**1. Introduction**

Ageing is a progressive biological process which a ffects the normal functions of cells and tissue, thereby imperiling the person towards diseases and mortality [1]. For a layman, it is the process of maturing and growing old. Both internal and external factors play an integral role in ageing [2]. Internal factors comprise the usual biological processes of the cell, whereas the external factors involve chronic sun-exposure, hormonal imbalance, nutritional deficiencies, ultraviolet (UV) irradiation and other factors such as pollution and smoking [3]. The hallmarks associated with ageing have been illustrated in Figure 1. Skin ageing, characterized by wrinkling, can be reduced via suitable preventive measures involving the consumption of antioxidant-rich supplements, a balanced diet and undertaking skincare [4]. By opting for these measures, the harmful e ffects induced by free radicals can be restrained [5].

**Figure 1.** Hallmarks contributing to ageing.

Over the past few decades, the relationship between nutrition and ageing has been extensively studied in both animals and humans [6]. Nutraceuticals are nutritional elements with medicinal characteristics; hence the name, where "Nutra" stands for food and "ceutical" means therapeutic properties [7]. As per the definition of Foundation for Innovation in Medicine (FIM), nutraceuticals are the "food and food products" that have medicinal value and provide health benefits, especially in preventing and treating age-related diseases [8]. These products include functional foods, dietary supplements and herbal extracts, which provide health benefits in the long-run when consumed as supplements in the diet [9]. Even researchers have suggested that antioxidants have propitious effects on both chronic as well as age-related diseases, especially neurodegenerative diseases and cancer [10]. Various food supplements that exhibit an antioxidant potential, such as carotenoids, flavonoids and vitamins, prevent and treat ROS-associated chronic conditions, which results in healthier and longer lifespans [10]. Food supplements produce antagonistic effects against the degenerative and inflammatory processes in the body, and have beneficial effects on the immune and digestive system, hence improving the quality of life [11].

The current review focusses on highlighting the manifestations of ageing and theories associated with ageing. Additionally, it also discusses the importance of diet managemen<sup>t</sup> in ageing and functional food, as well as nutraceuticals with anti-ageing potential.

#### **2. Manifestation of Ageing**

Clinical manifestations of intrinsic ageing can be determined by assessing the regenerative ability of the damaged tissues or organs [12]. All dividing and differentiating cells are vulnerable to insults causing intrinsic ageing [13]. The visual traits of ageing start appearing in the early 40s. Most cells, tissues and organs steadily undergo ageing and become incompetent [14]. A significant effect can be observed on the skin, which turns loose, thin wrinkled and inelastic [15]. The face fat also reduces, leading to hollowed eye sockets and cheeks.

Furthermore, the hair starts thinning from the armpits, pubic area and scalp [16]. As melanin content decreases, the hair strands become thinner grey, and the nails become thinner as well [17]. At over 80 years old, more noticeable visual changes can be observed, such as the compression of spinal disks, vertebrae and joints. The hearing abilities also diminish depending on the severity of the ageing phenomenon [18].

Other than this, the elderly population gets presbyopia and may require reading glasses [19]. In comparison to healthy adults, they lack deep sleep and are unable to take su fficient rest as required by the body at this stage [20]. The bone density decreases and becomes weaker, increasing the risk of fracture [21]. Due to slow metabolism and hormonal changes, there is a reduction in muscle mass and an increase in body fat [22]. Besides this, older adults also su ffer from lapses of memory and vagueness, preventing them from recalling names and memories [23]. The heart and lungs become less e fficient with time, and kidney functions are abated [24]. The accumulated harmful metabolic waste later appears as dangerous diseases and allergies, causing significant discomfort to older people [25]. Moreover, females at menopause produce reduced amounts of estrogen, due to which they experience various changes, such as vaginal dryness, hot flashes, chills, night sweats, sleeping problems, mood swings, weight gain and slowed metabolism [26]. Besides this, an unhealthy diet and indolent lifestyle further increase the risk of occurrence of chronic diseases in elderly people, such as cancer, osteoarthritis, type 2 diabetes, obesity, coronary artery disease osteoporosis and high blood pressure [27].

#### **3. Theories of Ageing**

Several theories have been formulated to define the ageing phenomenon. These theories have been postulated based on certain assumptions, but none of them provide a satisfactory explanation [28]. There are three major theories for ageing, i.e., genetic theories, dysfunction of interlinked organs and physiological approaches [29]. Of these, three physiological theories have been extensively studied, which comprise the cross-linking theory, the waste material accumulation theory and the free radical theory [30].

In 1950, Denham Harman stated that ageing is the result of the massive production of free radicals [31]. In general, free radicals are those atoms or molecules that have unpaired electrons and possess the ability to form electronic couples [32]. This explains the short life and high reactiveness of these molecules. These free radicals are usually formed during the metabolic reactions under normal conditions [33]. Moreover, the generation of these free radicals also takes place during exposure to cigarette smoke, UV rays and toxic substances, as well as during emotional stress [34]. Even though free radicals are involved in normal metabolic processes, but they do not generally infiltrate the cells. Still, when they do, they have harmful and deleterious e ffects on various organs [35].

Free radicals released from food are essential for energy production within the cell [36]. Additionally, their production also protects the body from opportunistic infections and elicits the synthesis of hormones involved in e ffective communication within the body [37]. However, the excessive production of free radicals has detrimental e ffects on DNA, collagen, elastin and blood vessels [38]. Oxidative damage to di fferent biomolecules, such as DNA, macromolecules and proteins, takes place over time [39]. It is considered a significant factor, but is not the only factor responsible for ageing [40]. Fundamentally, oxygen has a dual role in our body, i.e., it is necessary for life and is one of the chief components of harmful compounds like free radicals [41]. Free radicals are generated by the aerobic metabolism. They liberate di fferent types of reactive oxygen species, such as singlet oxygen (1[O2]), superoxide anion radicals (O2 –), hydroxyl radicals (OH–), hydroperoxyl radicals (HO2), peroxide radicals (R = lipid) (ROO–) and hydrogen peroxide (H2O2) [42]. The various sources involved in the generation of free radicals are illustrated in Figure 2.

**Figure 2.** Schematic representation of sources involved in the formation of free radicals and their association with the ageing process.

For example, if the free-radical-mediated DNA mutations are left uncorrected via repair mechanisms, this defect persists even after successive replication cycles, transcription and translation [43]. It is well-known that free radicals are formed by the aerobic metabolism for the synthesis of energy-rich molecules like ATP, which are synthesized in mitochondria (also known as cell factories) [44]. As humans start ageing, the efficacy of mitochondria in synthesizing ATP substantially decreases, thereby allowing the accumulation of free radicals in mitochondria as well as permitting the passage of free radicals through the mitochondrial membrane, thereby damaging other parts of the cell [45]. These alterations have helped to determine the key factors which favor ageing, i.e., increases in oxidative stress and a decrease in energy production [46]. Even the published literature has stated that a high degree of mutation is observed in mitochondrial DNA in contrast to nuclear DNA due to oxidative stress [47]. Therefore, calorie restriction (CR) impedes the process of ageing and increases the lifespans of flies, fish, spiders and mammals (mice and rats) [48]. This happens because CR decreases the oxidative load, which reduces the free radical formation in mitochondria [49]. The reduction in the free radical formation substantially reduces the number of oxidized proteins, lipids and mutated mitochondrial DNA [50]. Extensive studies have been conducted on rodent models to assess the effects of a diet enriched with minerals and vitamins in ageing [44]. As such, it is believed that calorie restriction and the consumption of food rich in antioxidants can considerably prolong the life span of individuals [51].

An important theory that explains the process of ageing is the shortening of the telomeres. Due to the end-replication problem, the telomeres are shortened in every generation of the cell till they reach a critical length in the crisis stage of ageing [52]. At this stage, the cell division slows down considerably, causing the cell to slowly die. This may be referred to as "replicative mortality". Cells involved in growth, development and reproduction express high levels of the enzyme telomerase, which maintains the length of the telomeric DNA [53]. These cells include the stem cells and reproductive cells

(eggs and sperms). However, most adult cells have low expressions or no expression of telomerase, which causes these cells to age and eventually die [54].

#### **4. Plant-Based Supplements with Anti-Ageing Potential**

Plants and their inherent components are well known to exhibit antioxidant potentials, sch as carotenoids, flavonoids and vitamins, that aid in the prevention and treatment of ROS-associated chronic conditions [55]. These supplements have antagonistic e ffects against the degenerative and inflammatory processes in the body and show beneficial e ffects on the immune and digestive system, hence improving the quality of life [56]. Some of the predominantly used plant-based supplements have been discussed below.
