Vitamin C - Ascorbic Acid Benefits
Vitamin C is chemically considered a simple carbohydrate with an endiol structure that functions as an acid in the body. Pure ascorbic acid is a white crystalline powder 4.
Unlike most mammals, humans cannot synthesize ascorbic acid in the liver. Humans lack the enzyme L-gulonolactone oxidase, which catalyzes the final step of ascorbic acid formation from D-glucose 5. Therefore, vitamin C must be consumed daily through diet since it cannot be stored in the body 6.
How much vitamin C is present in the body?
The range of vitamin C present in the body varies from 300 mg (close to scurvy, a deficiency disease caused by insufficient vitamin C intake) to 2g - on average, it is between 1.2 and 2 mg.7 8. Higher levels of vitamin C are found in the pituitary gland, adrenal glands, leukocytes (white blood cells), ocular tissues, body fluids, and generally in the brain 9. Additionally, vitamin C is a component of the skin, with significant amounts found in both the epidermis and the dermis 10.
Absorption of Ascorbic Acid
Vitamin C is absorbed through the intestinal mucosa of the small intestine. Approximately 70 to 90% of the vitamin C (30 to 180 mg per day) consumed through food is absorbed by the body 11. When the intake exceeds 500 mg per day, the effectiveness of absorption significantly decreases 12. In the body, ascorbic acid changes its form and becomes ascorbate 13.
Functions and Benefits
In the body, vitamin C plays an important role in its reduced form (ascorbate). It is mainly required for the functioning of numerous enzymes and serves as a reducing agent (electron donor for other molecules).
Vitamin C is involved in collagen formation. Collagen is the main structural protein for connective tissue, which holds together muscles, bones, dentin (tooth substance), and other tissue connections such as ligaments, tendons, cartilage, and blood vessels. Additionally, collagen is needed for wound healing. Therefore, ascorbic acid plays an important role, especially in older individuals, in the aging process and maintaining dental health 14. Vitamin C is required as a cofactor (essential for the function of some enzymes) for the iron (Fe2+)- and α-ketoglutarate-dependent enzymes involved in collagen synthesis 15. These enzymes catalyze the hydroxylation (chemical reaction to introduce one or more hydroxyl groups; hydroxyl group = -OH) of proline and lysine residues (both are amino acids and building blocks of the collagen triple helix structure) necessary for the formation of mature and stable collagen 16 17. Ultimately, vitamin C ensures that Fe2+ (non-heme iron) remains in its reduced state and does not oxidize to Fe3+ (heme iron).
Ascorbate is required as a co-factor for the formation of the catecholamine norepinephrine (also known as noradrenaline), which functions as a neurotransmitter, transmitting nerve impulses from dopamine 18.
Ascorbate is involved in the amidation (formation of amides) of peptide hormones (oxytocin, vasopressin, cholecystokinin, and alpha-melanotropin) to enable maximum hormone activity 19.
Ascorbate is also needed for the metabolism of tyrosine (an amino acid), which is a precursor for dopamine and melanin, among other things 20.
Ascorbate regulates the activity of HIF-1α (hypoxia-inducible factor 1α) through hydroxylation 21. HIF is a protein that responds to changes in available oxygen in cells and is involved in regulating cellular oxygen supply. When cell nuclei are poorly oxygenated (hypoxia), HIFs become active 22. It is assumed that HIFs are the target of 800 genes, which produce 60 gene products (protein products) 23. It is likely that ascorbate indirectly affects many cellular functions such as angiogenesis (growth of blood vessels), cell survival, glucose uptake (glucose for rapid energy production), glycolysis (part of glucose metabolism where glucose is broken down), iron homeostasis (maintenance of iron balance), and red blood cell production (erythropoiesis) through the protein products formed 24 25 26.
Additionally, ascorbate is needed with iron for the formation of carnitine 27. Carnitine contributes to the oxidation of fatty acids (fat burning) 28. It transports long-chain fatty acids into the mitochondria where they are burned and used for energy production.
Furthermore, vitamin C is required for the conversion of cholesterol to bile acids in the liver 29.
Ascorbate plays a crucial role in demethylation of DNA, regulation of histone demethylation, acting as a mediator between the genome (genetic material) and its environment, and maintaining the epigenome in early embryonic stage 30.
Vitamin C plays a role in the functionality of the enzyme endothelial nitric oxide synthase (eNOS) by recycling the cofactor of eNOS (Tetrahydrobiopterin) 31. The eNOS enzyme is relevant to arterial elasticity and blood pressure regulation.
Vitamin C as an antioxidant against free radicals
Vitamin C is an antioxidant that helps combat free radicals, thereby protecting cells. Free radicals are produced in the body when food is converted into energy (peroxidation of membrane phospholipids). Body stress, such as oxidative stress caused by cigarette smoke, air pollution, and ultraviolet light, also contributes to the formation of free radicals. They can cause damage to cells 32.
Due to its low redox potential (a measure of the ability to act as an oxidizing or reducing agent), ascorbate can react with almost all oxidized free radicals 33. In this process, vitamin C acts as a reducing agent (= scavenger), releasing electrons to reduce and neutralize other compounds.
In a redox reaction, electrons are transferred to the reactant (electron donor), thereby reducing other substances. Vitamin C (L-ascorbate) itself oxidizes to form an ascorbyl radical (relatively stable free radical), which can donate another electron 34. Ascorbate thus protects other cellular components from oxidation by free radicals 35. When two electrons are donated, dehydroascorbic acid is formed, which is the fully reduced and oxidized form of vitamin C. Dehydroascorbic acid is inactive in the body. Alternatively, two relatively stable and unreactive ascorbyl radical molecules can be converted into one ascorbate molecule and one dehydroascorbic acid molecule 36.
Moreover, vitamin C can be added to food as a preservative (E300). In the form of palmitoyl ascorbate (also known as ascorbyl palmitate), vitamin C is used as an antioxidant. Ascorbic acid and its fatty acid esters are used as food additives (E304) for various purposes such as antioxidants, anti-browning agents, reducing agents, flavor enhancers, and color stabilizers 37.
Vitamin C protects the skin
Vitamin C is the most abundant antioxidant in the skin, which, along with other antioxidants, protects the skin from reactive oxygen species (ROS) that are generated when the skin is exposed to UV light 43. Thus, vitamin C contributes to photoprotection and reduces cell damage caused by the impact of sunlight on the skin.
Together with vitamin E, vitamin C protects the hydrophilic and lipophilic compartments (defined space within the cell) of the cell, limiting damage caused by UV light. 44 45. According to scientific findings, the skin's protection against light works best with vitamin C in combination with vitamin E, which amplifies the benefits of vitamin C by four times. Therefore, both vitamins are typically added to sunscreens, which penetrate the skin and take effect there. However, vitamin C itself is not a sunscreen as it cannot block UVA or UVB rays. UVB rays can cause sunburn, reactive oxygen species (ROS), epidermal mutations, and skin cancer. 46. UVA rays penetrate the skin 30-40 times deeper than UVB rays, where they destroy collagen, elastin, proteoglycans, and other skin cell structures. They contribute to skin aging and may trigger the formation of melanoma.
Vitamin C also acts as a depigmentation agent in the body, interrupting important steps in melanogenesis 47. It interacts with copper ions at the tyrosinase, inhibiting the activity of the enzyme and reducing melanin production.
In addition, vitamin C has a potentially anti-inflammatory benefit that can be helpful in conditions such as acne vulgaris and rosacea 48.
Improvement of iron absorption
Vitamin C improves the absorption of non-heme iron from plant-based sources. 49 50 51. In the small intestine, vitamin C binds to ferric iron (Fe3+) and reduces it to ferrous iron (Fe2+). Ferrous iron can be much better absorbed in the small intestine. This increases the bioavailability of iron. Therefore, it is advisable to consume iron-rich foods along with foods high in vitamin C.
In addition, ascorbate is involved in iron metabolism by stimulating the synthesis of ferritin (stored iron), inhibiting the breakdown of stored iron, and reducing the efflux of cellular iron 52.
Vitamin C supports fat burning
According to a study, individuals with adequate vitamin C status burn (oxidize) 30% more fat during moderate exercise compared to those with low status 53. Therefore, individuals with depleted vitamin C levels may be more resistant to fat loss.
Vitamin C is present in high concentrations in immune cells 54. Therefore, it is believed that vitamin C can help during an infection. However, the mechanisms are still not clearly understood to this day.
However, it is certain that ascorbic acid stimulates the immune system by strengthening and protecting the body 55. The scientists add that vitamin C is suitable for areas of medicine such as immunology, toxicology, and radiobiology, due to its immunostimulatory, anti-inflammatory, antiviral, and antibacterial benefits.
Does Vitamin C prevent colds?
No. According to a study, supplementing with vitamin C (0.2 g per day) does not reduce the occurrence of colds 56.
However, vitamin C does help reduce the duration of cold symptoms, although the exact mechanism of action is still unclear 57.
Is Vitamin C helpful in cancer treatment?
Perimenopausal women with cases of breast cancer in the family had a 53-63% lower risk of breast cancer when consuming an average of 205 mg of vitamin C from food per day compared to those who consumed an average of 70 mg per day 60. In overweight women, consuming 110 mg of vitamin C daily resulted in a 39% lower risk of breast cancer compared to women with an average consumption of 31 mg per day 61. Further results suggest that consuming dietary vitamin C before breast cancer diagnosis could be helpful for cancer survival 62. However, other studies no longer show associations between vitamin C and breast cancer 63 64 65.
A systematic review of scientific studies does not support the hypothesis that supplementing with vitamin C would prevent and/or treat cancer. 66.
However, scientific findings suggest that intravenously administered gram doses could have benefits in cancer treatment with chemotherapy 67. These results are likely attributed to ascorbate autoxidation and the generation of hydrogen peroxide, which is selectively toxic to cancer cells.
Vitamin C is recycled
To reduce the body's loss of vitamin C, ascorbate can be regenerated from dehydroascorbic acid. In this process, dehydroascorbic acid is regenerated to ascorbate with the help of glutathione or the selenoenzyme thioredoxin reductase 68.
Do vegans need to consider anything?
With a vegan diet, it is very easy to meet the vitamin C requirement. According to two major British studies, vegans have the highest intake of vitamin C 69 70. The lowest intake was observed in omnivores.
According to a Danish study, vegans consume higher amounts compared to the general population. 71. Vegan women and men were found to consume 221 mg of vitamin C daily, while the general population consumed less than half of that amount, with 110 mg for women and 100 mg for men 72.
For a table of foods rich in vitamin C, click here.