Vitamin C: Sources, Functions, deficiency, and Toxicity

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In this article, we shall consider the food sources, functions, general uses, deficiency, and toxicity of vitamin C.

Vitamin C

L-Ascorbic acid, also known as L-xyloascorbic acid, 3-oxo-L-gulofuranolactone (enol form), L-3-ketothreohexuronic acid lactone, antisorbutic vitamin and vitamin C, has the chemical formula C6H8O6 and a molecular weight of 176.12.

Vitamin C is a water-soluble vitamin that required for normal growth and development. Moreover, it is an antioxidant that helps reduce oxidative damage to tissues by reactive oxygen species.

Vitamin C maintains the integrity of connective tissue protein collagen, protects against infection, and helps iron absorption.

Thus, the vitamin is necessary in the body to form collagen in bones, cartilage, muscle, and blood vessels and aids in the absorption of iron.

Food Sources of Vitamin C

All fruits and vegetables contain some amount of vitamin C.

Green peppers, citrus fruits and juices, strawberries, tomatoes, broccoli, turnip greens and other leafy greens, sweet and white potatoes, and cantaloupe, papaya, mango, watermelon, brussels sprouts, cauliflower, cabbage, winter squash, red peppers, raspberries, blueberries, cranberries, and pineapples.

Physiologic Functions of Vitamin C

Vitamin C as an antioxidant: Vitamin C as an antioxidant. It contributes to anti-oxidation by reducing oxidized vitamin E. Fat-soluble vitamin E helps to maintain cell membrane integrity by scavenging free radicals and thus breaking the oxidation chain reaction of unsaturated fatty acids in the phospholipid bilayer.

Vitamin C is known to reduce the vitamin E radical formed as a result of the scavenging activity back to the functional form of vitamin E. Thus, vitamin C plays an indirect role in anti-oxidation.

1. Vitamin C in the laying of Collagen: This water-soluble vitamin is important in forming collagen, a protein that gives structure to bones, cartilage, muscle, and blood vessels. It also helps maintain capillaries, bones, and teeth.

Vitamin C plays an indispensable role for in maintaining good-quality collagen. Vitamin C assists the post-translational modification of collagen by reducing iron in the participating enzymes, lysyl hydroxylase and prolyl hydroxylase. Modified/mature collagen undergoes important protein-protein interaction yielding a unique structure that is essential for its functions.

Complete collagen maturation cannot be achieved in vitamin C deficiency.

Many symptoms of scurvy can be explained by having poor-quality collagen. The importance of collagen is extensive. Collagen is the major protein from fibroblasts, osteoblasts and chondrocytes. A key symptom of scurvy is microvascular bleeding. In the absence of vitamin C in most tissues, internal hemorrhage is imminent due to loss of vessel wall integrity.

Because collagen is the main protein in the bone, skeletal defects were also found in scurvy.

Furthermore, high vitamin C intake has been found to be associated with lower bone loss in older adults.

Skin is the most sensitive tissue to vitamin C deficiency, which may relate to the function of collagen-producing fibroblasts. Poor wound healing is also seen in scurvy. Interestingly, higher vitamin C intake has been linked to a better appearance of aging skin.

Role of Vitamin C in Collagen Formation: Vitamin C, a reducing agent, is required to maintain the enzyme prolyl hydroxylase in an active form, most likely by keeping its iron atom in a reduced state.

The precursor molecule to collagen, procollagen, contains an unusual amino acid sequence in that every third amino acid is a glycine and contains a high frequency of two amino acids not found in any other proteins – hydroxyproline and hydroxylysine.

These latter two amino acids are converted from proline and lysine, respectively, after the procollagen molecule has been synthesized. The hydroxylation of proline and lysine in procollagen is carried out by the enzyme prolyl hydroxylase using ascorbic acid as a cofactor.

Vitamin C facilitates the absorption of iron.

General Uses of Vitamin C

Vitamin C is an important component of both basic body building processes and disease prevention. The benefits of Vitamin C include:

a. Collagen Production: Vitamin C assists the body in the production of collagen, a protein that serves as the building block of connective tissues throughout the body. In addition, collagen binds cells together in tissues.

Collagen is required for the formation and ongoing health of the skin, cartilage, ligaments, corneas, and other bodily tissues and structures. Vitamin C promotes faster healing of wounds and injuries because of its role in collagen production.

b. Allergy and asthma relief: Vitamin C is present in the lung’s airway surfaces, and insufficient vitamin C levels have been linked to bronchial constriction and reduced lung function.

c. Cancer prevention: Vitamin C is an antioxidant and has been associated with reduced risk of various forms of cancers – stomach, lung, colon, oral, and prostate. Studies have shown that cancer results mainly from oxidative stress.

d. Diabetes control: Vitamin C supplementation may assist diabetics in controlling blood sugar levels and improving metabolism.

e. Immune system booster: Vitamin C increases white blood cell production and is important to immune system balance. Low vitamin C levels have been associated with increased risk for infection. Vitamin C is frequently prescribed for HIV-positive individuals to protect their immune system.

f. Neurotransmitter and Hormone Production: Vitamin C is required for the conversion of certain substances into neurotransmitters. These are brain chemicals that facilitate the transmission of nerve impulses across a synapse (the space between neurons, or nerve cells). Such neurotransmitters as serotonin, dopamine, and nor epinephrine are responsible for the proper functioning of the central nervous system, and a deficiency of neurotransmitters can result in psychiatric illness. Furthermore, vitamin C helps the body manufacture adrenal hormones.

Vitamin C Deficiency

Severe deficiency of vitamin C causes scurvy. Although rare, scurvy includes potentially severe consequences, and can cause sudden death. Patients with scurvy are treated with vitamin C and should be under medical supervision.

Although scurvy is rare, symptoms appear when the serum level falls below 0.2 mg/dl. A total body pool of less than 300 mg is associated with symptoms of scurvy, while maximum body pools are limited to about 2 g.

Several symptoms of ascorbic acid deficiency have been recognized including follicular hyperkeratosis, swollen and inflamed gums, loosening of teeth, dryness of the mouth and eyes, loss of hair and dry itchy skin. These symptoms reflect the role of ascorbic acid in the maintenance of collagen and blood vessel integrity. Scurvy is an acute or chronic disease characterized by hemorrhagic manifestations and abnormal osteoid and dentin formation.

Scurvy has psychological manifestations which include depression and hysteria. This highlights the role of vitamin C in neurotransmitter production. This potentially fatal disease can be prevented with as little as 10 mg ascorbic acid per day, an amount easily obtained through consumption of fresh fruits and vegetables.

Factors that Influence Vitamin C Requirements

Bioavailability, nutrient-nutrient interactions, gender and antioxidant protection are important factors that affect ascorbic acid requirement.

The type of food consumed have a significant effect on the absorption of ascorbic acid.

In a particular study, the following findings were made about vitamin C bioavailability:

– absorption of ascorbic acid decreases to about 50% and less with the single doses above 1g,

– some 70-90% of usual dietary intake of ascorbic acid (30 – 180 mg/day) is absorbed.

– Bioavailability is complete for 200 mg of ascorbic acid as a single dose.

– No ascorbic acid is excreted in the urine of six of seven volunteers until the 100 mg dose. At single dose of 500 mg and higher, bioavailability declined and the absorbed amount was excreted.

It is important to note that ascorbic acid is very labile (unstable), and the loss of ascorbic acid upon boiling milk provides one dramatic example of a cause of scurvy in infants.

The ascorbic acid content of food is strongly influenced by season, transportation to market, shelf life, time of storage, cooking practices and chlorination of water.

Ascorbic acid is the most potent enhancer of non-heme iron absorption. Iron absorption from non-heme food sources can be increased significantly with a daily ascorbic acid intake of at least 25 mg for each meal (estimated for 3 meals/day).

Vitamin C Toxicity

This can occur due to long-term vitamin C supplementation. The small elevation in plasma concentration of vitamin C after supplementation may not affect collagen maturation but the chemical activity of vitamin C can increase dose-dependently. Although the Tolerable Upper Intake Level (UL) of vitamin C is set at 2 g/day for adults, the safety issue associated with long-term vitamin C supplementation at a dose lower than UL has emerged after various clinical trials drew to an end in recent years.

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