Vitamin C

Synonym: ascorbic acid

Characteristics: Vitamin C is a water-soluble vitamin that occurs naturally in some foods, is added to others, and is available as a dietary supplement. Humans, unlike most animals, are unable to synthesize vitamin C. Vitamin C is necessary for the biosynthesis of collagen (a basic component of connective tissue), L-carnitine (enables the transport of fatty acids to the mitochondria), some neurotransmitters, and is also involved in protein metabolism. Vitamin C is also an important physiological antioxidant that regenerates other antioxidants in the body, including alpha-tocopherol (vitamin E). In addition to its biosynthetic and antioxidant functions, vitamin C plays an important role in immune function and improves the absorption of non-heme iron, the form of iron present in plant-based foods. Inadequate intake of vitamin C causes scurvy, which is characterized by fatigue or malaise, weakness of connective tissue, and fragility of capillaries.

Absorption: Intestinal absorption of vitamin C is regulated by at least one specific active transporter in a dose-dependent manner. The body cells then accumulate vitamin C through a second specific transport protein. The body strictly controls the use of vitamin C. At intakes of 30–180 mg/day, approximately 70–90% of vitamin C is absorbed. However, at doses above 1 g/day, absorption drops to less than 50% and the absorbed, unmetabolized ascorbic acid is excreted in the urine.

Dietary supplements: They usually contain vitamin C in the form of ascorbic acid, which has equivalent bioavailability to naturally occurring ascorbic acid in food. Other forms of vitamin C include sodium ascorbate; calcium ascorbate; other mineral ascorbates; ascorbic acid with bioflavonoids; and combination products such as Ester-C®. However, simple ascorbic acid is the preferred source of supplemental vitamin C. Mineral ascorbates (also known as non-acidic vitamin C - buffered forms) are thought to be less irritating to the stomach than ascorbic acid. When taking mineral salts, both ascorbic acid and the mineral are absorbed. For example, sodium ascorbate generally provides 131 mg of sodium per 1000 mg of ascorbic acid. Calcium ascorbate provides 114 mg of calcium per 1000 mg of ascorbic acid. Many bioflavonoids are antioxidants, so they are added to some vitamin C products in the belief that this increases the bioavailability or effectiveness of vitamin C. Typically, bioflavonoids derived from citrus fruits are used. Ascorbyl palmitate, a fat-soluble form of vitamin C, is most used in topical creams.
Natural sources: Fruits and vegetables are the best sources of vitamin C. Citrus fruits, tomatoes and potatoes are the main sources. Other significant sources include red and green peppers, kiwi, broccoli, strawberries, Brussels sprouts, and cantaloupe. Although vitamin C is not naturally present in cereals, it is added to some fortified breakfast cereals. The vitamin C content of food is strongly influenced by many factors such as season, transport, storage conditions and time, cooking techniques and water chlorination. Slicing or crushing will reduce its content, however blanching or low pH storage will preserve it. During cooking and storage, up to 100% of the vitamin C content of food can be destroyed, as the vitamin is sensitive to light, heat, oxygen, and alkali. Additionally, using too much water during cooking can leach the vitamin from the food.

Effect: Because of its antioxidant function and its role in immune function, vitamin C has been promoted as an agent to help prevent and/or treat numerous health conditions. It is used in the prevention and treatment of cancer, cardiovascular diseases, age-related macular degeneration, cataracts, and colds. Vitamin C is important for effective wound healing. Vitamin C's antioxidant properties also contribute to its anti-aging effect, as reactive oxygen species are known to interact with fibroblasts (the basic cells of connective tissue) and destroy collagen and connective tissue structures. Thus, vitamin C is a well-known anti-aging agent due to its antioxidant effect and major role in collagen production.

Deficiency: Acute deficiency of vitamin C causes scurvy. Their development varies depending on the body's vitamin C stores, but symptoms may appear within 1 month of little or no vitamin C intake (below 10 mg/day). Initial symptoms may include fatigue (probably due to impaired carnitine biosynthesis), malaise, and gingivitis. As vitamin C deficiency progresses, collagen synthesis is impaired and connective tissue weakens, causing petechiae (small hemorrhages), ecchymoses (spotted bleeding into the skin and mucous membranes), purpura (bleeding), joint pain, poor wound healing, and hyperkeratosis (thickening of the upper layers of skin). Other symptoms of scurvy include depression as well as swollen, bleeding gums and loosening or loss of teeth due to fragility of tissues and capillaries. Iron deficiency anemia may occur due to increased bleeding and decreased absorption of non-heme iron. Bones can also be affected in children. If left untreated, scurvy is fatal. Studies consistently show that smokers have lower plasma and leukocyte levels of vitamin C than nonsmokers, in part due to increased oxidative stress.

Recommended daily dose: adult: 90 mg for men, 75 mg for women (85 mg during pregnancy, 120 mg during breastfeeding). Individuals who smoke need 35 mg/day more vitamin C than non-smokers.

Adverse effects: The most common problems are diarrhea, nausea, abdominal cramps, and other gastrointestinal disorders caused by the osmotic effect of unabsorbed vitamin C in the gastrointestinal tract.

Interactions: Patients undergoing chemotherapy or radiation should consult their oncologist before taking vitamin C or other antioxidant supplements, especially in high doses. Vitamin C can affect the results of many laboratory tests.

Pregnancy: Vitamin C is safe during pregnancy, but it is recommended not to exceed the therapeutic dose or to stop supplementation suddenly.

Breastfeeding: safe at usual doses.

Toxicity: Vitamin C has low toxicity and is not expected to cause serious adverse effects at high intakes. A high intake of vitamin C also has the potential to increase the urinary excretion of oxalate and uric acid, which could contribute to the formation of kidney stones, especially in individuals with kidney disorders. A high intake of vitamin C can cause excessive absorption of iron. Under certain conditions, vitamin C can act as a pro-oxidant and potentially contribute to oxidative damage. Several in vitro studies have suggested that oral vitamin C could act as a prooxidant and cause chromosomal and/or DNA damage and possibly contribute to cancer development. However, other studies have not shown increased oxidative damage or increased cancer risk with high vitamin C intake.