| Abstract|| |
Context: Antioxidant properties and Vitamin C.
Background: Vitamin C is a naturally occurring organic compound and a potent antioxidant preventing oxidative damage to lipids and other macromolecules. It can also exhibit bimodal activity as a pro-oxidant at a higher concentration. Vitamin C has a switch over role from being an antioxidant in physiologic conditions to a pro-oxidant under pathologic conditions. A systematic review of this role would help to elucidate whether it is an antioxidant or a pro-oxidant in the oral environment.
Objective: To review studies reported in the literature elucidating the activity of Vitamin C and determine whether it is an antioxidant or a pro-oxidant.
Materials and Methods: Articles were searched in PubMed, MEDLINE using appropriate key words like "Vitamin C," "antioxidant activity," "pro-oxidant activity," "oral health" "oral disease." Hand search of journals was also performed. Articles were reviewed and analyzed.
Results: Search strategy reviewed 10 relevant articles which studied the dual role of Vitamin C. 65% of authors analyzed antioxidant action of ascorbic acid compared to 35% of the pro-oxidant potential. Vitamin C acts as an antioxidant and a pro-oxidant by a plethora of mechanisms. Factors determining its bimodal activity were studied, and the frequencies of their occurrence in the literature were depicted in percentage.
Conclusion: The data validates the role of Vitamin C as an antioxidant under physiologic conditions exhibiting a cross over role as a pro-oxidant in pathological conditions. Further studies are required to substantiate its pro-oxidant activity to draw concrete conclusions.
Keywords: Antioxidant activity, oral disease, oral health, pro-oxidant activity, Vitamin C
|How to cite this article:|
Chakraborthy A, Ramani P, Sherlin HJ, Premkumar P, Natesan A. Antioxidant and pro-oxidant activity of Vitamin C in oral environment. Indian J Dent Res 2014;25:499-504
Vitamin C (chemical name ascorbic acid) is a naturally occurring water soluble organic compound, obtained exogenously by various dietary sources owing to the mutation of gene encoding for L-gulonolactone oxidase enzyme required for its synthesis. 
|How to cite this URL:|
Chakraborthy A, Ramani P, Sherlin HJ, Premkumar P, Natesan A. Antioxidant and pro-oxidant activity of Vitamin C in oral environment. Indian J Dent Res [serial online] 2014 [cited 2019 Oct 13];25:499-504. Available from: http://www.ijdr.in/text.asp?2014/25/4/499/142547
Reactive oxygen species, both oxygen centered radicals (superoxide, hydroxyl anions) and oxygen centered nonradicals (hydrogen peroxide, singlet oxygen) cause tissue damage.  Toxins in cigarette smoke contain numerous free radicals causing tissue damage in the oral cavity. Vitamin C is one of the several compounds that form part of the body's first line of defense' against the free radicals mediated cellular damage, directly neutralizing free radicals, implicated in the interaction between pathogenic bacteria and host immune response, contributing an increased risk of periodontal disease in smokers as compared to their normal counterparts. It also maintains systemic health by interfering with the production of free radicals and peroxides by regulating endothelium derived nitric oxide by its pivotal vasodilatory role to prevent cardiovascular disorders. 
At a higher concentration, it acts as a pro-oxidant inducing oxidative stress, either by generating reactive oxygen species or by inhibiting the antioxidant systems in the presence of iron, which in turn induces lipid peroxidation.  Whether Vitamin C has a net pro-oxidant or antioxidant effect depends on the concentration gradient and redox state of a cell. As normal tissue receives adequate blood flow and is rich in antioxidant enzymes, any hydrogen peroxide formed will be immediately destroyed. Meanwhile, tumor tissue is often associated with reduced blood flow and antioxidant enzymes, and consequently formed hydrogen peroxide remains active leading to cellular damage. This phenomenon is based on the ability of ascorbate metal dependent production of hydroxyl radicals by Fenton chemistry,  by reaction of the reduced metal ions like Iron or copper with hydrogen peroxide. Vitamin C when administered as a dietary supplements, exhibits a dual role in the oral environment depending on the concentration gradient.
Though the importance of Vitamin C is widely accepted, very few authors in previous studies were able to demark its action as a potent antioxidant or a pro-oxidant. Scarce information is available about its dual pharmacological effects and actions in the research literature about the same. Articles were selected on the basis of inclusion and exclusion criteria for the related topic using PubMed and published data. The purpose of the systematic review was to determine whether Vitamin C acts a potential antioxidant or a pro-oxidant under physiologic conditions in the oral environment.
| Materials AND METHODS|| |
0Search strategy for identification of studies
The search protocol was in consensus with the Cochrane Oral Health Group Specialized Trials Register and the Database of Abstracts and Reviews of Effectiveness were searched for systematic reviews. Articles were searched and selected for the related topic using PubMed; MEDLINE from 1993 to June 2013. Article search included only those published in the English literature. An internet search was also done to obtain the relevant articles of our interest. The title of the articles and abstracts were reviewed [Flow chart 1] [Additional file 1].
The search methodology through PubMed was done using the following keywords: (Vitamin C) OR (ascorbic acid) OR (sodium ascorbate) OR (dehydroascorbate) AND (Vitamin C antioxidant) OR (antioxidant potential) OR (antioxidant property) OR (antioxidant effect) OR (antioxidant activity) AND (pro-oxidant activity) OR (Vitamin C pro-oxidant) OR (pro-oxidant potential) OR (pro-oxidant property) OR (pro-oxidant effect) AND (Oral tissues) OR (oral health) AND (Oral neoplasms) OR (Oral tumors) OR (oral cancer) OR (gingivitis) OR (periodontitis). In addition, an internet search was also done using the key words "Vitamin C" and "ascorbic acid" and "antioxidant activity" and "pro-oxidant activity" and "oral tissues" "oral disease" "oral neoplasm" and "oral tumor" and "oral cancer." Journals evaluating the antioxidant and pro-oxidant effects of ascorbic acid were referred for review.
Published reviews were selected if they met the following criteria. Cross sectional studies to elucidate the role of Vitamin C in oral and systemic health of subjects under the studies and the mechanism by which it exhibits its properties.
- Human interventional studies with a follow up period of 6 months
- Studies published in English literature
- Studies that evaluated the activity of Ascorbic acid as an antioxidant in oral health
- Studies that evaluated the activity of Ascorbic acid as a pro-oxidant in oral disease
- Studies that analyzed the effects of ascorbic acid in combination with other antioxidants vitamins
- Studies that evaluated the evidence of ascorbic acid supplementation and combinations of antioxidants as interventions for the treatment of oral disease.
- Studies that have reported the effects of ascorbic acid in animals
- Botanical studies on ascorbic acid
- Studies involving more than two simultaneous interventions of Vitamin C
- Studies with missing data that could not be supplied by the study authors
- Studies are not exhibiting the antioxidant and pro-oxidant properties of ascorbic acid
- Case reports and case series
- Letters to editors.
Data extraction and analysis
All the included studies were based on the data extraction and analysis of the studies of quality and publication bias. The primary outcome was to elucidate the activity of Vitamin C on oral health and diseases correlating to its antioxidant and pro-oxidant properties. Potentially relevant articles for systematic review were obtained, data were extracted from each article was tabulated and cross checked.
| Results|| |
The selected articles were divided into subgroups of author name, journal name, and year of publication, country, and conclusion are shown in [Table 1]. All included studies have observed the action of Vitamin C, both as an antioxidant and a pro-oxidant depending on various factors. Three of the authors have found Vitamin C as a potential antioxidant promoting oral health status of individuals in various physiologic conditions by its vivid free radical neutralizing potential. Three authors each have advocated the strong antioxidant activity of Vitamin C in in vivo studies and glutathione dependent activity in the oral environment. One author has concluded the moderate potential of Vitamin C in in vivo conditions, whereas one author each have assessed the strong activity of Vitamin C among physiologic conditions and smokers, respectively. Five authors have concluded that Ascorbic acid is a pro-oxidant in iron induced Fenton reaction in pathologic conditions. Two studies have found the pro-oxidant action of Vitamin C in Copper induced oxidation. Vitamin C has been found to be a pro-oxidant under conditions of high plasma concentration, and among smokers by one author each substantiating its activity in various pathologic states. Three studies showed that Vitamin C acts as a pro-oxidant by inhibiting the growth of malignant cells exhibiting its chemo preventive potential.
[Table 2] reveals that 65% of authors have analyzed the antioxidant action of Ascorbic acid when compared to 35% to the pro-oxidant potential. It has been found that the antioxidant activity of Vitamin C is executed by the varied mechanism of action in physiologic conditions in the oral environment. The calculated percentage is 36%, 40%, 9%, 9%, 14%, 20%, 25%, 45% and 14% for mechanism of Vitamin C as an antioxidant by its free radical scavenging property, redox potential as an electron donor, co-administration with Glutathione, co-administration with tocopherol, prevention of lipid per oxidation, prevention of oxidative DNA damage, prevention of oxidative protein damage, chemo preventive action in the presence of iron overload, respectively. While 83%, 42% and 25% of authors have evaluated the pro-oxidant action of Vitamin C by the mechanism of iron induced oxidative damage, copper induced oxidative damage (Fenton reaction) and higher pharmacological concentration, respectively, elucidating its potential activity in pathologic conditions in the oral environment. A significant difference has been observed in the percentage of studies done to examine the action of Vitamin C as an antioxidant and a pro-oxidant, it has been observed that a greater percentage of studies have been done in favor of antioxidant property of Vitamin C under physiologic conditions as compared to that done for its pro-oxidant property inducing a compromised oral health. 70% authors have performed an in vivo and in vivo study on antioxidant property under various physiologic conditions as compared to only 30% done for pro-oxidant activity. Similar trend is observed for other study samples, 85%, 75%, 53% and 67%, in vivo , case control, cohort and placebo controlled, studies respectively have been done in comparison to 15%, 25%, 47% and 33%, studies respectively done for similar study samples mentioned above.
A marked potent action of Vitamin C as an antioxidant in oral health has been observed among authors in the included studies. The review found that 73% of authors successfully inferred the response of Vitamin C as an antioxidant in oral health compared to its nonconcrete action as a pro-oxidant which amounts to only about 27% in the pathologic state. 21% of authors have ascertained the potency of Vitamin C as a pro-oxidant in oral diseases compared to 79% authors who have witnessed the action of Vitamin C as an antioxidant in physiologic conditions improving the oral health of individuals. The tendency of shifting its role from an antioxidant to a pro-oxidant under pathological conditions has been observed by 78% of the authors as compared to 22% authors who have not observed any transformative role of Vitamin C as an antioxidant but revealing its individual activity as an antioxidant and pro-oxidant in oral health and disease respectively. Hence, from the previous reviewed literature, it can be concluded that Vitamin C has a high potential of transforming from an antioxidant in vivo under physiologic conditions to a pro-oxidant in pathologic conditions by the virtue of Fenton chemistry and at a higher pharmacological concentration.
It is seen from the present review that 23% of authors have mentioned the antioxidant action of Vitamin C in high plasma concentration compared to 30% authors in favor of the pro-oxidant action of Vitamin C in similar condition. Whereas 15% and 23%, of authors respectively, have concluded that Vitamin C is an antioxidant and a pro-oxidant, under low plasma concentration. Hence, it is observed from our review that the majority of studies have been done by various authors to study the antioxidant action of Vitamin C than the number of studies done to examine the pro-oxidant action of the same. The mechanism of action by which the antioxidant property of Vitamin C is carried out in the body is hence better studied and understood than its pro-oxidant property. Although its role as an antioxidant is well documented, there is little evidence that it serves as a pro-oxidant under physiological conditions, hence more studies is required to draw any concrete conclusions.
| Discussion|| |
There is a continuing debate over the best dose schedule (the amount and frequency of intake) of Vitamin C for maintaining optimal oral health in humans. An average intake by healthy adults ranges from 90 to 100 mg daily; while there is increased intake in pregnant and lactating women or individuals under stress. 
Vitamin C is an essential nutrient for humans and a cofactor for collagen synthesis for maintenance of all oral hard and soft tissues. The biochemical and molecular roles can be accounted by its function as a reducing agent. This was observed by Padayatty et al.  in 2003 focusing on its property as an electron donor to prevent diseases in reduced intake attributing to its Antioxidant potential by forming ascorbyl radical; which is comparatively stable and less reactive in nature.
The pathologies that accompany Vitamin C deficiency have prompted numerous investigators to determine a possible relationship between ascorbic acid and periodontal tissue health. Epidemiological studies also demonstrate an association between Vitamin C intake and pathologies, affecting the structural integrity of the oral hard and soft tissues. , Hence, various authors have extensively viewed the protective, formative and homeostatic benefits of Vitamin C in human oral environment.
Vitamin C by its protective role acts as a potent water soluble antioxidant as observed by 65% of authors from our systematic review of the literature [Figure 1]. ,,,,, Antioxidants play a pivotal role as free radical and nonradical oxidant scavengers. Depending on the circumstances, a compound may exhibit pro or antioxidant activity. The protective effect of Vitamin C was further reported by Lee et al.  in 2003, it was also stated that at a higher dosage, it had induced oxidative damage to DNA promoting to pro-oxidant and mutagenic potential in humans. 
|Figure 1: Pie chart showing percentage of antioxidant and pro-oxidant studies|
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Ascorbate is a strong antioxidant as suggested by 70% of authors among the included studies of our review of the literature, in various in vivo and in vitro conditions; however 30% of authors among the studies included, inferred it can also act as a pro-oxidant in vivo by the help of reducing transition metals; driving a Fenton reaction.  As reported by Joel Schwartz  in 1996 the beneficial or detrimental effect of a nutrient depends on the bimodal characteristic depending on the inorganic chemistry of the cell. Herbert  in 1996, observed the pro-oxidant effects of antioxidant vitamins at a higher concentration, later supported by Azmi et al.  in 2003, linking to their anticarcinogenic property by generating reactive oxygen species.
Although few studies have shown the pro-oxidant effects of Vitamin C in the presence of Cu 2+ and/or Fe 3+ , Retsky et al.  in 1993 contradicted that Vitamin C does not act as a pro-oxidant in the presence of transition metal ions, it rather exhibits its potentiality as an antioxidant under the same physiologic conditions demonstrating that Vitamin C was most effective in lipid peroxidation prevention. The evidence for antioxidant protection of lipids by Vitamin C, in both iron and noniron supplementation, was also reported by Carr and Frei  in 1999, suggestive of Vitamin C as a potential antioxidant under physiologic conditions, as also concluded by 73% of authors in our systematic review, compared to 27% authors who stated that Vitamin C exhibits a pro-oxidant by Fenton reaction ,, as also seen in studies investigating tumor growth and cancer inducing a compromised health status. 35% of authors in our review have assessed the pro-oxidant action of Vitamin C at high plasma concentration by the same mechanism. 
Nishida et al. in 2000,  in reference to NHAHES III survey, reported higher daily dosage of Vitamin C has the potential to scavenge free radicals in smokers which helps to prevent the progression of smoking associated progressive changes in the oral cavity. However, Naidu  in 2003 overviewed the mystical benefits of ascorbic acid in human health and disease stating that the ascorbic acid at a higher concentration can exhibits properties of antioxidant.
Carr and Frei  on the contrary stated that antioxidants inhibit the growth of transformed cells thus increasing their intercellular communication by the oxidative protective mechanism in a unique manner. Hence, the use of natural nutrients like Vitamin C is expected to provide the greatest benefit toward the inhibition of oral carcinogenesis.
Vitamin C plays a switch over from being an antioxidant in physiologic conditions to a pro-oxidant under pathological conditions as observed by 78% of the authors in our, as compared to 22% authors who have not observed the same stating the exhibition of its individual activity as an antioxidant and pro-oxidant in oral health and disease in physiologic and pathologic conditions respectively.
To summarize, from the previous review of the literature, it can be stated that under physiologic conditions, Vitamin C acts as a potential antioxidant due to its plethora of mechanisms like free radical and reactive oxygen species scavenging action, redox potential and prevention of oxidative damage of lipids, proteins and nuclear material. However, its switch over role as a pro-oxidant is paradoxical, stating the influence of iron and copper by the virtue of Fenton chemistry in pathological conditions. Further studies are warranted to explain, understand and determine its activity as a potential pro-oxidant in the oral environment.
| Conclusion|| |
Although the role of Vitamin C as an antioxidant is well documented, there is little evidence that emphasizes its pro-oxidant role in various conditions. Previous research and our systemic review confirm that under normal physiological conditions, it acts mainly as an antioxidant; however, the transition of a healthy oral status to a highly compromised pathologic state can trigger the pro-oxidant activity of Vitamin C. The observations of this systemic review also reflected the necessity for researchers to determine stronger theories that would elucidate the mechanisms and conditions that bring about the pro-oxidant attributes of Vitamin C to illustrate and demarcate its antioxidant and pro-oxidant properties more precisely.
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Department of Oral and Maxillofacial Pathology, Saveetha Dental College, Chennai, Tamil Nadu
Source of Support: None, Conflict of Interest: None
[Table 1], [Table 2]