If vitamins are defined as natural compounds that are indispensable for every living organism, it is understandable that all of the compounds within this group are equally important for the preservation of health. However, some vitamins are simply “more popular” than others.
One of them is the vitamin C (L-ascorbic acid or simply ascorbate), which raises new scientific debates on daily basis because of its multiple roles in many biological processes in the body. We already know a lot about its biological effects – it is well-known for its role in building collagen, as an antioxidant that is essential for maintaining the oxidative balance in the body, for its effects on the hormonal system, as well as many other useful functions.
In addition, an increasing number of scientific studies are attempting to provide the definitive answers to the question about the potentially harmful effects of vitamin C. This is an area filled with contradictions, some of which are likely to be clarified in the future. However, the fact that vitamin C continues to challenge the scientists because of its extreme importance remains valid.
Pure Vitamin C is a white crystalline solid, which is very soluble in water. It was first isolated as an “acid-carbohydrate” in 1928 from the adrenal glands, cabbage, and oranges. Although plants and most animals can synthesize vitamin C from glucose, humans and other primates have lost an enzyme required for the final stage of vitamin C synthesis (gulonolactone oxidase) during the evolution, and are therefore satisfying their needs for vitamin C through food. Being that the mentioned enzyme catalyzes the reaction in which, in addition to L-ascorbate, hydrogen peroxide is formed, it is somewhat ironic that a high-level synthesis can cause the oxidative stress.
Multiple biological effects
Vitamin C is involved in more than 300 biological processes in the body. Some of them are:
The biosynthesis of collagen – Vitamin C is required in vivo (in a living organism) as a cofactor for at least eight enzymes. The best known are the proline hydroxylase and lysine hydroxylase, which are involved in the biosynthesis of collagen. Both enzymes contain iron at their active sites.
Collagen synthesized in the absence of vitamin C is insufficiently hydroxylated and its fibers are not strong enough, which causes the fragility of blood vessels. Collagen is a protein that normally underlies the connective tissues, and is necessary for the construction and the protection of the blood vessels, bones, joints, and muscles.
Hormonal system – Vitamin C participates in the synthesis of the adrenal hormones. It is required for the synthesis of an enzyme called dopamine beta-hydroxylase, which contains copper and converts dopamine into noradrenaline.
Circulatory system – Vitamin C is important in the creation of cholesterol in the liver and its conversion into the bile acids. It promotes the normal cholesterol and LDL cholesterol levels. In addition, it affects the normal circulation, which is important for the proper heart function. Finally, vitamin C maintains the normal blood pressure levels.
Nervous system – Vitamin C is involved in the conversion of the amino acid tryptophan into serotonin, the transporter of the nerve stimulations that has a number of functions in the nervous system.
The importance of antioxidant activity
One of the most important biological functions of vitamin C is its ability to act as a reducing agent (for example, its ability to reduce ferric iron into ferrous iron, which is important for the stimulation of its absorption in the bowels). The observations that vitamin C can inhibit the cancerogenic actions of several nitroso compounds can be partially attributed to its ability to reduce these compounds into their less harmful, inactive forms.In this way, vitamin C acts as the body’s protector from the damaging effects of the free radicals and stands out as a very good antioxidant.
It is known that the free radicals react with the biological molecules in an organism (proteins, nucleic acids, cell lipids) and can cause damage to the biological system associated with many serious diseases (neurodegenerative diseases, cancer…). There is even a theory that their activities are associated with the aging process. Protecting the cells against the reactive molecules is provided through a number of compounds that are capable to act as “scavengers” of the free radicals, i.e. the antioxidants.
In vitro, vitamin C acts as an antioxidant in several ways: it removes the radicals created by certain medications; protects the lungs from damage, particularly from heavy contaminants from the air by reducing the peroxidation of fats from the cigarette smoke; removes peroxyl, sulfenyl, urate, and other nitroxide radicals. What is crucial for any antioxidant effects of vitamin C in the reaction with the reactive radicals is that it provides one electron from its molecule, which leads to the formation of the ascorbyl radicals, or it’s fairly inactive form.
The antioxidant synergies between vitamins C and E have also been observed. In other words, they complement each other’s activities, which can be seen in the reaction in which vitamin C regenerates the a-tocopherol (vitamin E), which also acts as an antioxidant by protecting the lipids in the membranes from the free radicals’ attack. When it comes to the in vivo antioxidant effects of vitamin C, many scientific confirmations still exist. Judging by it’s in vivo concentration (30-100 microns in plasma), it can be concluded that it is quite sufficient for the expression of an antioxidant activity in the body.
Hypotheses about the possible harmful effects
Despite the fact that vitamin C is one of the most effective reducing substances in the biological system, if it reacts with the metal ions (especially copper and iron), it may initiate the production of the active forms of oxygen, that is, it can act as a harmful pro-oxidant. The binding of vitamin C with ions of iron and copper encourages the DNA, lipid, and protein damage, because of the actions of the free radicals. Such adverse effects are well-known to the nutritionists.
This imposes the question of how such an effect of vitamin C is important in the physiological sense. This question is extremely important because of the efforts to determine the optimal doses of vitamin C from food, as well as the possible harmful effects of an additional vitamin C intake. Although there is no clear evidence that its toxicity is induced by excessive doses, the researchers do not recommend taking the multigram dosages of vitamin C. It is believed that the concentration of 200 mg of vitamin C per day is sufficient for the cells and fluids in the body. Any excess will simply get excreted from the body.
The pro-oxidant action is not only associated with vitamin C, but it can also be applied to any other reducing agent which gets surrounded by the transitional metal ions (e.g. vitamin E, GSH, NADP, and some plant phenolics). It is obvious that the critical availability of these metal ions raises the important question of the optimal daily intake of iron or copper. Undoubtedly, iron is extremely important for human health, especially for children and pregnant women, but it is known that the excessive concentrations of iron can be very harmful.
In vivo, iron and copper are in the forms that can facilitate the formation of the free radicals’ reactions, while the extracellular fluids do not contain these metal ions. However, the intracellular iron (probably copper) exists within the cells, and if it gets in contact with vitamin C, the pro-oxidant reaction will occur. Therefore, any tissue damage can increase the availability of metal ions and lead to the formation of free radicals. This issue is still full of hypotheses that future research will hopefully be able to explain more precisely.
Neither too much nor too little is good
The vitamin C deficit is very rare. Its deficiency leads to scurvy, a disease whose nature was described in detail by the French explorer, Jacques Cartier, in 1956. Its symptoms are associated with the poor collagen formation, capillary damage, loss of teeth, joint pains, and dry skin. The deficit of vitamin C can: increase our susceptibility to infections; cause infertility in men and genetic damage to the sperm cells; and increase the excretion of vitamin B6, which may result in a deficit of vitamin A. High doses of vitamin C can reduce the secretion of vitamin B6, reduce the levels of vitamin B12, copper, and selenium, and it is also considered that they can reduce the negative effects of birth control pills. Vitamin C protects the body against the toxic effects of cadmium, copper, vanadium, cobalt, mercury, and selenium.
Exposure to cigarette smoke or airborne pollutants, such as ozone or carbon monoxide, can deplete vitamin C from the lungs, while aspirin, alcohol, antibiotics, and steroids can increase the body’s need for vitamin C.
There is a lot of speculation about the harmful effects of vitamin C in the body. However, there is little clear scientific evidence about its actual harmful effects on the body, or its role in the development of some diseases, such as cancer.
Forms and availability to the target tissues
Vitamin C can be found in many different forms (powder, tablets …). It has been proven that the natural and synthetic forms of vitamin C are chemically identical, that is, they have the same bioavailability (the degree to which a nutrient becomes available to the target tissue after it enters the body) and biological activity, as well as the fact that the bioavailability does not change in relation to the form, whether we are talking about a pill, powder, or any other forms.
The absorption of vitamin C in the intestines occurs through the passive diffusion, but also through active transport. When the concentrations of vitamin C are low, active transport dominates in the intestines, while high vitamin C concentrations lead to the saturation of active transport, and only the passive diffusion remains. Theoretically, slowing the bowel movements can increase the absorption (for example, taking vitamin C with food or the forms which poorly release).
Mineral ascorbates are the mineral salts of vitamin C that are mildly acidic. They are often recommended to persons with digestive disorders, although there is little scientific evidence that they are any less irritating to the digestive system. They exist as sodium, calcium, potassium, magnesium, molybdenum, chromium, and manganese ascorbate.
Ester – C® is another form of vitamin C. It mainly contains the calcium ascorbate metabolites and small amounts of vitamin C (dehydroascorbic acid – the oxidized form of ascorbic acid), calcium threonate, and traces of xylonate. It is believed that calcium threonate enhances the bioavailability of vitamin C.
Vitamin C can also be consumed with bioflavonoids (a group of water-soluble plant pigments). Fruits and vegetables rich in vitamin C, especially citrus, are often a rich source of bioflavonoids.
One more vitamin C form is the ascorbyl palmitate, a fat-soluble antioxidant, which is occasionally used for the purposes of extending the shelf life of potato chips or vegetable oils. It is currently very popular because it is believed that it is more stable than the fluid forms of vitamin C, even though its oral administration does not have great results because it gets hydrolyzed into palmitate and ascorbic acid in the digestive system.