*4.1. Adaptogens*

Adaptogens are compounds obtained from herbal plants for maintaining homeostasis and stabilizing the physiological processes in humans [57]. These compounds reduce cellular sensitivity to stress and improve the ability of the body to resist the damage from other risk factors [58]. Moreover, they also help in restoring and promoting normal physiological function [59]. A few of the highly known adaptogens have been discussed below.

#### *4.2. Bacopa monnieri*

*Bacopa monnieri*, also known as Brahmi, is a perennial herb with small oblong leaves and purple flowers [60]. Highly valuable nootropic phytochemicals, such as bacosides, are found in this medicinal herb [61]. Brahmine and Herpestine are the two essential phytochemicals that are predominantly extracted from this herb [62]. The phytochemicals obtained from Brahmi aid in protecting the brain from the attack of free radicals and stimulating cognitive functioning and learning [63]. It has been comprehended that the regular consumption of Brahmi oil reduces the chance of various diseases like Alzheimer's disease and amnesia [64]. Bhattacharya et al. (2000) found that extracts of *Bacopa monnieri* enhance the activity of reactive oxygen species-scavenging enzyme catalase (CAT), glutathione peroxidase (GPX) and superoxide dismutase (SOD), in a dose-dependent manner. This study was carried out in the brain regions of rats and investigated after 14 and 21 days [65]. Shinomol and colleagues conducted an in vitro and in vivo study using 3-nitropropionic acid (NPA) (fungal toxin responsible for causing neurotoxicity in humans and animals) and *Bacopa monnieri* extract. The result obtained showed that NPA was e ffective in inducing the oxidative stress in dopaminergic (N27) cells and mitochondria of the striatum of rats, whereas *Bacopa monnieri* extract was found to be effective in regulating the NPA-induced oxidative reactions and reducing the Glutathione (GSH) and thiol levels [66]. Kumar and his colleagues also conducted a six-week randomized placebo-controlled trial to assess the e ffect of *Bacopa monnieri* extract on the cognitive functions of students studying medicine. The result obtained from the study showed significant improvement in the cognitive functioning of the students [67].

#### *4.3. Curcuma longa*

*Curcuma longa* is a plant of the ginger family that produces a compound known as curcumin [68]. It is known for diverse biological activities, such as its anti-cancerous, anti-inflammatory and antioxidant properties [69]. Due to these natural properties, curcumin is a potential therapeutic agen<sup>t</sup> for treating di fferent types of cancers [70]. Many studies have revealed that curcumin can suppress the expression or activity of cyclooxygenase-2 (COX-2), prostaglandin E2 (PGE2), pro-inflammatory cytokines and tumor necrosis factorα (TNFα) [71]. The antioxidant properties of curcumin can aid in the reduction of ROS production, the scavenging of free oxygen radicals and obstructing lipid peroxidation [72]. The consumption of curcumin via the oral route in rodents has been shown to ameliorate cystic fibrosis and block tumor progression; still, the evaluation of humans is pending [70]. A study reported that curcumin induces a cellular stress response in human fibroblasts via redox signaling and the phosphatidylinositol 3-kinase/Akt (Protein Kinase B; PKB) pathway. This provides evidence that curcumin-triggered cellular antioxidant defenses can serve as an e ffective approach to anti-ageing intervention [73].

Moreover, it has been reported to increase the life span of fruit flies, mice and nematodes [74–76]. In fact, curcumin has been stated to improve and regulate the symptoms of age-related diseases such as atherosclerosis, cancer and diabetes [77,78]. Other than this, curcumin has been reported to show protective activity against chemotherapy-induced side e ffects and radiation-induced dermatitis in breast cancer patients [79,80]. Some studies have claimed that curcumin has anti-ageing potential because it can delay cellular senescence [81]. Cox et al. conducted a study to assess the e ffects of solid lipid curcumin on mood and cognition in healthy adults aged 60–85. In this study, subjects were examined for the e ffects of solid lipid curcumin formulation, i.e., 400 mg of Longvida ® for acute (1 and 3 h after a single dose), chronic (4 weeks) and acute-on-chronic (1 and 3 h after a single dose following regular treatment) dosing. The results obtained showed significant improvements in the working memory for both acute and chronic dosing. Additionally, it also decreased physical fatigue (measured per Chalder Fatigue Scale) as well as total and LDL cholesterol [82].

#### *4.4. Emblica o*ffi*cinalis*

*Emblica <sup>o</sup>*ffi*cinalis*, also known as Amla, is a member of the Phyllanthaceae family [83]. The churn of Amla is known for reducing cholesterol level and improving memory potential [84]. The consumption of Amla in the diet is e ffective in lowering the cholesterol level in the brain as well as in the body [85]. It has also been stated as a beneficial functional food for treating Alzheimer's disease [86]. Draelos and colleagues conducted a double-blind study to evaluate the skin-lightening potential of a topical formulation comprising *E. o*ffi*cinalis* extract, glycolic acid and kojic acid. The study revealed that the topical formulation was 4% better than hydroquinone, due to which researchers claimed that the topical formulation could be an e ffective natural alternative for mild to moderate facial dyschromia [87]. Accumulation of free radicals in di fferent tissues is associated with various stress-induced conditions leading to the progression of the process of ageing [88]. Tannoids obtained from *E. o*ffi*cinalis* also show a protective e ffect because of their antioxidant potential against the tardive dyskinesia rat model [89]. Moreover, the extract of *E. o*ffi*cinalis* shows antidepressant properties by inhibiting the activity of Gamma Amino Butyric Acid (GABA) and Monoamine oxidase-A (MAO-A) in consort with antioxidant activity in mice models [90].

#### *4.5. Ginkgo biloba*

*Ginkgo biloba*, also known as Gingko, is a functional food which improves the availability of oxygen in the tissues [91]. The leaves of Ginkgo have been reported to play a significant role in maintaining the blood flow and glucose level in the brain [92]. Moreover, it also improves the mental functioning of the brain [93]. Ascorbic acid, catechin, shikimic acid, lactone derivatives (ginkgolides) and isorhamnetin are some of the flavone glycosides, which are active scavengers of free radicals and are obtained from the extract of ginkgo leaves [94]. Huang conducted a study to assess the e ffect of *Gingko biloba* extract on the liver of the aged rat. The result revealed that administration of *Gingko biloba* extract reduced the level of liver metalloproteinase as well as malondialdehyde, and improved the SOD activity to minimize the oxidative stress [95]. Another study has revealed that the administration of *Ginkgo biloba* extract improves the cognitive function in aged female rats [96]. Even clinical studies have been conducted to assess the e ffect of *Gingko biloba* extract in the treatment of Alzheimer's disease and cognitive function. Extensive analysis has revealed that the consumption of *Gingko biloba* extract improves the cognitive functioning of individuals who have mild dementia [97].

#### *4.6. Glycyrrhiza glabra*

*Glycyrrhiza glabra*, also known as licorice, is a member of the Fabaceae family [98]. The rhizomes, as well as roots of this plant, serve as a brain tonic which helps in regulating the blood sugar level [99]. Glycyrrhizin is the prime bioactive molecule obtained from this plant rich in antioxidants, which protects the brain from oxidative damage, maintains the normal functioning of the nervous system and improves the memory of the individual [100]. *Glycyrrhiza glabra* has a phenolic compound named "liquorice" which has antioxidant potential, due to which it is effective in the chelating of metal ions and the scavenging of free radicals [101]. It has been reported that *G. glabra* enhances the memory in the murine model of scopolamine-induced dementia [102]. Dhingra and colleagues also reported improvements in the memory of mice administered with *Glycyrrhiza glabra*. Three different doses, i.e., 75, 150 and 300 mg/kg p.o. of *Glycyrrhiza glabra* extracts, were administered for seven consecutive days. The result obtained showed that a dose of 150 mg/kg was effective in enhancing memory in the mice model [103].

#### *4.7. Panax ginseng*

*Panax ginseng*, also known as ginseng, is highly known for its medicinal value [104]. The bioactive molecule ginsenoside is obtained from the roots of this plant [105]. This bioactive molecule improves the resistance of the body against anxiety, fatigue, stress and trauma, and modulates the immune function [106]. Moreover, it also shows anti-stress properties and improves learning performance and memory [104]. A study reported an increase in the life span of juvenile mice with leukaemia upon the administration of ginseng [107]. Another study on *Panax ginseng* reported that it is able to decrease lipid peroxidation and improve antioxidant potential by reducing oxidative stress [108].

Moreover, double-blind clinical trials have confirmed that the consumption of ginseng improves the psychomotor performance of the individuals [109]. *Panax ginseng* has also been reported to have anti-melanogenic potential, and is associated with the activation of the foxo3a gene, also stated as the longevity gene [110]. Certain studies have reported that *Panax ginseng* prevents skin ageing. Furthermore, a randomized, placebo-controlled, double-blind study was conducted to assess the potential of both *Panax ginseng* and ginsenosides in preventing skin ageing. The result obtained from the study showed a significant reduction in wrinkle formation, and no participant showed an adverse reaction to the treatment [111].

#### **5. Plant-Based Metabolites with Anti-Ageing and Medicinal Properties**
