Ascorbic acid naturally occurring

For the measurement a moderate reduction potential between — 100 and + 100 mV vs. Ag/AgCl is appUed (Fig. 2.12). In this region the potential for electrochemical interferences is very low. However, the biggest problems arise from the high reactivity of compormds I and II with reducing substrates (electron donors), which compete with the electrode for the reduction of peroxidase. Ascorbic acid, naturally occurring phenolics and aromatic amines are among those compounds. The competitive reaction of reductants should be... 

Vitamin C occurs as L-ascorbic acid and dihydroascorbic acid in fruits, vegetables and potatoes, as well as in processed foods to which it has been added as an antioxidant. The only wholly undisputed function of vitamin C is the prevention of scurvy. Although this is the physiological rationale for the currently recommended intake levels, there is growing evidence that vitamin C may provide additional protective effects against other diseases including cancer, and the recommended dietary allowance (RDA) may be increased in the near future. Scurvy develops in adults whose habitual intake of vitamin C falls below 1 mg/d, and under experimental conditions 10 mg/d is sufficient to prevent or alleviate symptoms (Bartley et al., 1953). The RDA is 60 mg per day in the USA, but plasma levels of ascorbate do not achieve saturation until daily intakes reach around 100 mg (Bates et al., 1979). Most of the ascorbate in human diets is derived from natural sources, and consumers who eat five portions, or about 400-500 g, of fruits and vegetables per day could obtain as much as 200 mg of ascorbate. 

Cytochrome c is a heme containing protein which occurs in muscle at lower concentrations than does myoglobin. It was demonstrated some time ago (18) that oxidized cytochrome c reacts with gaseous nitrite oxide to produce a nltrosyl compound. Recent work (19, 20, 21) has examined the reactions of cytochrome c with nitrite and the contribution of the product formed to cured meat color in considerably more detail. The general conclusion is that even at the pH normally encountered in meat, the reaction can take place in the presence of ascorbic acid but probably does not affect meat color because of the unstable nature of the reaction product and the low concentration. 

Ascorbic acid retention in some fresh-cut produce is affected by the nature of the slicing method used (Lee and Kader 2000). Higher levels of ascorbic acid were retained in samples that had been prepared by manually tearing the lettuce into strips. Lettuce shredded using a sharp knife initially retained 18% less ascorbic acid than the torn samples. The retention of ascorbic acid in the products sliced by machine was 25-63% lower than that in lettuce shredded by manual tearing. Using a blunt machine blade resulted in 10% lower ascorbic acid levels than when a sharp blade was used (Barry-Ryan and O Beirne 1999). Losses in ascorbic acid occur when vegetables are severely cut or shredded (Lee and Kader 2000). 

Small amounts of organic radicals are formed continually in the skin during photolysis (in a process with rate constant k( )). The radicals are consumed immediately by natural substances in the skin, termed antioxidants (in a different process with rate constant k(2)). Vitamin C (L-(+)-ascorbic acid, IV) is one of the best naturally occurring antioxidants. Red wine and tea also contain efficient antioxidants. 

The body s natural defenses against overenthusiastic oxidation include a-lipoic acid, reduced glutathione, ascorbic acid, the tocopherols, the carotenoids, and a number of enzymes such as epoxide hydrolase and the like. Very efficient DNA repair systems also operate defensively. These various means are remarkably effective, but DNA assault is continuous, cumulative, and implacable. Thus, many degenerative diseases are associated with aging because of the gradual slippage in functional fidelity of cellular machinery which occurs with age. 

Ascorbic acid is widely distributed in nature, but it occurs in especially high concentration in citrus fruits and green plants such as green peppers and spinach. Ascorbic acid can be synthesized by all plants and animals with the exception of humans, other primates, and guinea pigs. Therefore, vitamin C must be present in our dietary substances. 

Nutrients frequently consumed in sub-optimal concentrations by humans are proteins, calcium, non, vitamin A, thiamin (vitamin Bl), riboflavin (vitamin B2) and ascorbic acid (vitamin C). Some of these nutrients occur in higher concentrations in fruit juices than hi other foods. There is experimental evidence that indicates that ascorbic acid of natural origin is apparently superior to that of synthetic origin. 

Supplementation with the antioxidant vitamins ascorbic acid (250 mg) and mixed natural tocopherols (50 IU on alternate days) may be beneficial. Higher doses may vitiate the impact of lipid lowering therapy. Other naturally occurring antioxidants such as resveratrol, 3-catechin, selenium, and various carotenoids found in a variety of fruits and vegetables may provide additional antioxidant defense. Homocysteine, which initiates proatherogenic changes in endothelium, can be reduced in many patients by restriction of total protein intake to the amount required for amino acid replacement. Daily supplementation with up to 2 mg of folic acid plus other B vitamins is also recommended. 

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