VITAMIN C IN FOODS
Most animals are able to synthesize vitamin C and so, do not require a dietary source. However, humans, other primates, guinea pigs, fruit bats, and a few birds and fish are unable to synthesize this water-soluble vitamin. They rely on their diets for a vitamin C source.
Vitamin C, also known as ascorbic acid, is involved in many processes in the human body, primarily as an electron donor. The term vitamin C actually refers not only to ascorbic acid but also to its oxidized form, dehydroascorbic acid. By adding or losing 2 hydrogens vitamin C undergoes reversible reduction and oxidetion. Both forms of vitamin C are found in the foods we eat.
Most fruits and vegetables contain some vitamin C, but the richtest sources are citrus fruits, peppers, and green vegetables. Animal products and grains are generally not good sources. An intake of 5 serving per day of fruits and vegetables can provide ample vitamin C, depending on the foods chosen.
FUNTIONS OF VITAMIN C
Collagen Synthesis
Collagen is the major fibrous protein that holds together the various structures of the body and gives strength to connective tissue. Collagen fibers are critical to the structure of bone and blood vessels, and they are essential in wound healing. A collagen molecule is like a 3-stranded rope-it consist of 3 polypeptide chains wound together to form a triple helix. Vitamin C is needed to get the 3 strands in the right shape to form the triple helix. Specifically, vitamin C helps convert the structure of 2 amino acids (lysine and proline) in collagen to hydroxyproline. The function of vitamin C in the formation of these unusual amino acids is to interact with the metalloenzymes involved in making the conversions.
Synthesis of Other Vital Compounds
Vitamin C is required for the synthesis of many impoprtant biological compounds. In each case, vitamin C keeps the copper or iron in the metalloenzyme in the reduced state. Some of the important compounds requiring vitamin C for synthesis are the amino acids tyrosine, the hormone thyroxine, the fatty acid transport compound carnitine, and the neurotransmitters norepinephrine, ephinephrine, and serotonin. In addition, vitamin C in involved in in the conversion of cholesterol to bile acids and biosynthesis of the hormones corticosteroids and aldosterone.
Antioxidant Activity
In vitro (in a test tube), vitamin C can be an antioxidant by donating electrons to free radicals. Recall that a free radical has an unpaired electron. A vitamin C molecule can donate electrons to free radicals so that they become stable. Researchers have proposed that vitamin C in the body’s water-based fluids, such as blood, acts just as vitamin E does in lipid-rich environments. It also has been suggested that vitamin C can recycle vitamin E and make it function more effectively.
Iron Absorption
Vitamin C with meals modestly facilitates the intestinal absorption of non-heme iron (iron that is not in hemoglobin) because of the conversion of iron in the GI tract to ferrous iron. Vitamin C also counters the action of certain food components that inhibit iron absorption.
Immune Function
White blood cells, part of the body’s immune defenses, contain the highest vitamin C concentration of all body constituents. This may protect against the oxidative damage associated with cellular respiration. Free radicals generated during phagocytosis and neutrophil activation, although intended to kill bacteria and damaged tissue, also can damage the body’s own immune cell.s Vitamin C may reduce this self-destruction through its antioxidant defense actions. Vitamin C also may have other roles in immune functions; however, supplemental vitamin C levels beyond the body’s needs may not improve immune functions.
VITAMIN C DEFICIENCY
A deficiency of vitamin C prevents the normal synthesis of collagen, thus causing widespread, significant changes in connective tissue throughout the body. The first signs and symptoms of scurvy, the vitamin C deficiency disease, appear after about 20 to 40 days on a diet free of vitamin C and include fatigue and pinpoint hemorrhages around hair follicles. These hemorrahges are the most characteristic sign of scurvy.
In addition, the gums and joints bleed, a classic sign of connective tissue failure. Other effects of scurvy include impaired wounds healing, bone paine, fractures, and diarrhea. Phychological problems, such as depression, are common in advanced scurvy. Scurvy is fatal if not treated.
VITAMIN C INTAKE ABOVE THE RDA
Some popular authors and speakers advocate the consumption of vitamin C at amounts higher than the RDA. Surprisingly, there is little research comparing different vitamin C intake levels. If vitamin C intake is above about 100 mg/day. much of the additional vitamin C is excreted in the urine. Some research indicates that 200 mg/day is the highest amount needed to maximize the health benefits of vitamin C intake. Eating several vitamin C-rich foods each day can boost intakes to 200 mg/day.
