**4. Ascorbic acid as a co-antioxidant**

Ascorbic acid has been used in combination with other vitamins and compounds to achieve a greater synergistic effect [26]. The synergistic effect of both the vitamins C and E has been reported earlier to prevent the phenomenon of autoxidation [27–29]. According to Shad *et al*. [30], ascorbic acid alone has little effect in preventing oxidation of lard oil. It was reported that a combination of ascorbic acid and α-tocopherol provides powerful synergistic antioxidative effect. The above researchers conclude that ascorbic acid role was to protect α-tocopherol from consumption. This characteristic of ascorbic acid is termed as co-antioxidant effect. Barcalay *et al*. [31] figure that ascorbic acid in sodium dodecyl sulfate (SDS) micelle system provides essential synergistic role in the autoxidation of linoleic acid acting from the aqueous phase to recreate tocopherol in the micellar phase. In a study by Gitto *et al*. [32], on the antioxidant action of melatonin with tocopherol, ascorbic acid, glutathione, and deferoxamine on homogenates of rat liver revealed combined effects with ascorbic acid and tocopherol. A similar study by Lui *et al*. [28] illustrated that the collaborative antioxidant effects of lycopene, vitamins E and C, and β-carotene were significantly higher than their antioxidant effects individually. An investigation on the fortification of water-in-extra virgin olive oil emulsion with vitamins by Cuomo *et al*. [26] showed that the presence of ascorbic acid controls the slowdown of oil oxidation. They also reported that the presence of vitamin C limited the peroxide value variations compared to the olive oil and to emulsions without vitamin fortification. The presence of ascorbic acid obstructs the pro-oxidant effect of α-tocopherol (vitamin E) radical since it could be changed back to α-tocopherol [33] as shown in the reaction below:

TocO • AscH TocOH • . + −→ + − *Asc*

The mechanism of vitamin E renewal in association with co-antioxidants makes use of greater concentrations of less potent α-tocopherol than lower concentrations. This demonstrates that food containing little amounts of vitamin E with co-antioxidants gives a higher health advantage compared to vitamin E supplements.

Tian *et al*. [34] observed that co-administration of araloside A and L-ascorbic acid produce coordinated antioxidant activity, and a relationship involving antioxidant cellular indexes with ROS clearing ability was identified. The pretreatment of araloside A and L-ascorbic acid singly or in combination improves both cell sustainability and function of antioxidant enzyme and obstructed the discharge of lactate dehydrogenase (LDH), the agglomeration of Malondialdehyde (MDA), lipid peroxidation (LPO) products, and H2O2; and the manufacturing of intracellular responsive oxygen species (ROS), protein carbonyls and 8-hydroxy-2-deoxy guanosine (8-OHdG).
