be upregulated in ISCs during adenoma formation [185,186]. Thus, informing on its potential role in the gut. Recently, ABT-263 was reported to induce apoptosis, decrease fibrotic gene expression (CTGF genes) in normal human intestinal myofibroblast, and mitigate inflammation (by reducing IL-6, TNF-α, and IL-12) as well as fibrotic gene expression in a dose-dependent manner in CBA/J mice S. typhimurium models [187]. However, side effects including neutropenia and thrombocytopenia were reported in patients who were administered with navitoclax daily for a week (Clinicaltrial.gov (NCT00406809)) [188]. Conversely, sole treatment using navitoclax in some individuals has an acceptable safety profile [186].

Dasatinib is a tyrosine kinase inhibitor that inhibits Src family tyrosine kinases and possesses antitumor activity [189]. Recently, it was reported to have tolerable toxicity in patients suffering from metastatic gastrointestinal stromal tumors who are notable to be resistant to imatinib and sunitinib therapy (Clinicaltrial.gov (NCT02776878)) [190]. Also, dasatinib inhibited ephrin A2-ephrinA1 complex to reduce intestinal inflammation, decreased leukocyte infiltration, reduced barrier leakages, and protected the intestines against radiation-mediated intestinal damage [191]. Although this class of drug improves longevity, the safety profile in the elderly is still under study. Moreover, single therapy of these drugs is less effective than combinational therapy in targeting multiple anti-apoptotic pathways of senescent cells [192]. This finding indicates that single senolytic therapy suffers a therapeutic drawback.

A summary of the recent findings of some senolytics has been presented in Table 1 below.

4.1.5. Perindopril

Perindopril is an ACE inhibitor used in hypertension management which could be repurposed as a gerotherapy intervention [196]. It improved exercise capacity and maintained the quality of life in the elderly [197]. Recently, Sayed et al. [198] reported that it could reduce proinflammatory cytokines via suppression of TLR4/NF-κB and c-Fos/c-Jun pathways coupled with the promotion of PPAR-/SIRT1 cytoprotective signals in intestinal injury. Previous studies reported dry cough, diarrhea, erectile dysfunction, headache, and hypotension as some common side effects of the drug [199,200]. Nonetheless, this drug could be used in the management of age-related gut disorders.

In all, Western medicine holds promise for treating gut-related diseases. Although the anti-aging mechanism of some of these drugs in the gut remains unclear, their use in animals as well as humans remains promising and encourages further development to avoid toxicity in the aged.

4.2.1. Berberine

Berberine is a natural alkaloid found in Chinese plants, often delivered in capsules and used for the treatment of several ailments including IBD [202]. Luo et al. [203] reported that Berberine activated Lgr5+ ISC and promoted the expression of Wnt genes in the resident stromal cells of the ISC niche. Also, proinflammatory cytokines (IL-1β and TNF-α) were significantly reduced while anti-inflammatory cytokine (IL-4) was increased in colon tissues via JAK/STAT3, TNF, and NF-κB pathways following Berberine treatment [202]. Similarly, other reports confirmed the antioxidant activity of Berberine and suggested it promoted Treg via the mTOR pathway, inhibited T-cell (TH1 and Th17 cells) differentiation, and played an anti-inflammatory role via IL6/STAT3/NF-κB pathway [204]. Hence, this mitigates the perturbed balance of the elevated T-cell proportion over Treg in the aged gut. Recently, this alkaloid has been reported to modulate gut microbiota and improve the intestinal barrier via TLR4/NF-κB/mTORC pathway and autophagy [205]. This suggests the multiple pathways through which Berberine exerts its gut protective effect. Although the use of this drug is considered safe in normal adults, it could result in toxicities in diabetic adults. Furthermore, high doses of the drug are associated with gastric lesions, dyspnea, and cardiac hypotension [206]. In a clinical phase 1 study, Berberine was reported to be safe for use and well tolerated when co-administered daily with mesalamine (ClinicalTrials.gov (NCT02365480)) [207].

4.2.2. Bazi bushen (BZBS)

Bazi bushen capsule is a clinically used and safe anti-aging drug, majorly comprising 14 herbs and possessing several bioactive compounds (1498 compounds) that delay stem cell senescence, improve telomerase activity, reduce β-galactosidase, and support balanced cell differentiation [[208], [209], [210]]. Recently, it has been reported to restore the gut with good microbes by increasing Firmicutes/Bacteroides ratio, reducing inflammatory cytokines (IL-1β and IL-18) secretion, improving intestinal barrier function by inhibiting NLRP inflammasome-mediated pyroptosis, restoring intestinal villi structure, and promoting Lgr5+ ISCs stemness in SAMP8 mice [211]. Similarly, Xu et al. [5] reported that this capsule reduced senescent-related markers (p16, p21, and p53), inhibited inflammation by downregulating proinflammatory cytokines (IL-1β, IL-6, and TNF-α), and suppressed the TLR4/NF-κB pathway. Hence, this suggests that Bazi bushen could be a good anti-aging drug for mitigating gut dysbiosis, preventing immunosenescence, and promoting ISC function. However, the therapeutic benefits of BZBS on smooth muscle cells and individually differentiated progenies are yet to be determined. Also, BZBS improved DNA repair pathways, and reduced cellular oxidative stress by upregulating Sirt3-mediated superoxide dismutase 2 (SOD2) deacetylation and FoxO1 to boost antioxidant activity (catalase and SOD2 activity) in naturally aging C57BL/6J mice [212]. These pieces of evidence suggest that BZBS could mitigate aging-associated cellular errors and boost the health span of the elderly. Yet, a large number of genes related to gut aging remains to be further studied and their discovery could offer avenues for a better understanding of other mechanisms by which BZBS mitigates age-related gut changes and repercussions. Although this drug is well tolerated and considered safe, minor adverse effects such as constipation and nasopharyngeal inflammation have been reported in elderly patients following 12 weeks of drug administration [210].

4.2.3. Fufang zhenshu TiaoZhi

Fufang zhenshu Tiaozhi (FZT) is a capsule made from 8 Chinese herbs and has been prescribed for treating glucose and lipid metabolism disorders [92,213]. Recently, it has been reported to delay intestinal aging by improving gut metabolites and flora, enhancing telomerase activity, and preventing gut inflammation mediators (COX-2, IL-6, TNF-α, and activator protein-1) in aged C57BL/6J mice [92]. Similarly, other studies confirmed that FZT could modulate the gut microbiota, mitigate intestinal inflammation, and prevent barrier disruption [214,215]. Furthermore, FZT improved intraepithelial lymphocytes and goblet cell numbers which tend to decrease during aging according to previous studies [92]. However, their therapeutic role in ISCs is yet to be studied. Recent reports indicate that FZT could decrease the expression of inflammatory genes (IL-1β, IL-6, TAK1-binding protein 1 (TAB-1), Ccl2, Tlr2, and Tlr4) and restore tight junction protein (ZO-1, E-cadherin, Cldn-2, and -4) in HFD-fed mice [215]. This evidence indicates that FZT could be used to treat similar gut changes observed with gut aging. Furthermore, its long-term use with no obvious toxicities warrants its investigation into the elderly suffering from gut diseases.

4.2.4. Ginseng (Ginsenoside Rb1)

Ginsenoside Rb1 (GRb1), a major constituent of ginseng (a famous herb in traditional Chinese medicine) has been shown to remarkably modulate gut microbiota and regulate the sirtuins family (SIRT-1, -3, and -6) to impede intestinal aging in C57BL/6 mice [216]. In line with this, gut tight junction protein (Cldn (2,3,7, and 15)) and ISC-related genes (Lgr5) among other genes such as Tert, proto-oncogene (c-Myc) and marker of proliferation ki67 were significantly increased following GRb1 administration [216]. Similarly, Wan et al. [217] reported that it can regulate microecological balance and reduce inflammatory cytokines. Of note, GRb1 could suppress intestinal inflammatory mediators (malondialdehyde (MDA), Il-6, IL-β, and Tnf-α), reduce intestinal injury by activating the phosphatidylinositol 3′-kinase (PI3K)/protein kinase B (Akt)/nuclear factor-erythroid 2-related factor 2 (Nrf2) pathway and mitigate oxidative stress [218]. The evidence above suggests that ginseng constituents may be beneficial in treating aged gut-associated ailments. However, some studies have reported the teratogenic effect of GRb1 (30–50 μg/ml) in mice and rats [219]. Nonetheless, administering this drug at a clinically accepted dose in humans does not exhibit these detrimental effects.

4.2.5. Liangxue-Guyuan-Yishen decoction

Liangxue-Guyuan-Yishen decoction is a medicinal preparation from 10 Chinese medicinal herbs and has gained much attention due to its effectiveness in protecting the gut against radiation with low toxicity [220]. Liangxue-Guyuan-Yishen decoction has been reported to promote ISC regeneration, improve tight junction protein expression via Wnt and MEK/ERK pathway, and enhance immune response via Tlr4/myeloid differentiation primary response 88 (MyD88)/NF-κB pathway [220,221]. Similarly, a recent study reported that the decoction promoted Akkermansia levels and improved ISC numbers to protect against radiation-induced intestinal injury in mice [222]. Furthermore, there were no apparent side effects associated with the use of the decoction [222]. Although gut aging studies remain limited, more studies are warranted to ascertain the pharmacological effect of this herbal mixture on the sub-epithelial niche components of the intestine.

4.2.6. Other nutraceuticals in TCM used globally with gut rejuvenation properties

Some nutraceuticals/herbal medicines in TCM have been reported to be used in many countries and served as a bridge connecting Western medicine based on a fair share of philosophical thoughts on aging gut repercussions [223]. Here, we focused on some of the worldwide commonly used nutraceuticals in TCM with significant therapeutic effects in the gut.

Vitamin D is a derivative of ergosterol which can be synthesized in the skin during exposure to ultraviolet light or obtained from food such as fish and eggs [224]. In TCM, sunlight and Wenyang herbs which supplemented the body with vitamin D shared treatment pathways with Vitamin D supplements administered in Western medicine [225]. Li et al. [226] reported that vitamin D and its receptor (VDR) complex protected ISCs and progenitors from apoptosis via the Pmaip1-mediated pathway. Recently, vitamin D receptors complexed with herbs containing vitamin D could maintain tight junction integrity, promote intestinal healing capacity, and prevent irritable bowel syndrome as reported by Yao et al. [227]. Vitamin D has been reported to modulate the gut microbiota. A dose-dependent concentration of vitamin D₃ was reported to modulate innate and adaptive immune cells, increase beneficial bacteria, and reduce pathogenic pathobionts in healthy vitamin D-deficient adults [228]. However, Yao et al. [229] have reported no changes in gut microbiota composition in the gut of the elderly following monthly oral supplementation of 60000 IU Vitamin D for five years (Australian New Zealand Clinical Trials Registry: ACTRN12613000743763). Although there is no special warning attached to the oral intake of vitamin D supplements, one could risk having vitamin D poisoning or hypervitaminosis D, and adverse effects in the intestine such as constipation and nausea may arise in combination therapy with calcium [230,231].

Proanthocyanidins (PAs) are a major class of phytochemicals in cranberry, cocoa, and grape seeds. They have been studied for their potential health benefits, including their effects on the intestine, and aging. PAs have been shown to exert antioxidant, and anti-inflammatory effects, which may help protect intestinal stem cells and their niche from damage caused by aging processes [232]. Recently, Zhu et al. [233] reported that procyanidin B2 (B-type proanthocyanidins) promoted Lgr5 expression and repressed oxidative stress via Wnt/β-catenin and Nrf2/antioxidant responsive element (ARE) pathways, respectively. Furthermore, Casanova-Martí et al. [234] have reported that long-term exposure to PAs improved epithelial differentiation of both secretory linage cells (paneth, goblet, and enteroendocrine cells (L-cells)) and enterocytes by modulating β-catenin pathway, promoting the early expression of gene transcription factors (Neurod1 and Pax6) and maintaining Notch-mediated lateral inhibition in-vitro. Hence, this suggests they could be used to mitigate the skewed cellular differentiation as seen in the aged gut. Additionally, they modulated the microbiome toward a healthy profile, promoted regeneration, repaired intestinal tissues, and strengthened barrier integrity [235]. These findings indicate that they could mitigate age-associated gut defects. Generally, PAs (2500 mg) are considered safe and well-tolerated in healthy adults, children, patients, and pregnant women [[236], [237], [238]]. Chen et al. [239] have reported that although the use of proanthocyanidins is encouraged, the safety profile and clinical evidence in the elderly must be ascertained.

Quercetin is a polyphenol with a low bioavailability and is normally found in most fruits and vegetables [240]. It is utilized in TCM as well as constitutes a major part of the Western diet (approximately 15 mg) [241,242]. It possesses antioxidant, anti-inflammatory, and antimicrobial activities which are beneficial properties of aging intervention drugs [243]. Yan et al. [244] reported that quercetin could prevent ISC aging by preventing ROS in Drosophila. Furthermore, it is known to significantly impact the intestinal microenvironment with many therapeutic benefits including abrogating intestinal flora disorder to protect the intestine in gut-free mice coupled with intestinal barrier function recovery and the increase in mucosal thickness [245]. Similarly, Lan et al. [243] reported that the antimicrobial activity of quercetin had a direct activity on modulating the gut microbiota, promoting beneficial SCFA production and reversing fecal metabolite anomalies. Also, quercetin has been reported to be administered as a senotherapeutic compound and inhibited BCL2/BCL2L1, PI3K/AKT, and TP53/P21 pathways. Therapeutically, it is administered in senolytic combination therapy with dasatinib to resolve the drawback associated with the single senolytic treatment that is not entirely efficient in targeting multiple anti-apoptotic pathways of senescent cells [192]. Recently, natural senotherapeutic repurposing candidates including piperlongumine found in long pepper and natural senomorphics (phloretin, Parthenolide, and curcumin) have been reported to be natural substitutes for dasatinib in the combination therapy following computational identification [246]. Furthermore, piperlongumine has been reported to inhibit Th17-differentiation without affecting Treg and could revert the aging-related increase in Th17/Treg ratio. Although quercetin is beneficial, some common side effects such as increased weight in rats, nephrotoxicity in animals with damaged kidneys, and reduced bioavailability of other drugs in combinational therapy in human and mice studies [247,248]. Although the reduced bioavailability could be beneficial in reducing the side effects of other drugs, it is advisable to consult the physician before its concomitant use with other drugs [248]. Furthermore, its long-term use in the elderly has not been assessed. Nonetheless, the above information underscores its beneficial use in maintaining gut health in the elderly.

Ginger (Zingiber officinale) is a popular herbal plant with many pharmacological benefits such as anti-inflammatory, antioxidant, and anticancer activity attributed to its rhizome [249]. In gut health maintenance, several studies reported that derived ginger products (Shogaol, ginger root powder, and ginger exosome-like nanoparticles (ELNs)) could modulate the gut microbiota composition, promote intestinal stem cell function, strengthen intestinal barrier function, and mitigate colitis via increasing IL-22 production [[250], [251], [252]]. Recently, 6-Shogaol was reported to promote apoptosis of colon cancer cells, and inhibited their migration via the NF-κB (IKKβ)/NF-κB/Snail signaling pathway [253]. Recent reports indicated that no adverse effects were associated with the use of ginger in breast cancer patients receiving chemotherapy [254]. However, vertigo, heartburn, and headache have been associated with ginger consumption in other studies and need to be monitored [253,254]. Given these gut therapeutic effects, its role in gut aging remains an exciting area yet to be explored.

Cinnamon is a common herb with several bioactive compounds and belongs to the family Lauraceae [255]. It possessed several polyphenolic compounds and volatile phenols with antioxidant, anti-cancer, and anti-inflammatory activities [256]. Other pharmacological effects include its neuroprotective and gastroprotective effects. Due to its therapeutic benefits, they are used in food preparation and as supplements. Several studies reported that products of cinnamon could be used to improve the intestinal flora imbalance following gut injury and the intestinal barrier by modulating claudin-2 expression in the gut [257,258]. The long-term use of cinnamon and high doses of it are associated with adverse effects such as nausea, rash, stomach ache, mucositis, and dermatitis and must be clinically monitored [259]. However, its role in the aged gut remains to be explored.

In summary, TCM has made major strides in academic research, such as being cited in medical journals, and has employed most herbal formulations, with many of the nutraceuticals globally accepted and used for treatment. Although these nutraceuticals exhibited tremendous therapeutic benefits, their efficacy and safety based on long-term clinical trials are needed to give credence to their use on a long-term basis. Thus, offering opportunities for exploration by researchers in their quest to promote healthy intestinal aging, and also bridging the biocultural disparity between Eastern and Western nutraceuticals.

4.3.1. Dietary intervention

Interestingly, extra dietary components such as seafood as an aging intervention are becoming popular globally. Recently, the consumption of ark clams (seafood) has been reported to alleviate gut barrier damage, reduce oxidative stress, modulate the gut microbiota, increase immunoglobulin A (IgA), and SCFA production to protect the gut in D‐galactose‐induced aging rats [260]. Also, a whey protein diet has been reported to be an effective measure in restoring key nutrients, especially increasing protein intake which is insufficient in the elderly. A systematic review showed that a whey protein diet improved the health span of the elderly while a metabolic analysis showed that whey protein supplementation was ideal for modulating the gut microbiome of the elderly [261,262]. However, adverse effects on the kidneys and liver have been associated with the indiscriminate use of whey protein and these effects are aggravated in people living sedentary lifestyles [263]. Furthermore, a low histamine diet could be a gold standard treatment that keeps excessive histamine secretion in check. Likewise, microbiota dietary targeted interventions such as probiotics, prebiotics, and post-biotics have been encouraged for use in the aged [264]. For example, bacterial species of genus Bifidobacteria could be considered for adequate supplementation as they are beneficial in the degradation of excessive histamine, maintenance of colonic tight junction, inhibition of pro-inflammatory cytokines, production of polyamines to scavenge reactive oxidative species, and promotion of lifespan in mice [26]. A recent meta-analyses study on the long-term use of probiotics including Bifidobacterium and Lactobacillus strains in the elderly showed a significant impact in improving constipation symptoms as well as their cognitive function which improved their quality of life [265]. Although dietary intervention has beneficial impact on the gut and is safer than pharmacological treatments, the side effects associated with it should not be overlooked [266]. Hence, this information strongly gives scientific credence that diet may mitigate gut biological aging and its repercussions.

4.3.2. Dietary restrictions and fasting

Dietary restrictions (such as reduced intake of a high-fat diet) and fasting have been reported to be beneficial in promoting stem cell function in the intestines [267]. Green et al. [268] reported that restricting an isoleucine-containing diet could improve metabolic health, reduce frailty, decrease the risk of cancer, and increase health span in old HET3 mice. In support, religiously practicing dietary or calorie restriction for two years could reduce cellular senescence biomarkers and SenMayo gene set responses in healthy young to middle-aged humans [269]. Currently, the clinical test results of dietary restriction in old adults expressing p16 and p21 markers remain to be reported. Recent reports on dietary restriction show that it is closely related to the risk of malnutrition in susceptible groups and further studies are required to understand their efficacy, and safety in different population groups [266]. Also, Jaime and Mank [270] have reported that dietary restriction could increase the risks of osteoporosis, cause depression, and result in weight loss. Therefore, this treatment will require that the patient understand the risks of the treatment. Furthermore, this treatment will be more suitable for obese individuals [270].

Also, 24-h fasting as a non-pharmacological intervention has been reported to promote ISC function by activating the PPAR-mediated fatty acid oxidation program in aged mice [271]. Furthermore, intermittent fasting including Ramadan 30 consecutive days of fasting was associated with increased microbiome diversity and provoked upregulation of butyric acid-producing Lachnospiraceae which promoted health benefits in middle-aged adults [272]. Similarly, the gut Bacteroides and Firmicutes increased by 21 % and 13 % respectively, after Ramadan fasting [273]. Although fasting may be beneficial, it may aggravate chronic gut disorders such as ulcerative colitis in older men [274]. Recently, a fasting-mimicking diet that mimics the benefits of traditional fasting could reduce the side effects of fasting to an extent [266]. However, there is a need to understand the associated risk of intermittent fasting or fasting-mimicking diets in the aged gut. Nevertheless, fasting remains beneficial, and further studies are required to affirm fasting-associated benefits in the aged gut of the elderly.

4.3.3. Exercise or physical activity

Physical activity or exercise has been one of the strategies reported to promote healthy aging in the elderly. Chae et al. [275] have reported that PA improved intestinal homeostasis by activating the apelin receptor-AMP-activated kinase (APJ-AMPK) pathway in epithelial cells to facilitate renewal. Furthermore, PA improved gut microbiota composition with some favorable intestinal flora including Bifidobacteria slowing the rate of biological aging in elderly people [276,277]. Recently, a systematic review indicated it could modulate the gut microbiota in the aged with no clear indication of which taxa are the most affected by this intervention [277]. However, SCFA metabolites were reported to decrease in elderly women after PA [276]. Also, other small-scale studies have conflicting reports on the impact of exercise on aging-related microbial α-diversity abundance [262,278]. Thus, making this intervention a bit controversial and necessitating further exploration. Recently, a large-scale study showed that regular exercise partially restored the abundance of Firmicutes, Bacteroidetes, and Verrucomicrobia among other microbes and significantly decreased α-microbial diversity in the elderly as confirmed in other studies [278,279].

Furthermore, regular endurance exercise could regulate the immune cells and prevent aging-associated repercussions such as inflammaging, immunosenescence, and maladaptive immune aging via decreasing exhausted T-cells, reducing TLR pathways, and increasing natural killer cell migration as well as naïve T cells [280]. Tarca et al. [281] have reported that dizziness, fatigue, and muscle pain are the side effects associated with PAs. Although these side effects are normal responses in PAs, embedding PAs in clinical treatment has been reported to be beneficial in people receiving peritoneal dialysis.

Furthermore, chronic exercise could reduce the risk of cardiovascular disease [282]. Conversely, PAs can increase the risk of sudden cardiac death in men above 40 years [282]. These shreds of evidence indicate that PAs in individuals suffering from cardiovascular diseases should be closely monitored. Nevertheless, they can be used in the elderly to promote gut health.

4.3.4. Fecal microbiota transplantation

Fecal microbiota transplantation (FMT) has been given credence to prolonging lifespan and reversing intestinal aging [6]. Gut microbiota restoration presents an auspicious strategy for gut health maintenance since it can delay aging along with beneficial effects including stem cell proliferation and immune system regulation [283]. Several studies on FMT in mice remain promising and clinical evidence in humans has to be carried out with a focus on safety and effectiveness [283]. Likewise, a pilot study revealed that FMT mitigated gut microbiota dysbiosis by increasing Clostridiales abundance while decreasing the pathogenic Enterobacteriales in patients suffering steroid-resistant lower gastrointestinal acute graft-versus-host disease for which there are limited treatment options (Trial registration number NCT# 03214289) [284]. Recently, the US FDA approved an oral FMT capsule Vowst (formerly known as SER-109) which is administered one dose daily for three consecutive days and can be used to effectively treat Clostridiodes difficile infections [285]. In the elderly, FMT has been considered safe and effective for treating severe, recurrent, and complicated C. difficile infections [286]. Furthermore, FMT can restore gut microbiota in the crypts of ulcerative patients [287]. Thus, it is of great importance to develop novel approaches that accurately regulate the gut microbiota to repair the disrupted internal equilibrium. Adverse effects of this treatment approach include abdominal pain, nausea, vomiting, and constipation [288].

4.3.5. Cell reprogramming and transplantation intervention

Most cellular intervention often comes as a last resort due to therapeutic failure using drugs and employs induced pluripotent stem cells or adult stem cells (mesenchymal cells or ISCs) [289]. Indeed, the inability of stem cells to repair damage is associated with many age-associated gut diseases and requires stem cell therapy using organoids derived from induced pluripotent stem cells or adult stem cells [290]. In intestinal aging, several studies using organoids derived from mice or rats make us understand the basic science of aging and related diseases. Moreover, the significant differences in genes across different species coupled with high failure in clinical translatability into human subjects make it imperative to employ human organoids derived from stem cells for efficient treatment [289]. The application of human organoids has been of interest as they mirror in-situ tissue characteristics of the gut of their donors and provide a more personalized treatment for intestinal diseases that may occur in them. As discussed above, the organoid derivation efficiency is reduced when the donor is an aged adult. Moreover, to the best of our knowledge, the use of human organoids in mitigating intestinal aging in aged adults without diseases has been understudied. Here, we reported the therapeutic benefits of organoids derived from both mice and humans used in mitigating age-related gut diseases in the context of stem cell potency.

Induced pluripotent stem cells are stem cells (with pluripotent abilities) derived from somatic cells after exposure to cues and simulate embryonic stages of development [291]. It is interesting to note that induced pluripotent stem cells (iPSC) could produce organoids with all the components in the intestines (epithelial cells, mesenchymal cells, fibroblast, immune cells, and smooth muscle cells) [292]. Also, they serve as therapeutic tools in gut rejuvenation, tissue regeneration, and personalized medicine. In tissue regeneration, induced pluripotent stem cells reconstituted the colonic epithelium in mice and humans after transplantation [293]. Although this approach addressed ethical limitations associated with the use of human embryonic stem cells, its clinical use is still limited due to the possibility of cells becoming oncogenic [294]. Furthermore, the dedifferentiation process leading to iPSC from aged somatic cells could have most of the genetic and epigenetic information on aging lost, thus limiting its use in aging preclinical studies to test drug efficacy [295].

Conversely, adult stem cells are limited to multipotent abilities and compared to induced pluripotent stem cells, they have been reported in many ongoing or even closed clinical trials [289,296,297]. Furthermore, they can accumulate and represent age-associated mutations that occur in both the small and large intestines and are very useful in precision therapy [292].

Adult mesenchymal stem cells (MSCs) have been widely used in pre-clinical (in-vitro and in-vivo) and clinical regenerative studies [298,299]. Moussa et al. [298] demonstrated the gut rescue effect following the co-injection of transplanted mesenchymal cells with colonic-derived epithelial cells to mimic clinical translatability. Moreover, they are considered a treatment option for mitigating IBD in patients [294]. In clinical trials, human umbilical cord-derived mesenchymal stem cell therapy is being used in patients with active ulcerative colitis (ClinicalTrials.gov ID, NCT02442037) and advanced colorectal cancer (ClinicalTrials.gov ID, NCT06446050). Furthermore, mesenchymal stem cells have been used in treating inflammatory bowel disease (ClinicalTrials.gov ID, NCT 03299413). However, the current status of the above clinical trials is unknown. Conversely, a Phase 2 clinical trial using adult human MSCs in treating moderate to severe Crohn's disease has been completed (ClinicalTrials.gov ID, NCT00294112). Although this treatment is beneficial, challenges due to the aging microenvironment persist and could hamper the efficacy of MSC transplantation therapy leading to unsatisfied results in some aged adults [300]. Chen et al. [300] reported that niche-targeted therapies such as senolytics and sirtuin activators could enhance MSC transplantation therapy.

The ISCs are multipotent and limited to only the development of epithelial cells on the crypt-villi axis. The ISC delivery in the form of organoids to damaged intestinal epithelial remains a promising strategy for gut restitution as reviewed by Rees et al. [45]. In detail, this strategy involves the long-term three-dimensional (3D) culture of enteroids (stem cells derived from the small intestine)/colonoids (stem cells derived from the colon). These stem cells are obtained from surgical resections or endoscopic biopsies and later transferred into the patient to regenerate the epithelial cells of the intestine [301,302]. In line with this, Jee et al. [303] demonstrated that the Lgr5+ stem cells are pivotal in reconstituting the epithelium of mice suffering radiation proctitis following enema delivery of colon organoids. Also, organoids derived from human ISCs mitigated intestinal failure and short bowel syndrome in rats [304]. Recently, human intestinal organoids generated to contain immune cells led to immune cell activation when transplanted into mice possessing humanized immune systems [305]. The results from the human organoids in mice experiments became the premier basis for clinical trials in rescuing patients suffering from ulcerative colitis using organoid transplantation therapy [306].

Furthermore, human intestinal organoid was used to mitigate colonic injury following delivery in a hydrogel vehicle to the injured intestinal mucosa [307]. Although this regeneration method is limited by scale, a protocol for engineering mucosa graft tissue from decellularizing native human tissue salvaged the limitation and mitigated intestinal failure in patients [308]. Currently, the collection of intestinal organoids from biopsies to establish biobanks and characterize them before therapeutic screening is still ongoing (ClinicalTrials.gov ID, NCT05294107). Of note, human organoids could be used in high throughput screening of drugs to understand an individual's response to therapy and safety whereas its grafting in animals offers insights into the role of organoids in treating gut diseases and could be translated into the elderly. The use of human organoids for clinical and translation therapy is limited due to the cost associated with organoid culture and the long time allocated to generate a comprehensive representative of a patient's organ. To date and to the best of our knowledge, intestinal organoid transplantation in the elderly without gut disease has not been explored yet.

Also, organoids derived from stem cells can be combined with gene therapy or reprogrammed by transient reprogramming, direct lineage reprogramming, and partial reprogramming to enhance them before being introduced into the affected area of the gut.

In gene therapy, modern sophisticated tools such as clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR-associated protein 9 (Cas9) (CRISPR/Cas9) are used to screen for regulators of intestinal cell fate, label colon stem cells, and modify or correct genes associated with diseases in organoids [[309], [310], [311]]. Also, CRISPR/Cas9 is used in lineage tracing system for human ISCs following human colonoid engraftment into the rectum surface of a mouse [312]. However, the translatability of this approach to improve an individual's intestinal function is yet to be reported despite their promise of retaining stemness to promote gut health. In cell reprogramming, Miura and Suzuki [293] have reported that pluripotent stem cells could be guided by direct lineage reprogramming to form human intestinal organoids for biomedical applications. In Table 2, we have summarized the findings of some gerotherapeutic strategies obtained from in-vivo gut studies, especially in mice ranging between mid-age and old age.