**3. Antioxidants**

An antioxidant can be described as any substance or compound capable of inhibiting the oxidation of suitable substrate even when present in low concentrations [42]. During the late 19th and early 20th century, the exploration of antioxidants resulted in a boom, due to their involvement in various industrial processes like the prevention of corrosion, the polymerization of fuels, fouling in combustion engines and the vulcanization of rubber [43]. The application of antioxidants was limited for the prevention of the oxidation of unsaturated fats, as it resulted in the rancidity of fats [44]. The general procedure to determine the antioxidant potential of any compound involves the assessment of the rate of oxygen consumption when fat is kept in an enclosed container with oxygen. The identification of vitamins A, C and E, which act as antioxidant agents, has revolutionized the field and highlighted the significance of antioxidants in the biochemistry of living beings [45,46].

Another way to understand the antioxidant is that it is a stable molecule, which donates an electron to unwanted free radical species and neutralizes it, and curbs its ability to cause damage. In general, these antioxidants either inhibit or delay the cellular damage because of their scavenging properties [47]. The low molecular weight of these antioxidants allows them to interact with ROS (free radicals) easily and terminate their chain reaction before damaging vital molecules. Glutathione, uric acid and ubiquinol are few antioxidant molecules that are generated by our body during normal metabolic processes [48]. There are various enzymes are present in our body that can scavenge free radicals, and micronutrients like ascorbic acid (vitamin C), β-carotene and α-tocopherol (vitamin E) [49]. The body cannot produce these micronutrients on its own, therefore, these molecules are obtained from the consumed food.

### **4. Sources of Antioxidants**

Dietary supplements are key source of antioxidants, which could aid in maintaining good health and prevent the onset of fatal diseases triggered by ROS. Even though some synthetic antioxidants have been developed, their carcinogenic and toxic nature has prompted the exploration for natural antioxidants like vitamins A, C and E [50]. Additionally, population studies have also revealed that the consumption of fruits, tea, vegetables and wine are a reliable source of natural antioxidants, and are effective in regulating the risk of cardiovascular diseases, which has intrigued researchers to exploring their potential as natural antioxidants [51]. Dietary supplements contain antioxidant compounds in the form of phytochemicals i.e., α-tocopherol, β-carotene, vitamin C, vitamin E and various phenolic compounds [52]. Numerous ethnomedicinal plants, fruits, vegetables, mushrooms and other spices have been well-documented as sources of natural antioxidants, which play a significant role in promoting healthy life and treating various fatal diseases [53].

Phenolic compounds obtained from natural sources are considered far better than synthesized chemicals. Microbes are also being explored for synthesizing organic compounds with antioxidant potential. Various fungal strains have been reported to produce compounds like ellagic acid, ferulic acid and gallic acid under solid-state fermentation and submerged fermentation conditions [54]. All these compounds are known to contain 2–4 reactive hydroxyl groups, which impart them the antioxidant potential. In addition, algae and lichens are active producers of different secondary metabolites, including antioxidants (Table 1).


