Vitamin C, also known as ascorbic acid and l-ascorbic acid, is a vitamin found in various foods and sold as a dietary supplement. It is used to prevent and treat scurvy. Vitamin C is an essential nutrient involved in the repair of tissue and the enzymatic production of certain neurotransmitters. It is required for the functioning of several enzymes and is important for immune system function. It also functions as an antioxidant.
Current evidence does not support its use for the prevention of the common cold. There is, however, some evidence that regular use may shorten the length of colds. It is unclear whether supplementation affects the risk of cancer, cardiovascular disease, or dementia. It may be taken by mouth or by injection.
Vitamin C is generally well tolerated. Large doses may cause gastrointestinal discomfort, headache, trouble sleeping, and flushing of the skin. Normal doses are safe during pregnancy. The United States Institute of Medicine recommends against taking large doses.
Vitamin C was discovered in 1912, isolated in 1928, and in 1933 was the first vitamin to be chemically produced. It is on the World Health Organization Model List of Essential Medicines, which lists the most effective and safe medicines needed in a health system. Vitamin C is available as an inexpensive generic and over-the-counter medication. Partly for its discovery, Albert Szent-Györgyi and Walter Norman Haworth were awarded the 1937 Nobel Prizes in Physiology and Medicine and Chemistry, respectively. Foods containing vitamin C include citrus fruits, kiwifruit, broccoli, Brussels sprouts, raw bell peppers, and strawberries. Prolonged storage or cooking may reduce vitamin C content in foods.
Vitamin C is an essential nutrient for certain animals including humans. The term vitamin C encompasses several vitamers that have vitamin C activity in animals. Ascorbate salts such as sodium ascorbate and calcium ascorbate are used in some dietary supplements. These release ascorbate upon digestion. Ascorbate and ascorbic acid are both naturally present in the body, since the forms interconvert according to pH. Oxidized forms of the molecule such as dehydroascorbic acid are converted back to ascorbic acid by reducing agents.
Vitamin C functions as a cofactor in many enzymatic reactions in animals (and humans) that mediate a variety of essential biological functions, including wound healing and collagen synthesis. In humans, vitamin C deficiency leads to impaired collagen synthesis, contributing to the more severe symptoms of scurvy. Another biochemical role of vitamin C is to act as an antioxidant (a reducing agent) by donating electrons to various enzymatic and non-enzymatic reactions. Doing so converts vitamin C to an oxidized state – either as semidehydroascorbic acid or dehydroascorbic acid. These compounds can be restored to a reduced state by glutathione and NADPH-dependent enzymatic mechanisms.
In plants, vitamin C is a substrate for ascorbate peroxidase. This enzyme utilizes ascorbate to neutralize toxic hydrogen peroxide (H2O2) by converting it to water (H2O).
Scurvy is a disease resulting from a deficiency of vitamin C, since without this vitamin, collagen made by the body is too unstable to perform its function.
Scurvy leads to the formation of brown spots on the skin, spongy gums, and bleeding from all mucous membranes. The spots are most abundant on the thighs and legs, and a person with the ailment looks pale, feels depressed, and is partially immobilized.
Vitamin C, also known as L-ascorbic acid, is a water-soluble vitamin that is naturally present in some foods, added to others, and available as a dietary supplement. Humans, unlike most animals, are unable to synthesize vitamin C endogenously, so it is an essential dietary component.
Vitamin C is required for the biosynthesis of collagen, L-carnitine, and certain neurotransmitters; vitamin C is also involved in protein metabolism. Collagen is an essential component of connective tissue, which plays a vital role in wound healing. Vitamin C is also an important physiological antioxidant and has been shown to regenerate other antioxidants within the body, including alpha-tocopherol (vitamin E). Ongoing research is examining whether vitamin C, by limiting the damaging effects of free radicals through its antioxidant activity, might help prevent or delay the development of certain cancers, cardiovascular disease, and other diseases in which oxidative stress plays a causal role. In addition to its biosynthetic and antioxidant functions, vitamin C plays an important role in immune function and improves the absorption of nonheme iron, the form of iron present in plant-based foods. Insufficient vitamin C intake causes scurvy, which is characterized by fatigue or lassitude, widespread connective tissue weakness, and capillary fragility.
The intestinal absorption of vitamin C is regulated by at least one specific dose-dependent, active transporter. Cells accumulate vitamin C via a second specific transport protein. In vitro studies have found that oxidized vitamin C, or dehydroascorbic acid, enters cells via some facilitated glucose transporters and is then reduced internally to ascorbic acid. The physiologic importance of dehydroascorbic acid uptake and its contribution to overall vitamin C economy is unknown.
Oral vitamin C produces tissue and plasma concentrations that the body tightly controls. Approximately 70%–90% of vitamin C is absorbed at moderate intakes of 30–180 mg/day. However, at doses above 1 g/day, absorption falls to less than 50% and absorbed, unmetabolized ascorbic acid is excreted in the urine. Results from pharmacokinetic studies indicate that oral doses of 1.25 g/day ascorbic acid produce mean peak plasma vitamin C concentrations of 135 micromol/L, which are about two times higher than those produced by consuming 200–300 mg/day ascorbic acid from vitamin C-rich foods. Pharmacokinetic modeling predicts that even doses as high as 3 g ascorbic acid taken every 4 hours would produce peak plasma concentrations of only 220 micromol/L.
The total body content of vitamin C ranges from 300 mg (at near scurvy) to about 2g. High levels of vitamin C (millimolar concentrations) are maintained in cells and tissues and are highest in leukocytes (white blood cells), eyes, adrenal glands, pituitary gland, and brain. Relatively low levels of vitamin C (micromolar concentrations) are found in extracellular fluids, such as plasma, red blood cells, and saliva.
Intake recommendations for vitamin C and other nutrients are provided in the Dietary Reference Intakes (DRIs) developed by the Food and Nutrition Board (FNB) at the Institute of Medicine (IOM) of the National Academies (formerly National Academy of Sciences). DRI is the general term for a set of reference values used for planning and assessing nutrient intakes of healthy people. These values, which vary by age and gender, include:
Recommended Dietary Allowance (RDA): Average daily level of intake sufficient to meet the nutrient requirements of nearly all (97%–98%) healthy individuals; often used to plan nutritionally adequate diets for individuals.
Adequate Intake (AI): Intake at this level is assumed to ensure nutritional adequacy; established when evidence is insufficient to develop an RDA.
Estimated Average Requirement (EAR): Average daily level of intake estimated to meet the requirements of 50% of healthy individuals; usually used to assess the nutrient intakes of groups of people and to plan nutritionally adequate diets for them; can also be used to assess the nutrient intakes of individuals.
Tolerable Upper Intake Level (UL): Maximum daily intake unlikely to cause adverse health effects.
Supplements typically contain vitamin C in the form of ascorbic acid, which has equivalent bioavailability to that of naturally occurring ascorbic acid in foods, such as orange juice and broccoli. Other forms of vitamin C supplements include sodium ascorbate; calcium ascorbate; other mineral ascorbates; ascorbic acid with bioflavonoids; and combination products, such as Ester-C®, which contains calcium ascorbate, dehydroascorbate, calcium threonate, xylonate and lyxonate.
A few studies in humans have examined whether bioavailability differs among the various forms of vitamin C. In one study, Ester-C® and ascorbic acid produced the same vitamin C plasma concentrations, but Ester-C® produced significantly higher vitamin C concentrations in leukocytes 24 hours after ingestion. Another study found no differences in plasma vitamin C levels or urinary excretion of vitamin C among three different vitamin C sources: ascorbic acid, Ester-C®, and ascorbic acid with bioflavonoids. These findings, coupled with the relatively low cost of ascorbic acid, led the authors to conclude that simple ascorbic acid is the preferred source of supplemental vitamin C.
Epidemiologic evidence suggests that higher consumption of fruits and vegetables is associated with lower risk of most types of cancer, perhaps, in part, due to their high vitamin C content. Vitamin C can limit the formation of carcinogens, such as nitrosamines, in vivo; modulate immune response; and, through its antioxidant function, possibly attenuate oxidative damage that can lead to cancer.
Most case-control studies have found an inverse association between dietary vitamin C intake and cancers of the lung, breast, colon or rectum, stomach, oral cavity, larynx or pharynx, and esophagus. Plasma concentrations of vitamin C are also lower in people with cancer than controls.
Evidence from many epidemiological studies suggests that high intakes of fruits and vegetables are associated with a reduced risk of cardiovascular disease. This association might be partly attributable to the antioxidant content of these foods because oxidative damage, including oxidative modification of low-density lipoproteins, is a major cause of cardiovascular disease.
Emerging research suggests that the route of vitamin C administration (intravenous vs. oral) could explain the conflicting findings. Most intervention trials, including the one conducted by Moertel and colleagues, used only oral administration, whereas Cameron and colleagues used a combination of oral and intravenous (IV) administration. Oral administration of vitamin C, even of very large doses, can raise plasma vitamin C concentrations to a maximum of only 220 micromol/L, whereas IV administration can produce plasma concentrations as high as 26,000 micromol/L. Concentrations of this magnitude are selectively cytotoxic to tumor cells in vitro. Research in mice suggests that pharmacologic doses of IV vitamin C might show promise in treating otherwise difficult-to-treat tumors.
AMD and cataracts are two of the leading causes of vision loss in older individuals. Oxidative stress might contribute to the etiology of both conditions. Thus, researchers have hypothesized that vitamin C and other antioxidants play a role in the development and/or treatment of these diseases.
A population-based cohort study in the Netherlands found that adults aged 55 years or older who had high dietary intakes of vitamin C as well as beta-carotene, zinc, and vitamin E had a reduced risk of AMD. However, most prospective studies do not support these findings. The authors of a 2007 systematic review and meta-analysis of prospective cohort studies and randomized clinical trials concluded that the current evidence does not support a role for vitamin C and other antioxidants, including antioxidant supplements, in the primary prevention of early AMD.
In the 1970s Linus Pauling suggested that vitamin C could successfully treat and/or prevent the common cold. Results of subsequent controlled studies have been inconsistent, resulting in confusion and controversy, although public interest in the subject remains high.
Most people in the United States get enough vitamin C from foods and beverages. However, certain groups of people are more likely than others to have trouble getting enough vitamin C:
People who smoke and those who are exposed to secondhand smoke, in part because smoke increases the amount of vitamin C that the body needs to repair damage caused by free radicals. People who smoke need 35 mg more vitamin C per day than nonsmokers.
Infants who are fed evaporated or boiled cow’s milk, because cow’s milk has very little vitamin C and heat can destroy vitamin C. Cow’s milk is not recommended for infants under 1 year of age. Breast milk and infant formula have adequate amounts of vitamin C.
People who eat a very limited variety of food. People with certain medical conditions such as severe malabsorption, some types of cancer, and kidney disease requiring hemodialysis.
Vitamin C deficiency is rare in the United States and Canada. People who get little or no vitamin C (below about 10 mg per day) for many weeks can get scurvy. Scurvy causes fatigue, inflammation of the gums, small red or purple spots on the skin, joint pain, poor wound healing, and corkscrew hairs. Additional signs of scurvy include depression as well as swollen, bleeding gums and loosening or loss of teeth. People with scurvy can also develop anemia. Scurvy is fatal if it is not treated.
While vitamin C (ascorbic acid) is an essential nutrient, it’s possible to have too much of it.
Vitamin C is a water-soluble vitamin that supports normal growth and development and helps your body absorb iron. Because your body doesn’t produce or store vitamin C, it’s important to include vitamin C in your diet. For most people, an orange or a cup of strawberries, chopped red pepper, or broccoli provides enough vitamin C for the day.
For adults, the recommended daily amount for vitamin C is 65 to 90 milligrams (mg) a day, and the upper limit is 2,000 mg a day. Although too much dietary vitamin C is unlikely to be harmful.
Vitamin C is an antioxidant that helps support the immune system and helps neutralize free radicals in the body. Additionally, vitamin C is necessary for the body to manufacture collagen, which is important for healthy skin. Vitamin C is ideal for those looking to supplement their diet, if it is low in fruit and vegetables, to meet daily vitamin C nutrient needs.
Vitamin C is found in citrus fruits, primarily oranges, lemons, grapefruit, limes. It is also found in tomatoes and strawberries.
Vitamin C is recommended for winter wellness and immunity. Vitamin Cs major role is to make collagen, an essential protein substance that holds connective tissues together in skin, bone, teeth, and other parts of the body. Vitamin C is also critical for the proper function of the immune system, for manufacturing certain nerve-transmitting substances and hormones, and for the absorption and utilization of other nutrients, such as vitamin E and iron. Vitamin C is also a very important and powerful antioxidant that works in the aqueous (water) environments of the body, such as the lungs and lens of the eye. Its primary antioxidant partners are vitamin E and the carotenes, though it also works with the antioxidant enzymes. Vitamin C regenerates oxidized vitamin E and restores the antioxidant potential of vitamin E in the body.
Abundant clinical research confirms Vitamin C’s fundamental support for the brain, eyes, heart and circulation, immune system, and all organ systems.
Helps promote immune response:
– Support the brain, eyes, heart and circulation, immune system, and all organ systems.
– Vitamin C is a key in the body’s ‘antioxidant network,’ a chain of synergistic, inextricability linked.
– Vitamin C can change this status, by regenerating vitamin E back to its native form, and is thought to ‘spare’ glutathione in the body as well.
– Promotes immune response, supports the cardiovascular system and is essential for joints and other connective tissues.
– Clinical research confirms vitamin C’s fundamental support for the brain, eyes, heart and circulation, immune system, and all the body’s other organ systems.
Vitamin C – Office of dietary supplements, Sept 18, 2018, https://ods.od.nih.gov/factsheets/VitaminC-HealthProfessional/, 2018