Vitamin C; sources, uses and importance

Vitamin C

Vitamin C is required by only a few species, which, by virtue of a single enzyme deficiency, cannot synthesize it. For most species, ascorbic acid is a normal metabolite of glucose, but it is not an essential dietary constituent. Whether it is synthesized or not, ascorbic acid is important for several physiological functions. Many, if not all, of these functions involve redox characteristics that allow ascorbic acid to play an important role, along with α-tocopherol, reduced glutathione, and other factors, in the antioxidant protection of cells. Thus, ascorbic acid represents the major water-soluble antioxidant in plasma and tissues. As such, it is thought to support the redox recycling of α-tocopherol, the bioavailability of non-heme iron, and the maintenance of enzyme-bound metals in oxidation states appropriate for several enzymatic functions. It is fairly well established that compromises of these effects underlie the pathophysiology of vitamin C deficiency. Other beneficial health effects of ascorbic acid have been reported: reductions in hypertension, atherogenesis, diabetic complications, colds and other infections, and carcinogenesis. Although some of these claims have become widely accepted, the empirical evidence remains incomplete for many.

Vitamin C


SOURCES OF VITAMIN C

Distribution in Foods

Vitamin C is widely distributed in both plants and animals, occurring mostly (80–90%) as ascorbic acid but also as dehydroascorbic acid. The proportions of both species tend to vary with food storage time, due to the time-dependent oxidation of ascorbic acid. Fruits, vegetables, and organ meats (e.g., liver and kidney) are generally the best sources; only small amounts are found in muscle meats. Plants synthesize l-ascorbic acid from carbohydrates; most seeds do not contain ascorbic acid, but start to synthesize it on sprouting. Some plants accumulate high levels of the vitamin (e.g., fresh tea leaves, some berries, guava, rose hips). Ascorbic acid-containing tissues of cruciferous vegetables of the family Brassicaceae typically contain β-thioglucopyranosides called glucosinolates, which degrade, when food is cut and cooked, to products2 that react spontaneously with ascorbic acid to form a stable adduct without vitamin C activity, ascorbigen. For practical reasons, citrus and other fruits are good daily sources of vitamin C, as they are generally eaten raw and are therefore not subjected to cooking procedures that can destroy vitamin C. Processed foods, such as cured meats and some beverages, can also contain the analog, erythorbic acid,3 which is used as a preservative. While that analog has no vitamin C activity in vivo, it can yield false positives in some analyses for plasma ascorbic acid.

Stability in Foods

The vitamin C contents of most foods decrease dramatically during storage owing to the aggregate effects of several processes by which the vitamin can be destroyed. Ascorbic acid is susceptible to oxidation to dehydroascorbic acid, which itself can be irreversibly degraded by hydrolytic opening of the lactone ring to yield 2,3-diketogulonic acid, which is not biologically active. These reactions occur in the presence of O2, even traces of metal ions, and are enhanced by heat and conditions of neutral to alkaline pH. The vitamin is also reduced by exposure to oxidases in plant tissues. Therefore, substantial losses of vitamin C can occur during storage and are enhanced greatly during cooking. For example, stored potatoes lose 50% of their vitamin C within 5 months, and 65% within 8 months, of harvest. Apples and cabbage stored for winter can lose 50% and 40%, respectively, of their original vitamin C content. Losses in cooking are usually greater with such methods as boiling, as the stability of ascorbic acid is much less in aqueous solution. For example, potatoes can lose 40% of their vitamin C content by boiling. Alternatively, quick heating methods can protect food vitamin C by inactivating oxidases.

Post a Comment

0 Comments