Vitamin C pharmacokinetics in healthy volunteers: evidence for a recommended dietary allowance

Vitamin C and E supplements improve the impaired antioxidant status and decrease plasma lipid peroxides in hemodialysis patients small star, filled

This study investigated the supplementation with vitamin C or/and E on the antioxidant system in hemodialysis patients. Thirty-eight hemodialysis patients (27 males and 11 females) with the average of 60 years old were divided into four groups: placebo (400 mg starch/time), vitamin C (400 mg/time)-, vitamin E (400 mg d,l- alpha-tocopheryl acetate/time)-, and vitamin C (400 mg/time) + E (400 mg d,l- alpha-tocopheryl acetate/time)-supplemented groups for 6-week supplementation. The patients orally received three capsules of placebo or antioxidant(s) three times a week after finishing hemodialysis. Thirty-six healthy subjects (22 males and 14 females) with the average of 58 years old were recruited as the control group. Hemodialysis patients significantly decreased plasma vitamin C by 32%, erythrocyte glutathione by 26%, and plasma total antioxidant status by 9%, but increased plasma lipid peroxide levels by 102% compared with the control group at the baseline. The levels of plasma vitamin C and total antioxidant status significantly decreased by 24% and 18%, respectively, from the post-dialysate compared with those from the pre-dialysate. At week 6, vitamin C + E-supplemented group significantly increased plasma vitamin C and E, erythrocyte glutathione, and plasma antioxidant status, and inhibited plasma lipid peroxides compared with placebo group. Additionally, vitamin C + E-supplemented group had higher plasma vitamin C, vitamin E, and total antioxidant status, and lower plasma lipid peroxides than placebo group even at least 2 weeks after the termination of the supplements. Therefore, antioxidant vitamin supplements could improve antioxidant status and decrease lipid peroxides of hemodialysis patients.

Effect of blackcurrant-, cranberry- and plum juice consumption on risk factors associated with kidney stone formation

OBJECTIVE

To evaluate the influence of plum-, cranberry- and blackcurrant juice on urinary stone risk factors.

DESIGN

Investigations were carried out in 12 healthy male subjects aged 18-38 y. All subjects received a standardized diet formulated according to the dietary recommendations of the German Society of Nutrition. The subjects provided 24 h urine collections in a control, three loading phases. In each loading phase a neutral mineral water was substituted for 330 ml of the particular juice.

RESULTS

Cranberry juice decreased the urinary pH, whereas the excretion of oxalic acid and the relative supersaturation for uric acid were increased. Blackcurrant juice increased the urinary pH and the excretion of citric acid. The excretion of oxalic acid was increased too. All changes were statistically significant. The plum juice had no significant effect on the urinary composition.

CONCLUSION

It is concluded that blackcurrant juice could support the treatment and metaphylaxis of uric acid stone disease because of its alkalizing effect. Since cranberry juice acidifies urine it could be useful in the treatment of brushite and struvite stones as well as urinary tract infection.

SPONSORSHIP

Funded by our own Division respectively the University.

Are vitamin and mineral deficiencies a major cancer risk?

Diet is estimated to contribute to about one-third of preventable cancers -- about the same amount as smoking. Inadequate intake of essential vitamins and minerals might explain the epidemiological findings that people who eat only small amounts of fruits and vegetables have an increased risk of developing cancer. Recent experimental evidence indicates that vitamin and mineral deficiencies can lead to DNA damage. Optimizing vitamin and mineral intake by encouraging dietary change, multivitamin and mineral supplements, and fortifying foods might therefore prevent cancer and other chronic diseases.

Vitamin C matters: increased oxidative stress in cultured human aortic endothelial cells without supplemental ascorbic acid

Because standard culture media for human aortic endothelial cells (HAEC) do not contain vitamin C, we hypothesized that HAEC may be under significant oxidative insult compared with the situation in vivo. To assess parameters of oxidative stress, intracellular vitamin C, glutathione (GSH), GSH/GSSG, and NAD(P)H/NAD(P)+ ratios, as well as oxidant appearance and oxidative damage, were measured in HAEC with or without vitamin C addition. The effect of vitamin C on eNOS activity was also determined. Results showed that HAEC without vitamin C treatment were essentially scorbutic. On addition of 100 mM vitamin C to the culture media, intracellular vitamin C levels increased and peaked at 6 h. A concomitant increase in the total GSH and the GSH/GSSG ratio was also observed; the NAD(P)H/NAD(P)+ ratio increased more slowly over the 24-h time course. Significantly lower (P <0.05) oxidant appearance and steady-state oxidative damage were also observed following vitamin C repletion. Vitamin C treatment increased eNOS activity by 600%. Thus, HAEC are scorbutic under normal culture conditions and exhibit higher oxidative stress than vitamin C repleted cells. Vitamin C supplementation should be considered when using cultured cells, especially when experimental endpoints are related to cellular redox status and eNOS activity.

Vitamin C prevents DNA mutation induced by oxidative stress

The precise role of vitamin C in the prevention of DNA mutations is controversial. Although ascorbic acid has strong antioxidant properties, it also has pro-oxidant effects in the presence of free transition metals. Vitamin C was recently reported to induce the decomposition of lipid hydroperoxides independent of metal interactions, suggesting that it may cause DNA damage. To directly address the role of vitamin C in maintaining genomic integrity we developed a genetic system for quantifying guanine base mutations induced in human cells under oxidative stress. The assay utilized a plasmid construct encoding the cDNA for chloramphenicol acetyl transferase modified to contain an amber stop codon, which was restored to wild type by G to T transversion induced by oxidative stress. The mutation frequency was determined from the number of plasmids containing the wild type chloramphenicol acetyl transferase gene rescued from oxidatively stressed cells. Cells were loaded with vitamin C by exposing them to dehydroascorbic acid, thereby avoiding transition metal-related pro-oxidant effects of ascorbic acid. We found that vitamin C loading resulted in substantially decreased mutations induced by H(2)O(2). Depletion of glutathione led to cytotoxicity and an increase in H(2)O(2)-induced mutation frequency; however, mutation frequency was prominently decreased in depleted cells preloaded with vitamin C. The mutation results correlated with a decrease in total 8-oxo-guanine measured in genomic DNA of cells loaded with vitamin C and oxidatively stressed. These findings directly support the concept that high intracellular concentrations of vitamin C can prevent oxidation-induced mutations in human cells.

Flavonoid inhibition of sodium-dependent vitamin C transporter 1 (SVCT1) and glucose transporter isoform 2 (GLUT2), intestinal transporters for vitamin C and Glucose

Vitamin C and flavonoids, polyphenols with uncertain function, are abundant in fruits and vegetables. We postulated that flavonoids have a novel regulatory action of delaying or inhibiting absorption of vitamin C and glucose, which are structurally similar. From six structural classes of flavonoids, at least 12 compounds were chosen for studies. We investigated the effects of selected flavonoids on the intestinal vitamin C transporter SVCT1(h) by transfecting and overexpressing SVCT1(h) in Chinese hamster ovary cells. Flavonoids reversibly inhibited vitamin C transport in transfected cells with IC(50) values of 10-50 microm, concentrations expected to have physiologic consequences. The most potent inhibitor class was flavonols, of which quercetin is most abundant in foods. Because Chinese hamster ovary cells have endogenous vitamin C transport, we expressed SVCT1(h) in Xenopus laevis oocytes to study the mechanism of transport inhibition. Quercetin was a reversible and non-competitive inhibitor of ascorbate transport; K(i) 17.8 microm. Quercetin was a potent non-competitive inhibitor of GLUT2 expressed in Xenopus oocytes; K(i) 22.8 microm. When diabetic rats were administered glucose with quercetin, hyperglycemia was significantly decreased compared with administration of glucose alone. Quercetin also significantly decreased ascorbate absorption in normal rats given ascorbate plus quercetin compared with rats given ascorbate alone. Quercetin was a specific transport inhibitor, because it did not inhibit intestinal sugar transporters GLUT5 and SGLT1 that were injected and expressed in Xenopus oocytes. Quercetin inhibited but was not transported by SVCT1(h). Considered together, these data show that flavonoids modulate vitamin C and glucose transport by their respective intestinal transporters and suggest a new function for flavonoids.

Ascorbic acid supplement during luteal phase in IVF

PURPOSE

To evaluate the impact of ascorbic acid of different doses as additional support during luteal phase in infertility treatment by means of a prospective, randomized, placebo-controlled, group comparative, double-blind study.

METHODS

Voluntary daily oral intake of either ascorbic acid (1, 5, or 10 g/day) or Placebo for 14 days after follicle aspiration for IVF-ET procedure. Data was obtained on 620 cases of women, age <40 years, undergoing first IVF-embryo transfer cycles in two private outpatient infertility clinics. All women were stimulated by the same protocol. The mean age was 31.73 (+/- 4.4 SD) years.

RESULTS

No differences in clinical pregnancy rate and implantation rate were noted in statistical logistic regression analysis between the four intake groups.

CONCLUSIONS

There was no clinical evidence of any beneficial effect, as defined by main outcome measures, of ascorbic acid on IVF-ET. Our data suggest there is no obvious value of high dosed intake of vitamin C during luteal phase in infertility treatment.

Stability of ascorbic acid in commercially available orange juices

OBJECTIVE

To compare the vitamin C content of reconstituted frozen orange juice concentrates at preparation to that of ready-to-drink orange juices purchased 4 to 5 weeks from expiration.

DESIGN

Juices were unsealed and analyzed for reduced and oxidized vitamin C content at the time of purchase and reanalyzed 3 times weekly for 4 to 5 weeks. Same-lot samples of the ready-to-drink juices remained sealed after purchase and were opened for analyses at 3, 2, 1, or 0 weeks before expiration. SAMPLES/SETTING: Orange juices were reconstituted frozen concentrates, ready-to-drink juice packaged in resealable, screw-top containers, or ready-to-drink juice packaged in nonresealable containers. Juices were obtained from local retailers and stored at 4 degrees C in their original containers when appropriate.

STATISTICAL ANALYSES PERFORMED

The reduced vitamin C content of juices analyzed repeatedly, 3 times weekly for 4 weeks, were compared using repeated measures analysis of variance. Linear regression lines were computed for reduced vitamin C in each juice over time, and differences between slopes were analyzed by oneway analysis of variance.

RESULTS

The orange juices from frozen concentrates contained 86 mg reduced vitamin C per fluid cup at initial preparation and 39 to 46 mg/c after 4 weeks of storage. Ready-to-drink juices averaged significantly lower reduced vitamin C: 27 to 65 mg/c at opening and 0 to 25 mg/c at expiration 4 weeks later. Ready-to-drink orange juices had twofold to threefold higher concentrations of oxidized vitamin C vs the orange juices reconstituted from frozen, and the decomposition rate of reduced vitamin C was similar for all juices, about 2% per day once opened.

APPLICATIONS

Ready-to-drink orange juices should be purchased 3 to 4 weeks before expiration date and consumed within 1 week of opening.

High-dose vitamin therapy stimulates variant enzymes with decreased coenzyme binding affinity (increased K(m)): relevance to genetic disease and polymorphisms

As many as one-third of mutations in a gene result in the corresponding enzyme having an increased Michaelis constant, or K(m), (decreased binding affinity) for a coenzyme, resulting in a lower rate of reaction. About 50 human genetic dis-eases due to defective enzymes can be remedied or ameliorated by the administration of high doses of the vitamin component of the corresponding coenzyme, which at least partially restores enzymatic activity. Several single-nucleotide polymorphisms, in which the variant amino acid reduces coenzyme binding and thus enzymatic activity, are likely to be remediable by raising cellular concentrations of the cofactor through high-dose vitamin therapy. Some examples include the alanine-to-valine substitution at codon 222 (Ala222-->Val) [DNA: C-to-T substitution at nucleo-tide 677 (677C-->T)] in methylenetetrahydrofolate reductase (NADPH) and the cofactor FAD (in relation to cardiovascular disease, migraines, and rages), the Pro187-->Ser (DNA: 609C-->T) mutation in NAD(P):quinone oxidoreductase 1 [NAD(P)H dehy-drogenase (quinone)] and FAD (in relation to cancer), the Ala44-->Gly (DNA: 131C-->G) mutation in glucose-6-phosphate 1-dehydrogenase and NADP (in relation to favism and hemolytic anemia), and the Glu487-->Lys mutation (present in one-half of Asians) in aldehyde dehydrogenase (NAD + ) and NAD (in relation to alcohol intolerance, Alzheimer disease, and cancer).

Vulnerable atherosclerotic plaque morphology in apolipoprotein E-deficient mice unable to make ascorbic Acid

BACKGROUND

Oxidative stress is thought to play an important role in atherogenesis, suggesting that antioxidants could prevent coronary artery disease. However, the efficacy of vitamin C in reducing atherosclerosis is debatable in humans and has not been tested rigorously in animals.

METHODS AND RESULTS

Gulo(-/-)Apoe(-/-) mice were used to test a hypothesis that chronic vitamin C deficiency enhances the initiation and development of atherosclerosis. These mice are dependent on dietary vitamin C because of the lack of L-gulonolactone-gamma-oxidase and are prone to develop atherosclerosis because of lacking apolipoprotein E. Beginning at 6 weeks of age, the Gulo(-/-)Apoe(-/-) mice were fed regular chow or Western-type diets containing high fat and supplemented with either 0.033 g or 3.3 g/L of vitamin C in their drinking water. This regimen produced mice with chronically low vitamin C (average 1.5 microg/mL in plasma) or high vitamin C (average 10 to 30 microg/mL in plasma). Morphometric analysis showed that within each sex, age, and diet group, the sizes of the atherosclerotic plaques were not different between low vitamin C mice and high vitamin C mice. However, advanced plaques in the low vitamin C mice had significantly reduced amounts of Sirius red-staining collagen (36.4+/-2.2% versus 54.8+/-2.3%, P<0.0001), larger necrotic cores within the plaques, and reduced fibroproliferation and neovascularization in the aortic adventitia.

CONCLUSIONS

Chronic vitamin C deficiency does not influence the initiation or progression of atherosclerotic plaques but severely compromises collagen deposition and induces a type of plaque morphology that is potentially vulnerable to rupture.

The placebo effect and randomized trials: analysis of alternative medicine

Randomized controlled trials are generally regarded as the gold standard of study designs to determine causality. The inclusion of a placebo group in these trials, when appropriate, is critical to access the efficacy of a drug or supplement. The placebo response itself has received some attention in the medical literature over the past fifty years. The recent increasing utilization of dietary supplements and herbal medications by patients makes it imperative to reevaluate the placebo response in conventional and alternative medicine. This article will review some of the negative and positive results from randomized trials utilizing dietary supplements (androstenedione, beta-carotene, CoQ10, garlic, soy, vitamin C and E...) for a number of non-urologic and urologic conditions, including cancer.

Decreased expression of the vitamin C transporter SVCT1 by ascorbic acid in a human intestinal epithelial cell line

Vitamin C (ascorbic acid) is an essential nutrient that is involved in a number of cellular processes. However, unlike most mammals, man is unable to synthesize vitamin C and it must therefore be acquired from the diet. Absorption of vitamin C is achieved by two transporters, SVCTI and SVCT2, recently cloned from rat and human kidney. SVCT1 is thought to be the predominant transporter in the intestine. Vitamin C supplements are increasingly common, thus contributing to an increased dietary load, and therefore the aim of the present study was to investigate the effect of high doses of ascorbic acid on SVCT1 expression. Using the Caco-2 TC7 cell model of small intestinal enterocytes, we measured the effects of ascorbic acid (4.5 mg/ml culture medium) on L-[14C]ascorbic acid uptake and SVCT1 expression (determined by reverse transcription-polymerase chain reaction). Ascorbic acid uptake was decreased significantly in Caco-2 TC7 cells exposed to ascorbate for 24 h (-50%, P<0.0005). Expression of SVCT1 was also significantly reduced by exposure to elevated levels of ascorbate for 24h (-77%, P<0.005). Taken together these results suggest that high-dose supplements might not be the most efficient way of increasing the body pool of vitamin C.

Ascorbic acid status in postmenopausal women with hormone replacement therapy

OBJECTIVES

Hormone oral contraceptives affected ascorbic acid status adversely in young women. In vitro, estrogens and progesterone inhibited ascorbic acid accumulation in intestinal cells. This is a pilot study to examine the relation between hormone replacement therapy (HRT) and plasma ascorbic acid levels among a group of healthy non-smoking postmenopausal women.

METHODS

Healthy non-smoking postmenopausal women aged 48-72 years, 34 with HRT and 21 without HRT, were recruited in summer, 1997. Their fasting plasma ascorbic acid levels were measured and information on ascorbic acid intakes (diet and supplements) was collected through questionnaires.

RESULTS

Women taking HRT in this study did not have significantly lower plasma ascorbic acid levels compared with non-HRT users. When subjects were further divided into groups based on ascorbic acid supplementation, HRT users without supplement had a lower mean plasma ascorbic acid level (54+/-16 microM, n=10) compared with non-HRT users (66+/-14 microM, n=12) (P=0.08 for the effect of therapy). HRT users and non-users taking ascorbic acid supplement had similar plasma levels (66+/-10 microM, n=24; 66+/-12 microM, n=9, respectively).

CONCLUSION

HRT does not affect ascorbic acid status of healthy well-nourished non-smoking postmenopausal women that are using ascorbic acid supplement. Future larger case-control or supplement intervention study is needed.

(2)H- and (13)C-labelled tracers compared for kinetic studies of ascorbic acid metabolism in man: a factor analytical approach

A recent report that a (13)C stable isotope method can be used to measure the kinetics of ascorbic acid uptake and distribution in man has raised some interesting questions with regard to the physiological interpretation of the data obtained, in particular the sizes of the ascorbate distribution spaces. In order to prove that this result is not a function of the label used we have compared the behaviour of two different tracers to see if they are likely to give comparable results for the kinetic parameters. Volunteers received an oral dose of ascorbic acid, half of which was labelled with (2)H, and half of which was labelled with (13)C. Blood samples were taken over the course of the next 48 h, and ascorbic acid mass spectra measured by gas chromatography/mass spectrometry (GC/MS). Principal component analysis was used to investigate the number of factors required to explain the total variations observed in the ratios of the molecular isotopomer cluster over the time course of the experiment. Theoretical cracking patterns were then used as test vectors in target transformation and as the basis for subsequent combination to determine tracer/tracee ratios. Two factors were found sufficient to account for the observed cracking pattern variations within experimental error. These were identified as (1) the spectrum of unlabelled (endogenous) ascorbic acid, and (2) a linear combination of the spectra of the two labelled species used. The absence of a third factor in the decomposition indicates that there is no difference in the behaviour of the (13)C- and (2)H-labelled tracers. Target testing allowed the tracer/tracee ratios to be determined using calculated cracking patterns, and produced equivalent results to conventional methods. Our experience in this work indicates that factor analysis has a useful place in many kinetic studies of this kind, either with one or many labelled species.

Antioxidant vitamin supplements do not reduce reactive oxygen species activity in Helicobacter pylori gastritis in the short term

Reactive oxygen species have been implicated in Helicobacter pylori-mediated gastric carcinogenesis, whereas diets high in antioxidant vitamins C and E are protective. We have examined the effect of vitamin C and E supplements in combination with H. pylori eradication on reactive oxygen species activity in H. pylori gastritis. H. pylori-positive patients were randomized into four groups: triple therapy alone (Bismuth chelate, tetracycline, and metronidazole for 2 weeks), vitamins alone (200mg vitamin C and 50mg vitamin E, both twice per day for 4 weeks), both treatments or neither. Plasma and mucosal ascorbic acid, malondialdehyde and reactive oxygen species were determined before and after treatment. Compared with normal controls (n 61), H. pylori-positive patients (n 117) had higher mucosal reactive oxygen species and malondialdehyde levels and lower plasma ascorbic acid. Plasma ascorbic acid doubled in both groups of patients receiving vitamins and mucosal levels also increased. Malondialdehyde and reactive oxygen species fell in patients in whom H. pylori was eradicated but vitamin supplements were not effective either alone or in combination with H. pylori eradication. Supplements of vitamins C and E do not significantly reduce mucosal reactive oxygen species damage in H. pylori gastritis.

Direct Serum Total Iron-binding Capacity Assay Suitable for Automated Analyzers

Background: Present methods for measuring serum total iron-binding capacity (TIBC) involve manipulation of samples or performance of two assays on each sample. We developed a direct automated assay (DTIBC) for TIBC.

Methods: We added to serum a saturating amount of iron bound to an excess of chelating dye at a low pH, recorded a blank reading that represented the sum of the saturating amount of iron plus the serum iron, and then added a strong neutral pH buffer. The decrease in absorbance (as transferrin extracts iron from the iron-dye complex) is directly proportional to the TIBC. TIBC values for 125 patients were determined by DTIBC, alumina column TIBC (AC), magnetic particle TIBC (MTIBC), and the UIBC method (UIBC) on Roche COBAS FARA and Mira chemistry analyzers. In a separate study, TIBC values for 128 patients were determined on an Olympus AU400 by the DTIBC and the MTIBC methods.

Results: Methods comparisons on the COBAS analyzers yielded the following results: DTIBC = 1.05(MTIBC) − 1.0 μmol/L (r = 0.987; Sy|x = 2.6 μmol/L); DTIBC = 1.07(AC) − 1.0 μmol/L (r = 0.982; Sy|x = 3.0 μmol/L); and DTIBC = 1.14(UIBC) + 3.4 μmol/L (r = 0.982; Sy|x = 3.0 μmol/L). A similar correlation study using the Olympus AU400 yielded DTIBC = 1.00(MTIBC) − 0.1 μmol/L (r = 0.983; Sy|x = 2.7 μmol/L). The assay was linear from 12.5 to 125 μmol/L (70–700 μg/dL) TIBC on the COBAS FARA. Within- and between-run imprecision (CV) was ≤4.8% at two concentrations. Plasma samples were unsuitable for the method. No interference was seen with common interferants other than ascorbate, deferoxamine, and ferrous sulfate, and only at concentrations well above normal.

Conclusion: The new DTIBC assay is suitable for routine use in clinical laboratories and may improve the quality of iron metabolism studies.

Extracellular matrix regulates induction of alkaline phosphatase expression by ascorbic acid in human fibroblasts

During wound healing and inflammation, fibroblasts express elevated alkaline phosphatase (ALP), but are not in contact with collagen fibrils in the fibronectin (FN)-rich granulation tissue. We hypothesized that the extracellular matrix (ECM) environment might influence the induction of ALP in fibroblasts. Here we tested this hypothesis by studying the ALP-inductive response of normal human gingival fibroblasts to ascorbic acid (AsA). AsA induced ALP activity and protein in cells in conventional monolayer culture. This induction was inhibited by blocking-antibodies to the FN receptor alpha 5 beta 1 integrin and by the proline analog 3,4-dehydroproline (DHP). DHP prevented cells from arranging FN fibrils into a pericellular network and reduced the activity of cell spreading on FN. Plating of cells on FN facilitated the up-regulation by AsA of ALP expression, but did not substitute for AsA. In contrast, AsA did not cause ALP induction in cells cultured on and in polymerized type I collagen gels. Collagen fibrils inhibited the up-regulation by AsA of ALP expression in cells plated on FN. These results indicate that the ECM regulates the induction of ALP expression by AsA in fibroblasts: FN enables them to express ALP in response to AsA through interaction with integrin alpha 5 beta 1, whereas type I collagen fibrils cause the suppression of ALP expression and overcome FN.

Antioxidant supplementation and exercise-induced oxidative stress in the 60-year-old as measured by antipyrine hydroxylates

The effects of 12 weeks of antioxidant supplementation on exercise-induced oxidative stress were investigated in older adults (60 (SE 1) years; BMI 26 (SE 1) kg/m(2)). Subjects were randomly divided in two groups: supplementation (n 11) with 100 mg dl-alpha-tocopheryl acetate, 200 mg ascorbic acid, and 2 mg beta-carotene, and placebo (n 9). Before and after the 12 week supplementation period, subjects cycled for 45 min at submaximal intensity (50 % maximal workload capacity). Antipyrine was used as marker for oxidative stress. Antipyrine reacts quickly with hydroxyl radicals to form para- and ortho-hydroxyantipyrine. The latter metabolite is not formed in man through the mono-oxygenase pathway of cytochrome P450. Daily supplementation significantly increased plasma concentrations of alpha-tocopherol and beta-carotene in the supplemented group (Delta 14.4 (SE 3.2) and 0.4 (se 0.1) micromol/l; P<0.001 and P<0.01). No significant differences, within and between groups, were observed in the exercise-induced increase in the ratios para- and ortho-hydroxyantipyrine to antipyrine. In addition, supplementation did not affect the exercise-induced increase in thiobarbituric acid reactive substances in plasma. In conclusion, in 60-year-old subjects antioxidant supplementation had no effect on the exercise-induced increase in oxidative stress as measured by free radical products of antipyrine.

Effect of dietary vitamin C on concentrations of ascorbic acid in plasma and milk

The addition of exogenous ascorbic acid to milk reduces the development of oxidized flavor. This experiment was conducted to determine whether feeding ascorbic acid to cows influenced vitamin C concentrations in milk. Thirty-two midlactation Holstein cows were fed a basal diet of 56% forage, 36.6% concentrate, and 7.4% roasted whole soybeans (dry basis) that was top-dressed with a premix that provided 0, 3, 16.5, or 30 g/d of L-ascorbic acid (provided by ascorbyl-2-polyphosphate) for 28 d. Supplementation had no effect on milk yield or composition or dry matter intake. Treatment linearly increased plasma concentrations of ascorbic acid (19.8, 22.3, 21.9, and 25.7 mumol/L, respectively) but had no effect on plasma dehydro-L-ascorbic acid (DHAA). Concentrations of ascorbic acid (103.7 mumol/L) and DHAA (9.5 mumol/L) in milk were not affected by treatment. Secretion of ascorbic acid into milk appeared to follow Michaelis-Menton kinetics, with a Vmax of 3.92 mmol/d and a Km of 3.59 mumol/L. Milk flavor as evaluated by a panel was normal for all samples after 1 d of storage. After 7 d of storage, the average flavor score was 2.5 (moderate oxidized flavor), but no differences among treatments were observed. Supplemental dietary vitamin C did not increase vitamin C concentration in milk, probably because the maximum potential secretion of the vitamin was occurring in unsupplemented cows.

Effects of nitrogen dioxide exposure and ascorbic acid supplementation on exhaled nitric oxide in healthy human subjects

BACKGROUND

Nitric oxide (NO) is detectable in the exhaled breath, is involved in airway defence and inflammation, and probably modulates bronchial smooth muscle tone. Given the sensitivity of nitrogen oxides to local redox conditions, we postulated that exposure to oxidant or antioxidant compounds could alter concentrations of NO in the exhaled breath (eNO). We assessed the effect of nitrogen dioxide (NO(2)) and ascorbic acid exposure on eNO in healthy human subjects.

METHODS

Ten healthy subjects were randomised to undergo a 20 minute single blind exposure to NO(2) (1.5 parts per million) or medical air in a crossover fashion. Exhaled NO and pulmonary function were measured before and for 3 hours after exposure. In a separate double blind crossover study 20 healthy subjects received ascorbic acid 500 mg twice daily or placebo for 2 weeks with a 6 week interim washout. Serum ascorbic acid levels and eNO were measured before and after each supplementation phase.

RESULTS

NO(2) induced a decrease of 0.62 (95% CI 0.32 to 0.92) ppb in the mean post-exposure eNO (p<0.01) with no change in forced expiratory volume in 1 second (FEV(1)). Oral supplementation with ascorbic acid increased the mean serum ascorbic acid concentration by 7.4 (95% CI 5.1 to 9.7) microg/ml (63%) but did not alter eNO.

CONCLUSIONS

NO(2) exposure causes a decrease in eNO, an effect which may be mediated through changes in epithelial lining fluid redox state or through a direct effect on epithelial cells. In contrast, ascorbic acid does not appear to play a significant role in the metabolism of NO in the epithelial lining fluid.

Effect of antioxidant supplementation on ozone-induced lung injury in human subjects

To determine whether antioxidants can influence human susceptibility to ozone (O(3))-induced changes in lung function and airway inflammation, we placed 31 healthy nonsmoking adults (18 to 35 yr old) on a diet low in ascorbate for 3 wk. At 1 wk, subjects were exposed to filtered air for 2 h while exercising (20 L/min/m(2)), and then underwent bronchoalveolar lavage (BAL) and were randomly assigned to receive either a placebo or 250 mg of vitamin C, 50 IU of alpha-tocopherol, and 12 oz of vegetable cocktail daily for 2 wk. Subjects were then exposed to 0.4 ppm O(3) for 2 h and underwent a second BAL. On the day of the O(3) exposure, supplemented subjects were found to have significantly increased levels of plasma ascorbate, tocopherols, and carotenoids as compared with those of the placebo group. Pulmonary function testing showed that O(3)-induced reductions in FEV(1) and FVC were 30% and 24% smaller, respectively, in the supplemented cohort. In contrast, the inflammatory response to O(3) inhalation, as represented by the percent neutrophils and the concentration of interleukin-6 recovered in the BAL fluid at 1 h after O(3) exposure was not different for the two groups. These data suggest that dietary antioxidants protect against O(3)-induced pulmonary function decrements in humans.

A new recommended dietary allowance of vitamin C for healthy young women

The recently released Recommended Dietary Allowance of vitamin C for women, 75 mg daily, was based on data for men. We now report results of a depletion-repletion study with healthy young women hospitalized for 186 +/- 28 days, using vitamin C doses of 30-2,500 mg daily. The relationship between dose and steady-state plasma concentration was sigmoidal. Only doses above 100 mg were beyond the linear portion of the curve. Plasma and circulating cells saturated at 400 mg daily, with urinary elimination of higher doses. Biomarkers of endogenous oxidant stress, plasma and urine F(2)-isoprostanes, and urine levels of a major metabolite of F(2)-isoprostanes were unchanged by vitamin C at all doses, suggesting this vitamin does not alter endogenous lipid peroxidation in healthy young women. By using Food and Nutrition Board guidelines, the data indicate that the Recommended Dietary Allowance for young women should be increased to 90 mg daily.

Intracellular ascorbic acid enhances the DNA single-strand breakage and toxicity induced by peroxynitrite in U937 cells

A well-established protocol to increase the intracellular content of ascorbic acid was used to investigate the effects of the vitamin on DNA single-strand breakage and toxicity mediated by authentic peroxynitrite (ONOO(-)) in U937 cells. This protocol involved exposure for 60 min to 100 microM dehydroascorbic acid, which was taken up by the cells and converted into ascorbic acid via a GSH-independent mechanism. At the time of exposure to ONOO(-), which was performed in fresh saline immediately after loading with dehydroascorbic acid, the vitamin present in the cells was all in its reduced form. It was found that, in cells that are otherwise ascorbate-deficient, an increase in their ascorbic acid content does not prevent, but rather enhances, the DNA-damaging and lethal responses mediated by exogenous ONOO(-). These results therefore suggest that acute supplementation of ascorbic acid can be detrimental for individuals with pathologies associated with a decrease in ascorbic acid and in which ONOO(-) is known to promote deleterious effects.

Evaluation of Aution Max AX-4280 automated urine test-strip analyser

Aution Max AX-4280, an automated urine test-strip analyser, was evaluated in three centres. Method comparison, imprecision, carry-over, linearity, detection limit and drift studies were performed for glucose, protein, blood and leukocytes using Uriflet S 9UB strips. These strips enable measurement of pH, glucose, protein, blood, leukocytes, ketones, bilirubin, urobilinogen and nitrite. Specific gravity is determined by the refractive index method. Within-run and between-day imprecision, assessed using pooled urines and quality control materials, were good. No drift over 24 h or sample carry-over was observed. Method comparison with quantitative methods for glucose, protein and specific gravity yielded good correlations. Ascorbate negatively interfered with haemoglobin, glucose and nitrite measurements. Acetylsalicylic acid lowered pH, the effect being greatest when protein was absent. During the assessment period no malfunction or breakdown was reported. The Aution Max is easy to use and needs minimal maintenance.

Vitamin C-induced decomposition of lipid hydroperoxides to endogenous genotoxins

Epidemiological data suggest that dietary antioxidants play a protective role against cancer. This has led to the proposal that dietary supplementation with antioxidants such as vitamin C (vit C) may be useful in disease prevention. However, vit C has proved to be ineffective in cancer chemoprevention studies. In addition, concerns have been raised over potentially deleterious transition metal ion-mediated pro-oxidant effects. We have now determined that vit C induces lipid hydroperoxide decomposition to the DNA-reactive bifunctional electrophiles 4-oxo-2-nonenal, 4,5-epoxy-2(E)-decenal, and 4-hydroxy-2-nonenal. The compound 4,5-Epoxy-2(E)-decenal is a precursor of etheno-2'-deoxyadenosine, a highly mutagenic lesion found in human DNA. Vitamin C-mediated formation of genotoxins from lipid hydroperoxides in the absence of transition metal ions could help explain its lack of efficacy as a cancer chemoprevention agent.

DNA damage from micronutrient deficiencies is likely to be a major cause of cancer

A deficiency of any of the micronutrients: folic acid, Vitamin B12, Vitamin B6, niacin, Vitamin C, Vitamin E, iron, or zinc, mimics radiation in damaging DNA by causing single- and double-strand breaks, oxidative lesions, or both. For example, the percentage of the US population that has a low intake (<50% of the RDA) for each of these eight micronutrients ranges from 2 to >20%. A level of folate deficiency causing chromosome breaks was present in approximately 10% of the US population, and in a much higher percentage of the poor. Folate deficiency causes extensive incorporation of uracil into human DNA (4 million/cell), leading to chromosomal breaks. This mechanism is the likely cause of the increased colon cancer risk associated with low folate intake. Some evidence, and mechanistic considerations, suggest that Vitamin B12 (14% US elderly) and B6 (10% of US) deficiencies also cause high uracil and chromosome breaks. Micronutrient deficiency may explain, in good part, why the quarter of the population that eats the fewest fruits and vegetables (five portions a day is advised) has about double the cancer rate for most types of cancer when compared to the quarter with the highest intake. For example, 80% of American children and adolescents and 68% of adults do not eat five portions a day. Common micronutrient deficiencies are likely to damage DNA by the same mechanism as radiation and many chemicals, appear to be orders of magnitude more important, and should be compared for perspective. Remedying micronutrient deficiencies should lead to a major improvement in health and an increase in longevity at low cost.

Helicobacter pylori infection reduces systemic availability of dietary vitamin C

OBJECTIVE

Helicobacter pylori infection is recognized to lower the concentration of vitamin C in gastric juice. The objective of this study was to assess the effect of the infection on the systemic availability of dietary vitamin C.

METHODS

The study involved 1,106 men and women aged 25-74 randomly recruited from the population of north Glasgow. Their H. pylori status, dietary vitamin C intake calculated from a food frequency questionnaire and plasma vitamin C concentration were measured. Correction was made for potential confounding factors such as age, sex, smoking and social status.

RESULTS

The mean plasma vitamin C concentration in those who were H. pylori-positive was only 65% of that in those classified negative. Although partly explained by differences in age, sex, social class, smoking and vitamin C intake, the systemic reduction was observed across almost all sub-groups after stratification. Correction for all these factors still gave a plasma vitamin C level for H. pylori positives which was only 80% of that for negatives (P < 0.0001).

CONCLUSIONS

H. pylori substantially impairs the bio-availability of vitamin C. This, together with the reduced vitamin C intake of H. pylori-positive subjects, markedly reduces the plasma vitamin C level of infected subjects. The reduced circulating levels of vitamin C in H. pylori-infected subjects may contribute to the aetiology of gastric cancer, as well as other diseases associated with anti-oxidant deficiency.

Reduction of dehydroascorbic acid by homocysteine

To determine the reductive process of extracellular dehydroascorbic acid (DHA), molecules (homocysteine, homocysteine thiolactone, methionine, cysteine, and homoserine) were tested to identify those with the potential to reduce DHA to ascorbic acid (AA). Homocysteine (Hcy) was the most potent of the molecules tested. The efficacy of Hcy was compared with that of other molecules able to reduce DHA (reduced glutathione (GSH) and cysteine (Cy)). Although all three molecules were able to reduce DHA, GSH and Cy were not to reduce DHA to AA at concentrations lower than 100 micromol/l, and only less than 5% DHA was reduced to AA at concentrations of 200-300 micromol/l. In contrast, Hcy reduced DHA to AA stoichiometrically at concentrations as low as 10 micromol/l. In Jurkat and U937 cells, the increasing concentrations of extracellular Hcy suppressed intracellular dehydroascorbic acid uptake, indicating that extracellular reduction of DHA by Hcy leads to decreasing extracellular DHA available for its intracellular uptake. Simultaneous oxidation and reduction of Hcy and DHA were accelerated extracellularly in the presence of quercetin, an inhibitor of DHA uptake, suggesting that extracellular ascorbic acid concentration increased via blocking DHA uptake by quercetin and reducing extracellular DHA by Hcy. The effect of homocysteine on DHA reduction and uptake was confirmed with human umbilical vein endothelial cells. The oxidation of Hcy also prevented the decrease in DNA synthesis in human umbilical vein endothelial cells, which would occur following exposure to Hcy.

Serum concentrations of carotenoids and vitamins A, E, and C in control subjects from five European countries

High intakes of fruits and vegetables, or high circulating levels of their biomarkers (carotenoids, vitamins C and E), have been associated with a relatively low incidence of cardiovascular disease, cataract and cancer. Exposure to a high fruit and vegetable diet increases antioxidant concentrations in blood and body tissues, and potentially protects against oxidative damage to cells and tissues. This paper describes blood concentrations of carotenoids, tocopherols, ascorbic acid and retinol in well-defined groups of healthy, non-smokers, aged 25-45 years, 175 men and 174 women from five European countries (France, UK (Northern Ireland), Republic of Ireland, The Netherlands and Spain). Analysis was centralised and performed within 18 months. Within-gender, vitamin C showed no significant differences between centres. Females in France, Republic of Ireland and Spain had significantly higher plasma vitamin C concentrations than their male counterparts. Serum retinol and alpha-tocopherol levels were similar between centres, but gamma-tocopherol showed a great variability being the lowest in Spain and France, and the highest in The Netherlands. The provitamin A: non-provitamin A carotenoid ratio was similar among countries, whereas the xanthophylls (lutein, zeaxanthin, beta-cryptoxanthin) to carotenes (alpha-carotene, beta-carotene, lycopene) ratio was double in southern (Spain) compared to the northern areas (Northern Ireland and Republic of Ireland). Serum concentrations of lutein and zeaxanthin were highest in France and Spain; beta-cryptoxanthin was highest in Spain and The Netherlands; trans-lycopene tended to be highest in Irish males and lowest in Spanish males; alpha-carotene and beta-carotene were higher in the French volunteers. Due to the study design, the concentrations of carotenoids and vitamins A, C and E represent physiological ranges achievable by dietary means and may be considered as 'reference values' in serum of healthy, non-smoking middle-aged subjects from five European countries. The results suggest that lutein (and zeaxanthin), beta-cryptoxanthin, total xanthophylls and gamma-tocopherol (and alpha- : gamma-tocopherol) may be important markers related to the healthy or protective effects of the Mediterranean-like diet.

Topical L-ascorbic acid: percutaneous absorption studies

BACKGROUND

Reactive oxygen species generated by ultraviolet light result in photocarcinogenic and photoaging changes in the skin. Antioxidants protect skin from these insults.

OBJECTIVE

This study defines formulation characteristics for delivering L-ascorbic acid into the skin to supplement the skin's natural antioxidant reservoir.

METHODS

L-ascorbic acid or its derivatives were applied to pig skin. Skin levels of L-ascorbic acid were measured to determine percutaneous delivery.

RESULTS

L-ascorbic acid must be formulated at pH levels less than 3.5 to enter the skin. Maximal concentration for optimal percutaneous absorption was 20%. Tissue levels were saturated after three daily applications; the half-life of tissue disappearance was about 4 days. Derivatives of ascorbic acid including magnesium ascorbyl phosphate, ascorbyl-6-palmitate, and dehydroascorbic acid did not increase skin levels of L-ascorbic acid.

CONCLUSIONS

Delivery of topical L-ascorbic acid into the skin is critically dependent on formulation characteristics.

Forbidden fruit

Although citrus fruits prevent and cure scurvy, they may not always be as good for you as you thought.

Antioxidant nutrients: current dietary recommendations and research update

OBJECTIVE

To review the importance of antioxidant nutrients in the maintenance of health and the prevention and treatment of disease, with a focus on data pertaining to vitamin C, vitamin E, selenium, and carotenoids. A secondary objective was to discuss the new Dietary Reference Intakes released by the Institute of Medicine (IOM) for these nutrients.

DATA SOURCES

IOM reports on the use of antioxidant vitamins were reviewed for nutrient recommendations. In addition, a MEDLINE search was performed to identify recent research and review articles on the topic, which were analyzed to identify key research findings in the area.

DATA SYNTHESIS

The review discusses the biologic processes of oxidation reactions and antioxidants in biologic systems, provides an overview of information on selected antioxidant nutrients, and explores their role in the prevention and treatment of cancer, cardiovascular disease, ocular disorders, and respiratory disorders.

CONCLUSION

There appear to be significant health benefits from dietary antioxidants, as can be found in fruits and vegetables. Some prospective assessment of the effect of supplemental antioxidants also suggests benefit, especially for vitamin E; however, there are conflicting results in this area. Overall, it appears that antioxidant nutrients, especially those from food sources, have important roles in preventing pathogenic processes related to cancer, cardiovascular disease, macular degeneration, cataracts, and asthma, and may enhance immune function.

Use of vitamin-mineral supplements by female physicians in the United States

BACKGROUND

Rates of vitamin-mineral supplement use by US female physicians are unknown but are of particular interest for several epidemiologic and clinical reasons.

OBJECTIVE

The objective was to determine rates of and variations in vitamin-mineral supplement use among US female physicians.

DESIGN

We used data from the Women Physicians' Health Study, a large (n = 4501) national, randomly sampled mail survey of female physicians aged 30-70 y.

RESULTS

Half of the physicians took a multivitamin-mineral supplement; 35.5% of these did so regularly. However, </=33% took any supplement other than calcium and <20% did so regularly. Regular vitamin-mineral supplement use increased with age, and antioxidant intake was higher in those at high risk of heart disease. Those with a history of osteoporosis were nearly 3 times as likely as those with no history to take supplemental calcium regularly. Those who took any supplement regularly also consumed more fruit and vegetables daily than did occasional users or nonusers (P: < 0.0001). Regular users of any supplement also consumed less fat than did occasional users or nonusers (P: < 0.01). Additionally, vegetarians were more likely than were nonvegetarians to regularly consume any supplement (59.9% compared with 46.3%; P: < 0.001) and those who regularly consumed any supplement were more likely to comply with US Preventive Services Task Force guidelines than were those who were occasional users or nonusers (72.4% compared with 66.5% and 60.2%; P: < 0.0001).

CONCLUSION

Female physicians, particularly those who were especially health conscious or at higher risk of heart disease or osteoporosis, used supplements at rates at least equal to those of women in the general population.

Reevaluation of ascorbate in cancer treatment: emerging evidence, open minds and serendipity

Some clinicians and alternative therapy practitioners advocate megadose intravenous and oral ascorbate treatment of cancer. Randomized control studies using oral ascorbate showed no benefit. Recent data show that intravenous but not oral administration of ascorbate can produce millimolar plasma concentrations, which are toxic to many cancer cell lines. We propose that ascorbate treatment of cancer should be reexamined by rigorous scientific scrutiny in the light of new evidence.

Dietary intake and bioavailability of polyphenols

The main dietary sources of polyphenols are reviewed, and the daily intake is calculated for a given diet containing some common fruits, vegetables and beverages. Phenolic acids account for about one third of the total intake and flavonoids account for the remaining two thirds. The most abundant flavonoids in the diet are flavanols (catechins plus proanthocyanidins), anthocyanins and their oxidation products. The main polyphenol dietary sources are fruit and beverages (fruit juice, wine, tea, coffee, chocolate and beer) and, to a lesser extent vegetables, dry legumes and cereals. The total intake is approximately 1 g/d. Large uncertainties remain due to the lack of comprehensive data on the content of some of the main polyphenol classes in food. Bioavailability studies in humans are discussed. The maximum concentration in plasma rarely exceeds 1 microM after the consumption of 10-100 mg of a single phenolic compound. However, the total plasma phenol concentration is probably higher due to the presence of metabolites formed in the body's tissues or by the colonic microflora. These metabolites are still largely unknown and not accounted for. Both chemical and biochemical factors that affect the absorption and metabolism of polyphenols are reviewed, with particular emphasis on flavonoid glycosides. A better understanding of these factors is essential to explain the large variations in bioavailability observed among polyphenols and among individuals.

Single cell gel electrophoresis assay: a new technique for human biomonitoring studies

Human biomonitoring using the single cell gel electrophoresis (SCGE) or comet assay is a novel approach for the assessment of genetic damage in exposed populations. This assay enables the detection of various forms of DNA damage in individual cells with ease and speed and is, therefore, well suited to the analysis of a large group in a population. Here, application of SCGE assay in the identification of dietary protective factors, in clinical studies and in monitoring the risk of DNA damage resulting from occupational, environmental or lifestyle exposures is reviewed. Also, the comparative sensitivity of SCGE assay and conventional cytogenetic tests to detect genetic damage is discussed. Finally, strengths and shortcomings of the SCGE assay are addressed.

The effect of vitamin E and vitamin C supplementation on LDL oxidizability and neutrophil respiratory burst in young smokers

OBJECTIVE

The purpose of this study was to determine the effect of vitamin E and/or vitamin C supplementation on low-density lipoprotein (LDL) oxidizability and neutrophil (PMN) superoxide anion production in young smokers.

METHODS

Thirty smokers with a <5 pack-year history were randomly assigned to take placebo; vitamin C (1 g/day); vitamin E (400 IU/day), or both vitamins in a double-blind fashion. Subjects took the supplements for 8 weeks. At weeks 0 and 8, blood was collected for isolation of LDL and PMN, and for antioxidant vitamin analysis. LDL was oxidized with a copper (Cu) catalyst, and oxidation was measured by formation of conjugated dienes over a 5-hour time course. Lag times and maximum oxidation rates were calculated from the time course data. PMN superoxide anion release was assessed by respiratory burst after stimulation with phorbol ester and opsonized zymosan, and their ability to oxidize autologous LDL following treatment with the above stimuli was measured with the conjugated diene assay.

RESULTS

Subjects who received vitamin E alone had a significant increase in the lag phase of Cu-catalyzed LDL oxidation (week 0, 118+/-31 min vs. week 8, 193+/-80 min, mean +/- SD, p < 0.05), whereas the vitamin C and placebo groups had no changes in LDL oxidation kinetics. The group receiving both vitamins E and C had a significant reduction in oxidation rate (week 0. 7.4+/-2.3 vs. week 8, 5.1+/-2.1, p < 0.05). There were no significant changes for any group in PMN superoxide anion production or PMN LDL oxidation after stimulation with either phorbol ester or opsonized zymosan. Plasma and LDL vitamin E concentrations were significantly increased in both groups that received vitamin E. The subjects who received vitamin C alone had no significant change in plasma vitamin C concentrations; however, when data were pooled from both groups who received vitamin C, the increases were significant.

CONCLUSION

Vitamin E supplementation of young smokers was effective in reducing Cu-catalyzed LDL oxidizability; however, vitamin E and/or C supplementation showed few significant effects on the more physiologically relevant PMN function. This casts doubt on the ability of antioxidant supplementation to reduce oxidative stress in smokers in vivo. Therefore, smoking cessation remains the only means by which young smokers can prevent premature coronary heart disease.

Direct correlation of glutathione and ascorbate and their dependence on age and season in human lymphocytes

BACKGROUND

Endogenous reactive oxygen species appear to contribute to aging and cancer and dietary antioxidants, present in fruit and vegetables, counteract these effects.

OBJECTIVE

The objective was to examine the association between intracellular glutathione, ascorbate (vitamin C), and alpha-tocopherol (vitamin E) in human lymphocytes.

DESIGN

The study group consisted of 240 healthy nonsmoking volunteers with an approximately equal number of male and female subjects subdivided into 3 age groups: 18-39, 40-59, and >/=60 y). Glutathione, glutathione disulfide, ascorbate, and alpha-tocopherol were measured in lymphocytes by HPLC.

RESULTS

The average concentration of antioxidants in lymphocytes was 27 +/- 8 nmol/mg protein for glutathione, 21 +/- 8 nmol/mg protein for ascorbate, and 0.4 +/- 0.2 nmol/mg protein for alpha-tocopherol. There was a strong positive correlation between glutathione and ascorbate (r = 0.62, P < 0.001). No correlation was observed for glutathione and ascorbate with alpha-tocopherol. The concentration of glutathione in lymphocytes was inversely correlated with age (r = -0.19, P < 0.01), as was that of ascorbate (r = -0.22, P < 0.01), with 10-20% lower values in elderly than in young and elderly subjects. The concentrations of glutathione in lymphocytes were as much as 25% higher and those of ascorbate were as much as 38% higher during the summer than during the winter. The seasonal variation of ascorbate in lymphocytes was described by a linear function for age and a periodic sine function for season.

CONCLUSION

Glutathione and ascorbate are directly correlated in human lymphocytes.

Oral vitamin C and endothelial function in smokers: short-term improvement, but no sustained beneficial effect

OBJECTIVES

To test the hypothesis that antioxidant therapy would improve endothelial function in smokers.

BACKGROUND

Several studies have documented a beneficial effect of short-term oral or parenteral vitamin C on endothelial physiology in subjects with early arterial dysfunction. Possible long-term effects of vitamin C on endothelial function, however, are not known.

METHODS

We studied the effects of short- and long-term oral vitamin C therapy on endothelial function in 20 healthy young adult smokers (age 36 +/- 6 years, 8 male subjects, 21 +/- 10 pack-years). Each subject was studied at baseline, 2 h after a single dose of 2 g vitamin C and 8 weeks after taking 1 g vitamin C daily, and after placebo, in a randomized double-blind crossover study. Blood samples were analyzed for plasma ascorbate levels and endothelial function was measured as flow-mediated dilation of the brachial artery, using high resolution ultrasound. Nitroglycerin-mediated dilation (endothelium-independent) was also measured at each visit.

RESULTS

At baseline, plasma ascorbate level was low in the smokers (42 +/- 21 micromol/liter; normal range, 50 to 150 micromol/liter), increased with vitamin C therapy after 2 h to 120 +/- 54 micromol/liter (p < 0.001) and remained elevated after eight weeks of supplementation at 92 +/- 32 micromol/liter (p < 0.001, compared with placebo). Flow-mediated dilation, however, increased at 2 h (from 2.8 +/- 2.0% to 6.3 +/- 2.8%, p < 0.001), but there was no sustained beneficial effect after eight weeks (3.9 +/- 3.2%, p = 0.26). Nitroglycerin-mediated dilation was unchanged throughout.

CONCLUSION

Oral vitamin C therapy improves endothelial dysfunction in the short term in healthy young smokers, but it has no beneficial long-term effect, despite sustained elevation of plasma ascorbate levels.

How does ascorbic acid prevent endothelial dysfunction?

Human coronary and peripheral arteries show endothelial dysfunction in a variety of conditions, including atherosclerosis, hypercholesterolemia, smoking, and hypertension. This dysfunction manifests as a loss of endothelium-dependent vasodilation to acetylcholine infusion or sheer stress, and is typically associated with decreased generation of nitric oxide (NO) by the endothelium. Vitamin C, or ascorbic acid, when acutely infused or chronically ingested, improves the defective endothelium-dependent vasodilation present in these clinical conditions. The mechanism of the ascorbic acid effect is unknown, although it has been attributed to an antioxidant function of the vitamin to enhance the synthesis or prevent the breakdown of NO. In this review, multiple mechanisms are considered that might account for the ability of ascorbate to preserve NO. These include ascorbate-induced decreases in low-density lipoprotein (LDL) oxidation, scavenging of intracellular superoxide, release of NO from circulating or tissue S-nitrosothiols, direct reduction of nitrite to NO, and activation of either endothelial NO synthase or smooth muscle guanylate cyclase. The ability of ascorbic acid supplements to enhance defective endothelial function in human diseases provides a rationale for use of such supplements in these conditions. However, it is first necessary to determine which of the many plausible mechanisms account for the effect, and to ensure that undesirable toxic effects are not present.