Vitamin C Urinary Loss in Fabry Disease: Clinical and Genomic Characteristics of Vitamin C Renal Leak.

BACKGROUND: Reduced plasma vitamin C concentrations in chronic diseases may result from abnormal urinary excretion of vitamin C: a renal leak. We hypothesized that vitamin C renal leak may be associated with disease-mediated renal dysregulation, resulting in aberrant vitamin C renal reabsorption and increased urinary loss. OBJECTIVES: We investigated the prevalence, clinical characteristics, and genomic associations of vitamin C renal leak in Fabry disease, an X-linked lysosomal disease associated with renal tubular dysfunction and low plasma vitamin C concentrations. METHODS: We conducted a non-randomized cross-sectional cohort study of men aged 24-42 y, with Fabry disease (n = 34) and controls without acute or chronic disease (n = 33). To match anticipated plasma vitamin C concentrations, controls were placed on a low-vitamin C diet 3 wk before inpatient admission. To determine the primary outcome of vitamin C renal leak prevalence, subjects were fasted overnight, and matched urine and fasting plasma vitamin C measurements were obtained the following morning. Vitamin C renal leak was defined as presence of urinary vitamin C at plasma concentrations below 38 µM. Exploratory outcomes assessed the association between renal leak and clinical parameters, and genomic associations with renal leak using single nucleotide polymorphisms (SNPs) in the vitamin C transporter SLC23A1. RESULTS: Compared with controls, the Fabry cohort had 16-fold higher odds of renal leak (6% vs. 52%; OR: 16; 95% CI: 3.30, 162; P < 0.001). Renal leak was associated with higher protein creatinine ratio (P < 0.01) and lower hemoglobin (P = 0.002), but not estimated glomerular filtration rate (P = 0.54). Renal leak, but not plasma vitamin C, was associated with a nonsynonymous single nucleotide polymorphism in vitamin C transporter SLC23A1 (OR: 15; 95% CI: 1.6, 777; P = 0.01). CONCLUSIONS: Increased prevalence of renal leak in adult men with Fabry disease may result from dysregulated vitamin C renal physiology and is associated with abnormal clinical outcomes and genomic variation.

The inhibition of intestinal glucose absorption by oat-derived avenanthramides.

Avenanthramides are phenolic compounds unique to oats and may contribute to health-promoting properties associated with oat consumption. This study used Xenopus laevis oocytes expressing the glucose transporters, glucose transporter 2 (GLUT2) or sodium-glucose transport protein 1 (SGLT1) and human Caco-2 cells models to investigate the effect of oat avenanthramides on human intestinal glucose transporters. The presence of avenanthramide reduced the glucose uptake in a dose-dependent manner in Caco-2 cells. Glucose uptake in oocytes expressing either GLUT2 or SGLT1 was nullified by oat avenanthramide. There was no significant difference between the inhibition potencies of avenanthramides C and B. Thus, our results suggest that avenanthramides may contribute to the antidiabetic properties of oats. PRACTICAL APPLICATIONS: The present research focus on the antidiabetic properties of avenanthramides, which are unique phenolic compounds found in oats. Inhibiting the activities of the glucose transport proteins expressed in the small intestine is a known strategy to improve the control of postprandial glucose level. We therefore examined the inhibitory effects of avenanthramides on two glucose transporters, glucose transporter 2 and sodium-glucose transport protein 1, predominantly found in the small intestine using the human small intestinal cell model Caco-2 cell line and by heterologously expressing these two transporters in the Xenopus laevis oocytes. Based on our results, we have confirmed for the first time that the glucose uptake is indeed inhibited by the presence of avenanthramides, suggesting the possibility of incorporating avenanthramides in foods to enhance postprandial glucose response, and ultimately improve the management of diabetes. Therefore, future research could consider utilizing this evidence in the development of diabetic-friendly functional foods or nutraceuticals containing avenanthramides.

Valorization of Heat-Treated Brewers' Spent Grain Through the Identification of Bioactive Phenolics by UPLC-PDA and Evaluation of Their Antioxidant Activities.

Thermal processing not only disrupts cell membranes and cell walls, but also cleaves covalent bonds releasing low molecular phenolic. This study examined the impact of various heat treatments (100, 140, and 160°C) on the composition of phenolic acids and antioxidant activities in extracts obtained from defatted brewers spent grain (BSG) meal. Heating BSG at 160°C resulted in a 2-fold increase in total phenolic content [TPC, 172.98 ± 7.3 mg Gallic acid equivalent (GAE)/100 g defatted meal] and total flavonoid content [TFC, 16.15 ± 2.22 catechin equivalents (CE)/100 g defatted meal] compared to the untreated BSG extracts. The antioxidant activities of treated BSG extracts, determined by radical scavenging and ferric reducing antioxidant power (FRAP) were significantly (p < 0.5) higher than the corresponding untreated BSG extracts. Eleven phenolic acids were identified and quantified in BSG extracts by Ultra Performance Liquid Chromatography with Photodiode Array (UPLC-PDA). The amounts varied significantly (p < 0.05) depending on the degree of toasting BSG was subjected to. Chlorogenic acid, an ester of caffeic and quinic acid was the predominant phenolic acid present in all fractions. Significant (p < 0.05) increases in TPC, TFC, individual phenolic acids and antioxidant activity were observed in BSG extracts exposed to increasing oven temperatures. These results confirm the ability of heat processing to release bioactive phenolic from their bound forms thereby enhancing the phenolic acids and the digestibility of BSG meal in the intestinal tract.

The Manitoba Personalized Lifestyle Research (TMPLR) study protocol: a multicentre bidirectional observational cohort study with administrative health record linkage investigating the interactions between lifestyle and health in Manitoba, Canada.

INTRODUCTION: Lifestyle factors, such as diet, physical activity and sleep, are associated with the development of many chronic diseases. The objective of The Manitoba Personalized Lifestyle Research study is to understand how these lifestyle factors interact with each other and with other factors, such as an individual's genetics and gut microbiome, to influence health. METHODS: An observational study of adults, with extensive phenotyping by objective health and lifestyle assessments, and retrospective assessment of early life experiences, with retrospective and prospective utilisation of secondary data from administrative health records. STUDY POPULATION: A planned non-random convenience sample of 840 Manitobans aged 30-46 recruited from the general population, stratified by sex (equal men and women), body mass index (BMI; 60% of participants with a BMI>25 kg/m2) and geography (25% from rural areas). These stratifications were selected based on Manitoba demographics. MEASUREMENTS: Lifestyle factors assessed will include dietary pattern, physical activity, cardiovascular fitness, and sleep. Factors such as medical history, socioeconomic status, alcohol and tobacco consumption, cognition, stress, anxiety, and early life experiences will also be documented. A maternal survey will be performed. Body composition and bone density will be measured by dual energy X-ray absorptiometry. Blood pressure, pulse wave velocity, and augmentation index will be measured on two consecutive days. Chronic disease risk biomarkers will be measured in blood and urine samples. DNA will be extracted for genetic analysis. A faecal sample will be collected for microbiome analysis. Participants may provide their Manitoba personal health information number to link their study data with administrative health records. ETHICS AND DISSEMINATION: Ethics approval has been obtained from the University of Manitoba Health Research Ethics Board (protocol # HS18951; 05/01/2016). Data analysis, release of results and publication of manuscripts are scheduled to start in early 2019. Additional information at www.TMPLR.ca. TRIAL REGISTRATION NUMBER: NCT03674957; Pre-results.

Inhibition of Intestinal Cellular Glucose Uptake by Phenolics Extracted from Whole Wheat Grown at Different Locations.

Whole grain consumption is associated with reduced risk of type 2 diabetes, and the underlying mechanism might be related to the actions of polyphenols. Dietary polyphenols contribute to low glycemic indices through inhibition of intestinal glucose transport proteins. This study has two objectives: (1) to evaluate how the contents of phenolic acids in wheat vary by genetic background and growth condition and (2) to evaluate how these changes translate into physiologic relevance by investigating cellular glucose transporter inhibitions. Phenolic acids were extracted from wheat varieties grown at different locations over two crop years. The degree of inhibition of glucose uptake into human Caco-2E cells was determined. Free and bound phenolic acid extracts of all wheat genotypes inhibited glucose uptake. Degree of glucose uptake inhibitions positively correlated with the contents of free and bound phenolic acids, and the correlation coefficients were R2=0.91 and R2=0.89, respectively. Genotype and environment influenced the content of free and bound phenolic acids which linearly translated to the degree of glucose uptake inhibition in a model of intestinal absorption (P < 0.05). Results of this work mechanistically support the hypothesis that dietary phenols positively influence the glycemic index and therefore the health properties of whole grain consumption.

Dietary Vitamin C in Human Health.

Vitamin C is essential to prevent scurvy in humans and is implicated in the primary prevention of common and complex diseases such as coronary heart disease, stroke, and cancer. This chapter reviews the latest knowledge about dietary vitamin C in human health with an emphasis on studies of the molecular mechanisms of vitamin C maintenance as well as gene-nutrient interactions modifying these relationships. Epidemiological evidence indicates 5% prevalence for vitamin C deficiency and 13% prevalence for suboptimal status even in industrialized countries. The daily intake (dose) and the corresponding systemic concentrations (response) are related in a saturable relationship, and low systemic vitamin C concentrations in observational studies are associated with negative health outcomes. However, there is no evidence that vitamin C supplementation impacts the risks for all-cause mortality, impaired cognitive performance, reduced quality of life, the development of eye diseases, infections, cardiovascular disease, and cancers. This might be related to the fact that prevention would not be realized by supplementation in populations already adequately supplied through dietary sources. Recent genetic association studies indicate that the dietary intake might not be the sole determinant of systemic concentrations, since variations in genes participating in redox homeostasis and vitamin C transport had been associated with lowered plasma concentrations. However, impact sizes are generally low and these phenomena might only affect individual of suboptimal dietary supply.

The SLC2A14 gene, encoding the novel glucose/dehydroascorbate transporter GLUT14, is associated with inflammatory bowel disease.

Background: Variations in intestinal antioxidant membrane transporters are implicated in the initiation and progression of inflammatory bowel disease (IBD). Facilitated glucose transporter member 14 (GLUT14), encoded by the solute carrier family 2 member 14 (SLC2A14) gene, is a putative transporter for dehydroascorbic acid and glucose. Although information on the gene is limited, shorter and longer GLUT14 isoforms have been identified. We hypothesized that GLUT14 mediates glucose and dehydroascorbic acid uptake. If this function could be validated, then genetic variations may associate with IBD.Objective: This study aimed to determine the substrate(s) for the GLUT14 protein and interrogated genetic associations of SLC2A14 with IBD.Design: The uptake of radiolabeled substrates into Xenopus laevis oocytes expressing the 2 GLUT14 isoforms was assessed. Examination of gene-targeted genetic association in the Manitoba Inflammatory Bowel Disease Cohort Study was conducted through the genotyping of single nucleotide polymorphisms (SNPs) representing linkage blocks of the SLC2A14 gene.Results: Both GLUT14 isoforms mediated the uptake of dehydroascorbic acid and glucose into X. laevis oocytes. Three alleles in the SLC2A14 gene associated independently with IBD. The odds of having ulcerative colitis (UC) or Crohn disease (CD) were elevated in carriers of the SLC2A14 SNP rs2889504-T allele (OR: 3.60; 95% CI: 1.95, 6.64 and OR: 4.68; 95% CI: 2.78, 8.50, respectively). Similarly, the SNP rs10846086-G allele was associated with an increased risk of both UC and CD (OR: 2.91; 95% CI: 1.49, 5.68 and OR: 3.00; 95% CI: 1.55, 5.78, respectively). Moreover, the SNP rs12815313-T allele associated with increased susceptibility to CD and UC (OR: 2.12; 95% CI: 1.33, 3.36 and OR: 1.61; 95% CI: 1.01, 2.57, respectively).Conclusion: These findings strengthen the hypothesis that genetically determined local dysregulation of dietary vitamin C or antioxidants transport contributes to IBD development. These transporter proteins are targetable by dietary interventions, opening the avenue to a precision intervention for patients of specific genotypes with IBD. This trial was registered at clinicaltrials.gov as NCT03262649.

Effect of Daily Iron Supplementation in Healthy Exclusively Breastfed Infants: A Systematic Review with Meta-Analysis.

BACKGROUND: The literature on the iron requirements of exclusively breastfed infants contains conflicting data and contrary views. OBJECTIVE: The purpose of this study was to summarize the evidence for both benefits and risks of daily oral iron supplementation with regard to hematologic, growth, cognitive parameters, and adverse effects in exclusively breastfed infants. MATERIALS AND METHODS: Structured electronic searches were conducted using PubMed, Cochrane Library databases, and Google Scholar for randomized controlled trials (RCTs) involving daily iron supplementation in full-term healthy exclusively breastfed infants. Random- and fixed-effects models were used for calculating the pooled estimates. RESULTS: Four RCTs with 511 infants were included in the meta-analysis. Iron interventions had no significant effect on iron deficiency or iron deficiency anemia, serum ferritin level, or hemoglobin level. Iron interventions did result in a significant increase in Bayley psychomotor developmental indices in later life (mean difference [MD] = 7.00, confidence interval [95% CI] 0.99-13.01) and mean corpuscular volume (MD = 2.17 fL; 95% CI 0.99-3.35 fL). Iron supplementation was associated with slower growth during the exclusive breastfeeding period, but the long-term effect is unclear. CONCLUSIONS: Limited available evidence suggests that daily iron supplementation has beneficial effects on hematologic parameters and cognitive development, but may delay physical growth in healthy exclusively breastfed infants. There was no evidence to suggest that iron supplementation could cause other adverse effects.

Gene Expression Profiles Suggest Iron Transport Pathway in the Lactating Human Epithelial Cell.

OBJECTIVES: The molecular background of iron excretion into breast milk has not been determined in humans. We determined the expression of known iron transporters in mRNA extracted from human milk fat globules to deduce which known transporters are responsible for iron excretion into human milk. METHODS: The expression of iron transporters in mRNA from human milk fat globules and mouse mammary epithelial cell lines was determined by quantitative real-time polymerase chain reaction. RESULTS: The expression of the transferrin receptor 1 (TFRC), divalent metal transporter 1 (SLC11A2), transferrin (TF), and lactoferrin (LTF) was confirmed in RNA isolated from the human milk fat globule. Similar expression was observed in the mouse mammary epithelial cell line HC11 in resting and lactating phenotypes. No iron export protein could be determined in the RNA isolated from fat globules in human breast milk and a human mammary epithelial cell line. CONCLUSIONS: The lack of iron exporters in the human mammary epithelia, in conjunction with the presence of lactoferrin suggests that transmembrane transport is not a major route of iron excretion into human milk.

Interactions between dietary oil treatments and genetic variants modulate fatty acid ethanolamides in plasma and body weight composition.

Fatty acid ethanolamides (FAE), a group of lipid mediators derived from long-chain fatty acids (FA), mediate biological activities including activation of cannabinoid receptors, stimulation of fat oxidation and regulation of satiety. However, how circulating FAE levels are influenced by FA intake in humans remains unclear. The objective of the present study was to investigate the response of six major circulating FAE to various dietary oil treatments in a five-period, cross-over, randomised, double-blind, clinical study in volunteers with abdominal obesity. The treatment oils (60 g/12 552 kJ per d (60 g/3000 kcal per d)) provided for 30 d were as follows: conventional canola oil, high oleic canola oil, high oleic canola oil enriched with DHA, flax/safflower oil blend and corn/safflower oil blend. Two SNP associated with FAE degradation and synthesis were studied. Post-treatment results showed overall that plasma FAE levels were modulated by dietary FA and were positively correlated with corresponding plasma FA levels; minor allele (A) carriers of SNP rs324420 in gene fatty acid amide hydrolase produced higher circulating oleoylethanolamide (OEA) (P=0·0209) and docosahexaenoylethanolamide (DHEA) levels (P=0·0002). In addition, elevated plasma DHEA levels in response to DHA intake tended to be associated with lower plasma OEA levels and an increased gynoid fat mass. In summary, data suggest that the metabolic and physiological responses to dietary FA may be influenced via circulating FAE. Genetic analysis of rs324420 might help identify a sub-population that appears to benefit from increased consumption of DHA and oleic acid.

Single-nucleotide polymorphisms in SLC22A23 are associated with ulcerative colitis in a Canadian white cohort.

BACKGROUND: SLC22A23 is an orphan gene in the SLC22 family of organic membrane transporters, and its single-nucleotide polymorphism rs17309827-T was recently nominally associated with intestinal inflammation in a genome-wide association study. Other polymorphisms in the SLC22A23 gene have been associated with diseases with an inflammatory component, and polymorphisms in related genes in the SLC22 family have been repeatedly associated with inflammatory bowel disease (IBD). OBJECTIVE: In a candidate-gene study using a well-phenotyped, highly monitored, Manitoban white cohort, we investigated whether variations in SLC22A23 were associated with intestinal inflammation. DESIGN: Selected genetic variations were genotyped by using fluorescent-based assays or a polymerase chain reaction-restriction fragment length polymorphism analysis in 160 individuals with Crohn disease, 149 individuals with ulcerative colitis, and 142 healthy control subjects to determine genetic associations. RESULTS: Homozygocity for single-nucleotide polymorphisms rs4959235-TT and rs950318-GG was associated with IBD, whereby 6% of patients (18 of 311 cases) carried these genotypes, but they were not seen in healthy controls. CONCLUSION: Associations reported in this article add to the emerging evidence that SLC22A23 variants could modify IBD risk. However, the biology of the gene and impact of variations on the gene's functions need to be tested to validate a causative role.

Polymorphisms in the sodium-dependent ascorbate transporter gene SLC23A1 are associated with susceptibility to Crohn disease.

BACKGROUND: Crohn disease (CD) and ulcerative colitis (UC) are 2 common inflammatory bowel diseases (IBDs) associated with intestinal inflammation and tissue damage. Oxidative stress is suggested to play a major role in the initiation and progression of IBD. Vitamin C (ascorbate, ascorbic acid) supplementation has reduced oxidative stress in persons with IBD. The role of ascorbate transporters in IBD remains to be determined. SLC23A1 is a major ascorbate transporter in the intestinal tract, and some of its genetic variants have been associated with severely decreased ascorbate transport and lower systemic concentrations. OBJECTIVE: This study aimed to determine whether common genetic variants in the vitamin C transporter SLC23A1 are associated with the risk of IBD. DESIGN: Genomic DNA samples from patients with CD (n = 162) and UC (n = 149) from the Manitoba IBD Cohort Study and ethnically matched controls (n = 142) were genotyped for 3 SLC23A1 polymorphisms (rs6596473, rs33972313, and rs10063949) by using TaqMan assays. RESULTS: Variation at rs10063949 (G allele for heterozygote and homozygote) was associated with increased susceptibility to CD (OR: 2.54; 95% CI: 1.38, 4.66; OR: 4.72; 95% CI: 2.53, 8.81; P < 0.0001; respectively). A strong linkage disequilibrium (LD) was observed across the SLC23A1 region (variation rs6596473 with rs10063949) for CD and UC (D' = 0.94 and 0.96, respectively). The risk alleles confirmed a haplotype (CGG) that is carried more in CD patients (65.3%, P < 0.0001) than in controls (43.5%). CONCLUSIONS: A genetic variant (rs10063949-G) in the SLC23A1 ascorbate transporter locus was identified and is associated with an increased risk of CD in a white Canadian IBD cohort. The presented evidence that SLC23A1 variations can modulate the risk of CD has implications for understanding ascorbate transport in CD patients and provides a novel opportunity toward individualized nutritional therapy for patients carrying the disease-associated genotype.

Toward preparation of antibody-based imaging probe libraries for dual-modality positron emission tomography and fluorescence imaging.

Two novel imaging agents trastuzumab-Cy5.5-CHX-A''1 and cetuximab-Cy7-CHX-A''2, bearing both a chelating moiety (CHX-A'') for sequestering metallic radionuclides ((86)Y or (111)In) and the near infrared dye Cy5.5/Cy7, were prepared by a novel modular synthetic strategy as examples of dual-labeled, antibody-based imaging probe library. Fluorescent microscopy illustrated that 1 and 2 strongly bind to HER2-expressing cancer cells (e.g., NIH3T3-HER2(+), SKOV-3) and to EGFR-expressing cancer cells (e.g., A431), respectively, thereby demonstrating that the functionality of the targeting moiety is conserved. Hence, the described novel synthesis strategy can be applied to engineer other tumor-targeted monoclonal antibody based probes for multimodality imaging.

Genetic variation in sodium-dependent vitamin C transporters SLC23A1 and SLC23A2 and risk of advanced colorectal adenoma.

Previous observational studies suggest that vitamin C may reduce risk of colorectal cancer. Vitamin C transport is facilitated by membrane bound sodium-dependent transporters, SVCT1 (encoded by SLC23A1) and SVCT2 (encoded by SLC23A2). To investigate if common genetic variants in these two genes are associated with risk of colorectal tumor development, we conducted a case-control study of 656 Caucasian advanced distal colorectal adenoma cases and 665 Caucasian sigmoidoscopy-negative controls nested within the screening arm of the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial. The analysis of common single nucleotide polymorphisms in SLC23A1 revealed no association. For SLC23A2, overall, there was no association with haplotypes, but two SNPs located in intron 8 and exon 11 could be associated (odds ratio = 0.49, 95% confidence interval = 0.25-0.95 for haplotype G-C vs. haplotype C-C). The findings should be confirmed in follow-up studies, and further investigation is required to probe the functional basis of this finding.

Synthesis, characterization, and biological evaluation of integrin alphavbeta3-targeted PAMAM dendrimers.

Ligand size and valency strongly influence the receptor uptake and clearance of tumor angiogenesis imaging agents. The structures of successful imaging agents exhibit a high degree of variability, encompassing small monovalent arginine-glycine-aspartic acid (RGD)-containing peptides, multivalent RGD-oligomers, and a monoclonal antibody against integrin alpha-v-beta-3 (alpha-v-beta-3). We have pursued a nanoscale approach to imaging of angiogenesis using rationally designed polyamidoamine (PAMAM) dendrimers covalently adorned with RGD-cyclopeptides. An orthogonal oxime-ligation strategy was applied to chemoselectively effect conjugation of the PAMAM dendrimers with RGD-cyclopeptides for targeting alpha vbeta 3. Fluorescent dyes for optical imaging and chelates for gadolinium-based magnetic resonance (MR) imaging were subsequently appended to create robust multimodal macromolecular imaging agents. Fluorescence microscopy revealed selective binding of the resulting RGD peptide-bearing dendrimer with empty chelates to alpha-v-beta-3-expressing cells, but somewhat reduced selectivity was observed following Gd(III) complexation. The expected incomplete saturation of chelates with Gd(III) ions permitted radiometal complexation, and an in vivo tissue distribution of the resulting agent in M21 melanoma tumor-bearing mice showed mostly renal and reticuloendothelial accumulation, with the tumor:blood ratio peaking (3.30+/-0.03) at 2 h postinjection.

Inhibition of the intestinal glucose transporter GLUT2 by flavonoids.

We tested whether the dominant intestinal sugar transporter GLUT2 was inhibited by intestinal luminal compounds that are inefficiently absorbed and naturally present in foods. Because of their abundance in fruits and vegetables, flavonoids were selected as model compounds. Robust inhibition of glucose and fructose transport by GLUT2 expressed in Xenopus laevis oocytes was produced by the flavonols myricetin, fisetin, the widely consumed flavonoid quercetin, and its glucoside precursor isoquercitrin [corrected]. IC50s for quercetin, myricetin, and isoquercitirin [corrected]were approximately 200- to 1000-fold less than glucose or fructose concentrations, and noncompetitive inhibition was observed. The two other major intestinal sugar transporters, GLUT5 and SGLT1, were unaffected by flavonoids. Sugar transport by GLUT2 overexpressed in pituitary cells and naturally present in Caco-2E intestinal cells was similarly inhibited by quercetin. GLUT2 was detected on the apical side of Caco-2E cells, indicating that GLUT2 was in the correct orientation to be inhibited by luminal compounds. Quercetin itself was not transported by the three major intestinal glucose transporters. Because the flavonoid quercetin, a food component with an excellent pharmacology safety profile, might act as a potent luminal inhibitor of sugar absorption independent of its own transport, flavonols show promise as new pharmacologic agents in the obesity epidemic.

Vitamin C as an antioxidant: evaluation of its role in disease prevention.

Vitamin C in humans must be ingested for survival. Vitamin C is an electron donor, and this property accounts for all its known functions. As an electron donor, vitamin C is a potent water-soluble antioxidant in humans. Antioxidant effects of vitamin C have been demonstrated in many experiments in vitro. Human diseases such as atherosclerosis and cancer might occur in part from oxidant damage to tissues. Oxidation of lipids, proteins and DNA results in specific oxidation products that can be measured in the laboratory. While these biomarkers of oxidation have been measured in humans, such assays have not yet been validated or standardized, and the relationship of oxidant markers to human disease conditions is not clear. Epidemiological studies show that diets high in fruits and vegetables are associated with lower risk of cardiovascular disease, stroke and cancer, and with increased longevity. Whether these protective effects are directly attributable to vitamin C is not known. Intervention studies with vitamin C have shown no change in markers of oxidation or clinical benefit. Dose concentration studies of vitamin C in healthy people showed a sigmoidal relationship between oral dose and plasma and tissue vitamin C concentrations. Hence, optimal dosing is critical to intervention studies using vitamin C. Ideally, future studies of antioxidant actions of vitamin C should target selected patient groups. These groups should be known to have increased oxidative damage as assessed by a reliable biomarker or should have high morbidity and mortality due to diseases thought to be caused or exacerbated by oxidant damage.

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.