Anitra C Carr, Balz Frei, Towards a brand new advisable dietary allowance for vitamin C primarily based on antioxidant and well being results in people, The American Journal of Scientific Vitamin, Quantity 69, Challenge 6, June 1999, Pages 1086–1107, https://doi.org/10.1093/ajcn/69.6.1086
ABSTRACT
INTRODUCTION
Vitamin C (ascorbic acid) is a necessary micronutrient required for regular metabolic functioning of the physique (1). People and different primates have misplaced the power to synthesize vitamin C on account of a mutation within the gene coding for l-gulonolactone oxidase, an enzyme required for the biosynthesis of vitamin C by way of the glucuronic acid pathway (2). Thus, vitamin C should be obtained by the weight-reduction plan. The vitamin is very plentiful in recent fruit, particularly citrus fruit, and greens (3). A scarcity of vitamin C within the weight-reduction plan causes the deficiency illness scurvy (4). This probably deadly illness may be prevented with as little as 10 mg vitamin C/d (5), an quantity simply obtained by consumption of recent fruit and greens.
The present advisable dietary allowance (RDA) for vitamin C is 60 mg/d for wholesome, nonsmoking adults (6). The RDA is set by the speed of turnover and price of depletion of an preliminary physique pool of 1500 mg vitamin C and an assumed absorption of ≈85% of the vitamin at ordinary intakes (7). This quantity offers an enough margin of security: 60 mg/d would stop the event of scurvy for ≈1 mo with a weight-reduction plan missing vitamin C (7). The RDAs are decided totally on the idea of prevention of deficiency; as a result of scurvy shouldn’t be a serious well being downside in the USA, this purpose is clearly achieved by the present RDA for vitamin C. Nonetheless, ≈25% of women and men in the USA eat <60 mg vitamin C/d and ≈10% of adults eat <30 mg/d (3). The molecular mechanisms of the antiscorbutic impact of vitamin C are largely, though not fully, understood. Vitamin C is a cofactor for a number of enzymes concerned within the biosynthesis of collagen, carnitine, and neurotransmitters (8, 9). Procollagen-proline dioxygenase (proline hydroxylase) and procollagen-lysine 5-dioxygenase (lysine hydroxylase), 2 enzymes concerned in procollagen biosynthesis, require vitamin C for maximal exercise (10). Posttranslational hydroxylation of proline and lysine residues by these enzymes is important for the formation and secretion of steady collagen helixes. A deficiency of vitamin C leads to a weakening of collagenous buildings, inflicting tooth loss, joint pains, bone and connective tissue problems, and poor wound therapeutic, all of that are traits of scurvy (8). Two dioxygenases concerned within the biosynthesis of carnitine additionally require vitamin C as a cofactor for maximal exercise (8). Carnitine is important for the transport of activated long-chain fatty acids into the mitochondria; because of this, vitamin C deficiency leads to fatigue and lethargy, early signs of scurvy. As well as, vitamin C is used as a cofactor for catecholamine biosynthesis, particularly the conversion of dopamine to norepinephrine catalyzed by dopamine β-monooxygenase (8). Melancholy, hypochondria, and temper modifications ceaselessly happen throughout scurvy and might be associated to poor dopamine hydroxylation. The actions of a number of different enzymes are recognized to be depending on vitamin C, though their connection to scurvy has not but been clearly established. These enzymes embody the mono- and dioxygenases concerned in peptide amidation and tyrosine metabolism (8, 9). Vitamin C has additionally been implicated within the metabolism of ldl cholesterol to bile acids by way of the enzyme ldl cholesterol 7α-monooxygenase and in steroid metabolism within the adrenals (8, 9). Hydroxylation of fragrant medication and carcinogens by hepatic cytochrome P450 can also be enhanced by lowering brokers akin to vitamin C (9). The function of vitamin C within the above metabolic pathways is to cut back the lively heart metallic ion of the varied mono- and dioxygenases (8, 9). Not like different water-soluble nutritional vitamins, vitamin C acts as a cosubstrate in these reactions, not as a coenzyme. The power to take care of metallic ions in a decreased state is expounded to the redox potential of vitamin C (11). Discount of iron by vitamin C has additionally been implicated in enhanced gastrointestinal absorption of dietary nonheme iron (8, 12). Different proposed actions of vitamin C embody upkeep of enzyme thiols in a decreased state and sparing of glutathione, an essential intracellular antioxidant and enzyme cofactor (13), and tetrahydrofolate, a cofactor required for catecholamine biosynthesis (9). Many biochemical, medical, and epidemiologic research have indicated that vitamin C could also be of profit in continual illnesses akin to heart problems, most cancers, and cataract (5, 14, 15). The quantity of vitamin C required to forestall scurvy could also be lower than the quantity crucial to take care of optimum well being and cut back the incidence of continual illness morbidity and mortality. The Meals and Vitamin Board of the US Nationwide Academy of Sciences has modified the factors for establishing RDAs from prevention of deficiency illness to prevention of continual illnesses. Due to this fact, on this evaluate we tackle whether or not the present RDA of 60 mg vitamin C/d is ample for optimally lowering the danger of continual illnesses akin to heart problems and most cancers, in addition to cataract. We additionally tackle whether or not the exercise of vitamin C in these circumstances is because of its antioxidant properties or to different proposed mechanisms.
VITAMIN C AS AN ANTIOXIDANT
Vitamin C is a vital water-soluble antioxidant in organic fluids (16, 17). An antioxidant has been outlined as “any substance that, when present at low concentrations compared to those of an oxidizable substrate (e.g., proteins, lipids, carbohydrates and nucleic acids), significantly delays or prevents oxidation of that substrate” (11). The definition proposed by the Panel on Dietary Antioxidants and Associated Compounds of the Meals and Vitamin Board is that “a dietary antioxidant is a substance in foods that significantly decreases the adverse effects of reactive oxygen species, reactive nitrogen species, or both on normal physiological function in humans” (18). Vitamin C readily scavenges reactive oxygen and nitrogen species, akin to superoxide and hydroperoxyl radicals, aqueous peroxyl radicals, singlet oxygen, ozone, peroxynitrite, nitrogen dioxide, nitroxide radicals, and hypochlorous acid (11), thereby successfully defending different substrates from oxidative injury. Though vitamin C additionally reacts quickly with hydroxyl radicals (price fixed >109 L • mol−1•s−1), it’s however unable to preferentially scavenge this radical over different substrates (19). The explanation for that is that hydroxyl radicals are extraordinarily reactive and can mix indiscriminately with any substrate of their rapid atmosphere at a diffusion-limited price. Vitamin C may also act as a coantioxidant by regenerating α-tocopherol (vitamin E) from the α-tocopheroxyl radical, produced by way of scavenging of lipid-soluble radicals (20, 21). This can be a probably essential operate as a result of in vitro experiments have proven that α-tocopherol can act as a prooxidant within the absence of coantioxidants akin to vitamin C (21, 22). Nevertheless, the in vivo relevance of the interplay between vitamin C and vitamin E is unclear. Vitamin C has additionally been proven to regenerate urate, glutathione, and β-carotene in vitro from their respective one-electron oxidation merchandise, ie, urate radicals, glutathiyl radicals, and β-carotene radical cations (11, 23).
Vitamin C has been acknowledged and accepted by the US Meals and Drug Administration (FDA) as considered one of 4 dietary antioxidants, the opposite 3 being vitamin E, the vitamin A precursor β-carotene, and selenium, a vital part of the antioxidant enzymes glutathione peroxidase and thioredoxin reductase. The Panel on Dietary Antioxidants and Associated Compounds of the Meals and Vitamin Board (26) has, in precept, concurred with this definition, and as well as will think about different carotenoids. New rules have been just lately revealed during which the FDA acknowledged that vitamin C serves as an efficient free radical scavenger to guard cells from injury by reactive oxygen molecules (27). Statements of the antioxidant properties of vitamin C are additionally showing on meals labels and in nutrient content material claims all through the USA.
Though there is no such thing as a convincing proof for a prooxidant impact of vitamin C in people, there’s substantial proof for vitamin C’s antioxidant exercise. Curiously, the antioxidant exercise of vitamin C shouldn’t be straight associated to its antiscorbutic impact. Due to this fact, if the antioxidant exercise of vitamin C is accepted as occurring in vivo and thought of to be related to human well being by the Panel on Dietary Antioxidants and Associated Compounds of the Meals and Vitamin Board, then scurvy shouldn’t be used as the only real criterion for dietary adequacy or to find out the required or optimum quantity of vitamin C. On this part of the evaluate, subsequently, we tackle the next questions: 1) Is oxidative injury to organic macromolecules related to human continual illnesses? 2) What’s the proof that vita-min C acts as an antioxidant in people? 3) Are there different mechanisms, in addition to these straight associated to the antiscorbutic and antioxidant actions of vitamin C, by which vitamin C may have an effect on continual illness incidence, mortality, or each? and 4) Does vitamin C decrease continual illness incidence, mortality, or each?
Is oxidative injury to organic macromolecules related to human continual illnesses?
Oxidative injury to biomolecules, akin to lipids, DNA, and proteins, has been implicated in lots of continual illnesses, particularly, heart problems, most cancers, and cataract, respectively (30).
LDL oxidation and heart problems
Oxidative processes have been strongly implicated in atherosclerosis, myocardial infarction, and stroke (31). The oxidative modification speculation of atherosclerosis is at the moment probably the most extensively accepted mannequin of atherogenesis. LDL, the most important service of ldl cholesterol and lipids within the blood (32), infiltrates the intima of lesion-prone arterial websites, the place it’s oxidized over time by oxidants generated by native vascular cells or enzymes (33) to a kind that displays atherogenic properties. Minimally oxidized LDL and cytokines can activate endothelial cells to specific floor adhesion molecules, primarily vascular cell adhesion molecule 1 and intercellular adhesion molecule 1 in addition to monocyte chemotactic protein 1, which trigger circulating monocytes to stick to the endothelium and migrate into the artery wall (31). The monocytes subsequently differentiate into macrophages in response to macrophage colony stimulating issue, the expression of which by vascular cells is also enhanced by modified LDL. The oxidized LDL additional inhibits the egress of macrophages from the artery wall, the place the cells acknowledge and readily take up the oxidized LDL by a course of mediated by scavenger receptors (31). Not like the traditional apolipoprotein B/E LDL receptor recognizing native LDL, the scavenger receptors on macrophages that acknowledge modified LDL are usually not tightly regulated; because of this, the macrophages are transformed into foam cells, a element of fatty streaks and the hallmark of atherosclerosis.
It’s nonetheless unsure which components are answerable for the oxidation of LDL in vivo. LDL may be oxidized right into a probably atherogenic kind in vitro by metal-ion-dependent oxidation of its lipid element with subsequent modification of apolipoprotein B-100 by reactive aldehyde merchandise of lipid peroxidation, notably malondialdehyde and 4-hydroxynonenal (34). Whether or not catalytic metallic ions can be found within the early lesion in vivo stays a matter of debate (28). A number of metal-ion-independent mechanisms which might be primarily enzymatic in nature have been proposed; these embody mechanisms involving 15-lipoxygenase and myeloperoxidase (33, 35). The issues of evaluating LDL oxidation in vitro with LDL oxidation in vivo have been mentioned in 2 latest evaluations (36, 37).
A number of traces of direct proof level to the formation and existence of oxidized LDL in vivo. Antibodies to aldehyde-modified LDL acknowledge epitopes in human plaques (38) and LDL extracted from these lesions reacts with antibodies to oxidized LDL and has traits an identical to these of LDL oxidized in vitro. Aldehyde-modified LDL has additionally been detected in plasma, as have autoantibodies to oxidized LDL (31). Antibodies to hypochlorous acid–modified protein have detected epitopes in lesions, suggesting another or further mechanism of LDL oxidation (39). F2-isoprostanes, steady markers of lipid oxidation, have been detected in lesions (40, 41). Different oxidized lipids additionally improve with age and severity of atherosclerosis (42). Oblique proof for in vivo oxidation has come from antioxidant supplementation research in animals that present decreased lesion formation and decreased LDL oxidation (43, 44). As well as, quite a few epidemiologic research have indicated that dietary antioxidants cut back the incidence of heart problems in people, as mentioned beneath.
DNA oxidation and mutagenesis and carcinogenesis
Carcinogenesis is a multistage course of. Free radicals and oxidative processes have been implicated in each the initiation and the promotion of carcinogenesis (5, 45). The oxidative speculation of carcinogenesis asserts that many carcinogens can generate free radicals that injury cells, predisposing these cells to malignant modifications (46). Antioxidants, by neutralizing free radicals and oxidants, can stop cell injury and subsequent improvement of most cancers. DNA accommodates reactive teams in its bases which might be extremely prone to free radical assault (47) and oxidative DNA injury can result in deleterious mutations (2). It has been proposed that oxidative injury to DNA happens repeatedly in vivo at a price of ≈104 oxidative hits per cell per day (2). Most oxidative lesions are effectively repaired by particular DNA glycosylases, however restore shouldn’t be 100% environment friendly and the variety of lesions accumulates with age. When the cells divide, the lesions turn out to be fixated and mutations and most cancers might outcome (2). The existence of quite a few DNA glycosylases particular for oxidized bases (47) signifies that the latter are deleterious to human survival and subsequently should be repaired.
There are >20 completely different oxidative DNA lesions, of which 8-oxoguanine is among the most considerable and finest studied (2). 8-Oxoguanine is shaped by hydroxyl radical, peroxynitrite, or singlet oxygen assault of guanine within the C-8 place and is extremely mutagenic (2). Usually guanine kinds a base pair with cytosine, however 8-oxoguanine tends to pair with adenine. After replication, a transverse mutation happens whereby G-C base pairs are changed by A-T base pairs. This may increasingly contribute to the formation of most cancers if the lesion happens at a vital place in an essential gene, akin to a tumor suppressor or progress issue gene (2). People who smoke, who’ve the next threat of lung most cancers than nonsmokers, have elevated concentrations of 8-oxo-2′-deoxyguanosine (8-oxodG) (48–50). 8-Oxoguanine has additionally been detected in girls with breast most cancers (51, 52). Though direct proof for the hyperlink between DNA oxidation and most cancers remains to be missing (53), many epidemiologic research have recommended that dietary consumption of antioxidant nutritional vitamins, primarily from fruit and greens, protects in opposition to several types of most cancers.
Protein oxidation and cataract
Cataract is a dysfunction of the lens ensuing from opacification, which impedes the transmission of sunshine (54). About 98% of the strong mass of the lens is protein, predominantly crystallins. These proteins are lengthy lived and bear minimal turnover; because of this, cataract formation is primarily age associated. Oxidation of the lens proteins on account of continual publicity to ultraviolet gentle and oxygen has been implicated on this course of (54, 55). Smoking, which is understood to provide oxidative stress, can also be related to enhanced cataract threat (56, 57). Proof of lens protein oxidation contains lack of sulfhydryl and tryptophan residues with age in addition to formation of disulfides and different covalent cross-linkages. Deamination and acidification additionally happen, in addition to formation of superior glycation finish merchandise, notably in individuals with diabetes (54, 58). The oxidized proteins accumulate, combination, and ultimately precipitate, producing the sequelae of cataract.
The lens accommodates a number of antioxidant defenses, akin to excessive concentrations of vitamin C and glutathione, and antioxidant enzymes akin to superoxide dismutase, catalase, and the glutathione peroxidase-reductase system (54, 55). Secondary defenses embody proteolytic enzymes that selectively degrade broken proteins. With ageing, nonetheless, antioxidant concentrations within the lens, together with concentrations of vitamin C, could also be decreased (54) and the antioxidant enzymes are liable to inactivation, leading to elevated protein oxidation in older lenses. Proteolytic exercise can also be decreased in older lenses, leading to accumulation of broken proteins (54). Due to this fact, supplementation with antioxidants akin to vitamin C might cut back the danger of cataract.
What’s the proof that vitamin C acts as an antioxidant in people?
Essentially the most conclusive proof that vitamin C acts as an antioxidant in people has come from supplementation research utilizing particular biomarkers of oxidative injury to lipids, DNA, and proteins. As a result of these particular oxidative biomarkers have solely just lately been developed and proceed to be evaluated, solely comparatively few research have investigated the consequences on these biomarkers of supplementation with antioxidant micronutrients, together with vitamin C.
Lipid oxidation
Essentially the most generally used assay for lipid oxidation, though maybe the least particular, measures the aldehyde peroxidation product malondialdehyde and different aldehydes by response with thiobarbituric acid (the so-called thiobarbituric acid–reactive substances, or TBARS, assay) (59, 60). Different merchandise of lipid peroxidation, akin to conjugated dienes and lipid hydroperoxides, are sometimes measured (61, 62). TBARS and conjugated dienes are additionally generally used to measure the oxidizability, or susceptibility to oxidation, of LDL (63). The oxidizability of LDL is set by measuring the lag time and propagation price of lipid peroxidation in LDL uncovered in vitro to copper ions or different oxidants and depends on the antioxidant content material and lipid composition of LDL (32, 63). Particular biomarkers of lipid peroxidation are the F2-isoprostanes [in particular 8-epi-prostaglandin F2α (8-epi-PGF2α)], that are shaped from nonenzymatic, radical-mediated oxidation of arachidonyl-containing lipids (59). Elevated concentrations of F2-isoprostanes have been detected in individuals with diabetes, in people who smoke, in individuals with hypercholesterolemia (64, 65), and in LDL uncovered in vitro to varied varieties of oxidative stress (66).
Quite a few research in people have investigated the consequences on the oxidizability of LDL of vitamin C supplementation together with vitamin E or β-carotene or each (67). Research have been carried out in people who smoke (68–70), nonsmokers (71, 72), and individuals with hypercholesterolemia or heart problems (73, 74). In all circumstances, a major discount in LDL oxidizability was noticed. It’s, nonetheless, troublesome to find out the relative contribution of vitamin C in these research due to the presence of the cosupplements, of which vitamin E seems to be the most important contributor to safety of LDL. It’s because vita-min E is probably the most considerable lipid-soluble antioxidant related to LDL (32).
A number of research of LDL oxidizability have additionally been carried out with vitamin C as the one complement (Desk 1). Though 2 research discovered no results (80, 81), 2 different research (76, 84) discovered a major discount within the oxidizability of LDL obtained from individuals supplemented with vitamin C. It’s troublesome to rationalize these findings, nonetheless, as a result of vitamin C, being water soluble, is faraway from the LDL throughout isolation from plasma. One doable rationalization is the postulated function of vitamin C in both sparing or recycling vitamin E (15, 84). As talked about above, this exercise is instantly noticed in vitro, however proof for sparing or recycling of vitamin E by vitamin C in vivo is inconclusive (15, 83, 87).
A number of research investigated the consequences of vitamin C supplementation on lipid oxidation markers in people who smoke (Desk 1). People who smoke have greater concentrations of lipid oxidation merchandise (65, 77) and decrease plasma concentrations of vitamin C than do nonsmokers (88, 89). It has subsequently been proposed that supplementation with antioxidant nutritional vitamins might inhibit smoking-related lipid oxidation. Three of 6 research in people who smoke reported a discount within the markers of lipid oxidation with vitamin C supplementation (75–77). The examine by Reilly et al (77) is especially noteworthy as a result of it measured lipid oxidation with the particular biomarker 8-epi-PGF2α. Reilly et al discovered that supplementation of heavy people who smoke with 2000 mg vitamin C/d for less than 5 d considerably decreased the urinary excretion of 8-epi-PGF2α.
An equal variety of research with vitamin C supplementation have been carried out in wholesome people or nonsmokers. A discount in lipid oxidation was noticed with some markers (83–85), whereas a nonsignificant lower (82) or no change was noticed with others (79, 83). Lastly, a latest examine in coronary artery illness sufferers supplemented with 500 mg vita-min C/d for 1 mo discovered no change in plasma 8-epi-PGF2α concentrations (86). With one exception (80), not one of the research listed in Desk 1 discovered a rise in lipid oxidation markers with vitamin C supplementation.
An antioxidant function of vitamin C in vivo can also be recommended by animal research in guinea pigs or genetically scorbutic (Osteogenic Dysfunction Shionogi, or ODS) rats, which, like people, are unable to synthesize vitamin C (90–93). Guinea pigs fed a weight-reduction plan marginally poor in vitamin C have considerably greater concentrations of endogenous malondialdehyde than do animals fed diets supplying 20–40 occasions the quantity of vitamin C required to keep away from scurvy (90). Vitamin C–poor guinea pigs exhale considerably greater quantities of breath ethane when uncovered to an oxidative stress than do vitamin C–ample animals (92). Be aware that human people who smoke supplemented with 600–1000 mg vitamin C/d together with vitamin E and β-carotene for 3–8 wk exhaled decrease quantities of breath pentane and ethane (68, 94). Genetically scorbutic ODS rats fed a vitamin C–free weight-reduction plan have greater concentrations of plasma and liver TBARS and lipid peroxides than do rats fed vitamin C–supplemented diets (91). In a newer examine (93), nonetheless, supplementation of ODS rats with vitamin C, in distinction with vitamin E, didn’t shield in opposition to the endogenous formation of TBARS.
Lastly, in vitro experiments with human plasma have proven that the formation of F2-isoprostanes and lipid hydroperoxides by aqueous peroxyl radicals, stimulated neutrophils, gas-phase cigarette smoke, or redox-active iron doesn’t happen till most or all the endogenous vitamin C has been depleted (29, 66, 95). Vitamin C has additionally been proven to guard remoted LDL in vitro from oxidation by numerous radicals and oxidants (31). It isn’t shocking that vitamin C successfully prevents oxidation of remoted LDL and lipoproteins in plasma as a result of, as defined above, vitamin C successfully scavenges most aqueous reactive oxygen and nitrogen species earlier than they will work together with and oxidize different substrates, together with lipids.
DNA oxidation
Of the >20 recognized oxidative DNA lesions, probably the most generally measured markers of in vivo DNA oxidation are 8-oxoguanine and its respective nucleoside 8-oxodG (96). These modified bases have been detected in cells akin to lymphocytes and are additionally excreted in urine (96). The tissue pool represents a gradual state between oxidation and mobile restore mechanisms, whereas the excreted merchandise symbolize the whole web injury and restore. The two strategies mostly used to detect 8-oxoguanine and 8-oxodG are gasoline chromatography–mass spectroscopy (GC-MS) and HPLC with electrochemical detection (HPLC-EC) (97). These strategies, nonetheless, can generate artificially excessive quantities of oxidation merchandise due to prolonged extraction, hydrolysis, and derivatization procedures, particularly with GC-MS (97). Oblique strategies for measuring DNA injury through the use of particular restore endonucleases together with single-cell gel electrophoresis (the comet assay) have proven baseline concentrations of DNA oxidation merchandise as much as 1000-fold decrease than these measured by GC-MS (96, 97). Antibodies in opposition to 8-oxopurines are one other probably helpful methodology for detecting DNA oxidation (98).
DNA oxidation, as decided by 8-oxodG in cells, is elevated in circumstances of oxidative stress akin to smoking and is correlated with decreased plasma concentrations of the antioxidant nutritional vitamins C and E (99). It’s subsequently conceivable that supplementation with vita-min C might ameliorate oxidative injury to DNA. Six of seven research proven in Desk 2 (82, 100–102, 104, 105) symbolize cell measurements, ie, regular state injury. 5 of those research confirmed a major discount in ≥1 marker of oxidative DNA injury in vitamin C–supplemented topics (100–102, 104, 105).
In one of many newer research, a major lower in lymphocyte 8-oxoguanine concentrations was noticed after vitamin C supplementation with 500 mg/d; in distinction, a rise within the much less established marker 8-oxoadenine was noticed (104). These authors thus recommended that vitamin C supplementation might have a prooxidant impact. Nevertheless, lymphocyte 8-oxoguanine concentrations on this examine have been 30 lesions/105 guanine bases, which is ≥30-fold greater than at the moment accepted values (97), and 8-oxoadenine concentrations seemed to be extraordinarily excessive as effectively. Due to this fact, the info appear to largely mirror ex vivo oxidation of the DNA earlier than or throughout GC-MS evaluation. As well as, different main issues with this examine have been recognized (106, 107).
One other latest examine utilizing GC-MS confirmed a major discount in each 8-oxoguanine and 8-oxoadenine in wholesome individuals supplemented for 12 wk with both 60 or 260 mg vitamin C/d together with 14 mg Fe/d, though different modified bases have been elevated, together with 5-hydroxycytosine and thymine glycol (105). As soon as once more, nonetheless, baseline concentrations of 8-oxoguanine and 8-oxoadenine have been excessive and comparable with these measured by Podmore et al (104). Moreover, there have been no management teams included given iron alone or placebo, and vitamin C supplementation within the examine didn’t all the time lead to vital modifications in plasma vitamin C concentrations (105). Due to this fact, the interpretation of the info from these 2 research (104, 105) is unsure.
The examine by Anderson et al (82) indicated no vital change in DNA injury as assessed by the comet assay after 2 wk of supplementation with 60 or 6000 mg vitamin C/d, both earlier than or after an ex vivo problem by hydrogen peroxide (Desk 2). In distinction, 2 different research during which the comet assay was used confirmed decreased susceptibility of lymphocytes to ex vivo oxidation of DNA after supplementation with vitamin C (101, 102). Comparable findings have been reported in one other examine during which vitamin E and β-carotene got as cosupplements (108). Utilizing HPLC-EC, Fraga et al (100) additionally confirmed a major lower in sperm 8-oxodG after replenishment with vitamin C. A latest animal examine of vitamin C (and vitamin E) supplementation of guinea pigs, nonetheless, confirmed no impact on 8-oxodG concentrations within the liver as decided by HPLC-EC (109), regardless of a 60-fold distinction in liver vitamin C concentrations.
The remaining examine in Desk 2 (103), which represents urinary measurements (ie, the online price of injury and restore), confirmed no vital impact of vitamin C supplementation on 8-oxodG concentrations as decided by HPLC-EC. Two different research during which vitamin C was given within the presence of cosupplements additionally reported related findings (49, 110). Nevertheless, a number of investigators have questioned the appropriateness of utilizing urinary concentrations of 8-oxoguanosine or 8-oxodG as markers of nucleic acid injury as a result of these merchandise may be derived from nonspecific degradation of RNA or DNA, respectively, from lifeless cells (47), and the most important 8-oxoguanine DNA glycosylase restore product is 8-oxoguanine, not 8-oxodG.
Protein oxidation
Protein oxidation is mostly measured by figuring out carbonyl teams, oxidized amino acids, and superior glycation finish merchandise (111, 112). Protein carbonyls may be shaped by direct oxidative cleavage of the peptide chain or by oxidation of particular amino acid residues, akin to lysine, arginine, proline, and threonine (111). Carbonyls can be shaped not directly by modification of lysine, histidine, and cysteine residues by α,β-unsaturated aldehydes akin to 4-hydroxynonenal in a course of known as Michael addition, or by way of Schiff base formation between lysine residues and dialdehydes akin to malondialdehyde (111, 113). A wide range of oxidized amino acids have been recognized in proteins uncovered to oxidizing circumstances, together with methionine sulfoxide, o- and m-tyrosine, o,o′-dityrosine, 3-chlorotyrosine, and 3-nitrotyrosine (112). Superior glycation finish merchandise are generated by response of lowering sugars with lysine residues beneath oxidizing circumstances within the Maillard response to kind carboxymethyl- and carboxyethyl-lysine (111). Superior glycation finish merchandise have been implicated in illnesses akin to diabetes and in ageing (58, 111). The oxidative merchandise of vitamin C might bear related reactions, though whether or not these happen in vivo is unsure (114).
Few research have been carried out to research the consequences of vitamin C supplementation on the in vivo merchandise of protein oxidation. Two animal research (90, 115) indicated decreased protein carbonyl formation in guinea pigs supplemented with vitamin C, both with (115) or with out (90) an endotoxin problem. Vitamin C supplementation (2000 mg/d for 4–12 mo) of sufferers with Helicobacter pylori gastritis led to a major discount in nitrotyrosine concentrations (116). Nevertheless, one other latest human examine discovered no change in urine o,o′-dityrosine or o-tyrosine concentrations in coronary artery illness sufferers supplemented with 500 mg vitamin C/d for 1 mo (86). Additional research investigating the impact of vitamin C supplementation on the markers of protein oxidation are clearly required.
Taken collectively, the proof reviewed above means that vitamin C acts as an antioxidant in people. Biomarker research confirmed decreased lipid oxidation after vitamin C supplementation in each people who smoke and nonsmokers (Desk 1). A number of research additionally confirmed decreased regular state DNA oxidation after vitamin C supplementation, though different research confirmed both no change or combined outcomes (Desk 2). The latter seems to be primarily because of the technical difficulties of precisely measuring DNA oxidation merchandise by GC-MS and HPLC-EC and eliminating ex vivo artifacts. As well as, the consequences of vitamin C supplementation on oxidative markers additionally critically rely on baseline concentrations of the vitamin. In a examine by Levine et al (117) of the pharmacokinetics of vitamin C, it was discovered that in wholesome grownup males tissue saturation occurred at vitamin C intakes of ≈100 mg/d, as assessed by vitamin C concentrations in lymphocytes, monocytes, and neutrophils. Kallner et al (118) additionally reported that the physique pool of vitamin C was saturated by an consumption of ≈100 mg/d in wholesome, nonsmoking males. If tissues are already saturated earlier than vitamin C supplementation due to an consumption ≥100 mg/d, then supplementation with any vitamin C dose can’t additional cut back oxidative injury, a indisputable fact that seemingly explains a few of the discrepant outcomes of the biomarker research mentioned. Extra consideration to this vital situation and extra detailed biomarker research on vitamin C intakes close to tissue-saturating concentrations are warranted.
Are there different mechanisms by which vitamin C may have an effect on continual illness incidence, mortality, or each?
As mentioned above, vitamin C has many features within the physique, akin to appearing as a cosubstrate for a number of biosynthetic enzymes (8). Due to this fact, vitamin C might have an effect on continual illness incidence, mortality, or each by mechanisms that is probably not straight associated to its function as an antioxidant.
Heart problems
Hypercholesterolemia is a major threat issue for heart problems (43, 119). The relation between vitamin C supplementation, or plasma vitamin C concentrations, and complete serum ldl cholesterol has been investigated in a number of research (82, 119–123). In a single supplementation examine, consumption of 1000 mg vitamin C/d for 4 wk resulted in a discount in complete serum ldl cholesterol (120), whereas in one other examine, supplementation with 60 or 6000 mg/d for two wk had no impact (82). Two observational research discovered an inverse correlation between vitamin C standing and complete serum ldl cholesterol concentrations (122, 123). The mechanism for the doable modulating impact of vitamin C on serum ldl cholesterol concentrations shouldn’t be totally sure. One putative pathway is thru vitamin C’s function as a cofactor for ldl cholesterol 7α-monooxygenase, an enzyme concerned within the in vivo hydroxylation of ldl cholesterol to kind bile acids (8). Vitamin C may additionally modulate the exercise of hydroxymethylglutaryl-CoA reductase, the rate-limiting enzyme within the biosynthesis of ldl cholesterol (43).
The plasma lipoprotein profile can also be an essential consideration for heart problems, with decreased concentrations of HDL and elevated concentrations of LDL being vital threat components (43, 119). Quite a few observational research have discovered a major affiliation between elevated plasma vitamin C concentrations and elevated concentrations of HDL ldl cholesterol and decreased concentrations of LDL ldl cholesterol (119, 121–125). Findings indicated that with each 30-μmol/L improve in plasma vitamin C, HDL was elevated by 4–10% and LDL was decreased by 4% (125). Comparable modulatory results have been reported after supplementation with 1000 mg vitamin C/d for 4 wk (120). Ness et al (121) additionally discovered an inverse correlation between vitamin C standing and triacylglycerol concentrations. Vitamin C might modulate the exercise of lipoprotein lipase (43), though the mechanism is unknown.
The thrombotic threat of heart problems is related to elevated concentrations of the coagulation issue fibrinogen (126). Two research discovered an inverse affiliation between serum vitamin C concentrations and coagulation components in addition to a optimistic affiliation between low serum vitamin C and elevated fibrinogen and coagulation activation markers (126, 127). Two early research indicated that supplementation of coronary heart illness sufferers with 2000–3000 mg vitamin C/d for 1–6 wk elevated fibrino-lytic exercise and decreased platelet adhesiveness (128, 129). In a newer examine (130), wholesome volunteers have been supplemented with 250 mg vitamin C/d for 8 wk and a nonsignificant lower in platelet aggregation and an elevated sensitivity to PGE1 have been reported. In vitro research confirmed that physiologic concentrations of vitamin C might improve PGE1 and PGI1 (prostacyclin) manufacturing, leading to a discount in platelet aggregation and thrombus formation (31), though whether or not this mechanism is related in vivo has but to be established. Low concentrations of vitamin C are additionally related to elevated concentrations of plasminogen activator inhibitor 1, a protein that inhibits fibrinolysis (131).
Adhesion of leukocytes to the endothelium is a vital initiating step in atherogenesis (31). People who smoke have decrease plasma vitamin C concentrations than do nonsmokers (88), and monocytes remoted from people who smoke exhibit elevated adhesion to endothelial cells (132, 133). Supplementation of people who smoke with 2000 mg vitamin C/d for 10 d elevated plasma vitamin C concentrations from 48 to 83 μmol/L and considerably decreased monocyte adhesion to endothelial cells (132). In one other examine, nonetheless, supplementation of people who smoke with 2000 mg vitamin C 2 h earlier than serum was collected had no vital impact on ex vivo monocyte or endothelial cell adhesion, regardless of a rise in vitamin C concentrations from 34 to 115 μmol/L (133). Curiously, supplementation with 7000 mg l-arginine, the physiologic substrate for nitric oxide (NO) synthase, considerably decreased monocyte and endothelial cell adhesion, suggesting an essential function for NO. A number of in vivo animal research additionally recommended an essential function for vitamin C in modulating leukocyte and endothelial cell interactions in hamsters uncovered to cigarette smoke (134, 135) or injected with oxidized LDL (136).
Impaired vascular operate and leisure are extremely related to the medical expression of atherosclerosis, ie, angina pectoris, myocardial infarction, and stroke. Hypertension is a acknowledged threat issue for heart problems (43) and low concentrations of plasma vitamin C have been related to hypertension (122, 125, 137–139). A number of research reported optimistic results of excessive doses of vitamin C, administered both orally or by intraarterial infusion, on vasodilation (Desk 3). 4 research (86, 141, 146, 150) investigated vasodilation in sufferers with heart problems and located will increase of 45–220% in vasodilation after administration of vitamin C (1000–2000 mg oral or 25 mg/min infusion). Certainly one of these research discovered a 128% improve in brachial artery dilation in coronary artery illness sufferers, however a nonsignificant 27% improve in continual coronary heart failure sufferers (150). Kugiyama et al (148) noticed a 100% reversal of epicardial artery vasoconstriction in
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