Abnormal urinary loss of vitamin C in diabetes: prevalence and clinical characteristics of a vitamin C renal leak

BACKGROUND

Diabetes is associated with low plasma vitamin C concentrations.

OBJECTIVES

We investigated the contribution of dysregulated vitamin C renal physiology, its prevalence, and associated clinical characteristics.

METHODS

An essential prerequisite was determination of normal vitamin C renal threshold, the plasma concentration at which vitamin C first appears in urine. Using data from 17 healthy participants who underwent vitamin C depletion-repletion studies with a vitamin C dose range of 15-1250 mg daily, renal threshold was estimated using physiology-based pharmacokinetics modeling. Applying renal threshold 95% CIs, we estimated the minimal elimination threshold, the plasma concentration below which no vitamin C was expected in urine of healthy people. Renal leak was defined as abnormal presence of vitamin C in urine with plasma concentrations below the minimal elimination threshold. Criteria were tested in a cross-sectional cohort study of individuals with diabetes (82) and nondiabetic controls (80) using matched plasma and urine samples.

RESULTS

Vitamin C renal thresholds in healthy men and women were [mean (SD)] 48.5 (5.2) µM and 58.3 (7.5) µM, respectively. Compared with nondiabetic controls, participants with diabetes had significantly higher prevalence of vitamin C renal leak (9% compared with 33%; OR: 5.07; 95% CI: 1.97, 14.83; P < 0.001) and 30% lower mean plasma vitamin C concentrations (53.1 µM compared with 40.9 µM, P < 0.001). Fasting plasma glucose, glycosylated hemoglobin A1c, BMI, micro/macrovascular complications, and protein/creatinine ratio were predictive of vitamin C renal leak.

CONCLUSIONS

Increased prevalence of vitamin C renal leak in diabetes is associated with reduced plasma vitamin C concentrations. Glycemic control, microvascular complications, obesity, and proteinuria are predictive of renal leak.

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

Objectives

Reduced plasma vitamin C concentrations in chronic diseases may result from abnormal urinary excretion of vitamin C excretion: a renal leak. We hypothesized that vitamin C renal leak may be the consequence of disease-mediated dysregulation affecting the renal tubules, resulting in aberrant vitamin C renal reabsorption and increased urinary loss. The study objective was to investigate the prevalence, clinical characteristics, and genomic associations of vitamin C renal leak in Fabry disease, an X-linked lysosomal storage disease associated with renal tubular dysfunction and low plasma vitamin C concentration.

Methods

This was a non-randomized cross-sectional cohort study of males with Fabry disease (n = 34) and healthy male controls (n = 33). To determine primary outcome of vitamin C renal leak, matched urine and fasting plasma vitamin C measurements were obtained following an overnight fast. Based on data from healthy men, 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. Proof-of-concept studies were conducted using two mouse models.

Results

Compared with healthy men, Fabry cohort had 16-fold higher odds of renal leak (6% vs 52%: OR16, P < 0.001). Renal leak was associated with higher protein creatinine ratio (p = 0.01), lower hemoglobin (p = 0.002) and hematocrit (p = 0.008). Renal leak, but not plasma vitamin C, was associated with a non-synonymous single nucleotide polymorphism in vitamin C transporter SLC23A1 (p = 0.01 and p = 0.47 respectively). Using two mouse models, we recapitulated the relationships between renal leak, dysregulated vitamin C renal reabsorption and low plasma vitamin C concentrations.

Conclusions

Increased prevalence of vitamin C renal leak in Fabry disease may result from dysregulated vitamin C renal physiology, with significant clinical and genomic associations. Renal leak may be more sensitive than plasma vitamin C in evaluating genomic associations in small cohorts.

Funding Sources

Intramural Research Program, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda MD 20,892–1372.

Vitamin E catabolism in women, as modulated by food and by fat, studied using 2 deuterium-labeled α-tocopherols in a 3-phase, nonrandomized crossover study

BACKGROUND

Human vitamin E (α-tocopherol) catabolism is a mechanism for regulating whole-body α-tocopherol.

OBJECTIVES

To determine the roles of the intestine and liver on α-tocopherol catabolism as affected by fat or fasting, 2 deuterium-labeled (intravenous d6- and oral d3-) forms of α-tocopherol were used.

METHODS

Healthy women received intravenous d6-α-tocopherol and consumed d3-α-tocopherol with a 600-kcal defined liquid meal (DLM; 40% or 0% fat, n = 10) followed by controlled meals; or the 0% fat DLM (n = 7) followed by a 12-h fast (0% fat-fast), then controlled meals ≤72 h. The order of the 3-phase crossover design was not randomized and there was no blinding. Samples were analyzed by LC/MS to determine the α-tocopherol catabolites and α-carboxyethyl hydroxychromanol (α-CEHC) in urine, feces, and plasma that were catabolized from administered oral d3- and intravenous d6-α-tocopherols.

RESULTS

Urinary and plasma d3- and d6-α-CEHC concentrations varied differently with the interventions. Mean ± SEM cumulative urinary d6-α-CEHC derived from the intravenous dose excreted over 72 h during the 40% fat (2.50 ± 0.37 μmol/g creatinine) and 0% fat (2.37 ± 0.37 μmol/g creatinine) interventions were similar, but a ∼50% decrease was observed during the 0% fat-fast (1.05 ± 0.39 μmol/g creatinine) intervention (compared with 0% fat, P = 0.0005). Cumulative urinary d3-α-CEHC excretion was not significantly changed by any intervention. Total urinary and fecal excretion of catabolites accounted for <5% of each of the administered doses.

CONCLUSIONS

Differential catabolism of the intravenous d6-α-tocopherol and oral d3-α-tocopherol doses shows both liver and intestine have roles in α-tocopherol catabolism. During the 40% fat intervention, >90% of urinary d3-α-CEHC excretion was estimated to be liver-derived, whereas during fasting <50% was from the liver with the remainder from the intestine, suggesting that there was increased intestinal α-tocopherol catabolism while d3-α-tocopherol was retained in the intestine in the absence of adequate fat/food for α-tocopherol absorption.This trial was registered at clinicaltrials.gov as NCT00862433.

SUN-555 Vitamin E Sequestration by Liver Fat in Vitro and in Women with Hepato-Steatosis

BACKGROUND: The global obesity epidemic has sobering consequences to human health. Especially concerning is obesity-associated hepato-steatosis (HS), a common cause of chronic liver disease in the Americas and Western Europe that precedes non-alcoholic steatohepatitis (NASH). Maintenance of normal body weight is the only current means to prevent HS and NASH. We hypothesized that excess liver fat in obesity-associated HS could act as a pathophysiologic chemical depot for fat-soluble vitamins and alter normal physiology. Because clinical trials with Vitamin E (α-T) have shown that NASH partially responds to this supplement, we selected α-T as a model vitamin to test the sequestration hypothesis.

INTERVENTIONS: Under an IND and IRB-approved protocol, two deuterium-labeled α-tocopherols (d₃-α-T and d₆-α-T) were administered orally and intravenously, respectively, to 10 healthy women and 6 women with HS. Serial blood samples obtained over 72 h were analyzed by LC-MS/MS. In parallel, we performed studies in hepatocytes in cell culture and mouse model.

RESULTS: In healthy women who received oral d₃- and intravenous d₆-α-T, 85% of the initial plasma peak d₆-α-T disappeared within 20 minute and reappeared in the plasma peaking between 6-8 h. Compared to healthy subjects, subjects with HS had similar d₆-α-T entry rates into liver, but reduced release rates into plasma (p<0.001). Similarly, pharmacokinetics parameters (AUC and Maximum Concentration [C_max]), were reduced (AUC_0-8,p<0.01;C_max p<0.02) in HS subjects, indicating reduced hepatic d₆-α-T output. Consistently, livers of mice fed with a high fat diet (42% fat) had more vitamin E compared to controls diet (5% fat), with both diets having the same α-T content

CONCLUSION: These findings suggest the unique role of the liver in vitamin E physiology which is dysregulated by excess liver fat (measured by magnetic resonance spectroscopy). Considered together, the findings imply that obesity-associated HS may produce unrecognized hepatic α-T sequestration, which might subsequently drive liver disease. The data here raise the intriguing possibility that timely α-T supplementation might attenuate progression of HS to NASH, perhaps by correcting an unrecognized fat-induced, localized, hepatic vitamin E deficiency prior to onset of inflammation, hepatitis, and fibrosis. Additionally, our findings raise the possibility that HS may similarly alter hepatic physiology of other fat-soluble vitamins.

Vitamin E absorption and kinetics in healthy women, as modulated by food and by fat, studied using 2 deuterium-labeled α-tocopherols in a 3-phase crossover design

BACKGROUND

Determining the human vitamin E [α-tocopherol (α-T)] requirement is difficult, and novel approaches to assess α-T absorption and trafficking are needed.

OBJECTIVE

We hypothesized that the dual-isotope technique, using 2 deuterium-labeled [intravenous (IV) d6- and oral d3-] α-T, would be effective in determining α-T fractional absorption. Further, defined liquid meal (DLM) fat or fasting would modulate α-T fractional absorption and lipoprotein transport.

METHODS

A 3-phase cr ossover design was used. At 0 h, participants received IV d6-α-T and consumed d3-α-T with a 600-kcal DLM (40% or 0% fat) followed by controlled meals or by the 0% fat DLM, a 12-h fast, and then controlled meals. Blood samples and fecal samples were collected at intervals and analyzed by LC-MS. Pharmacokinetic parameters were calculated from plasma tracer concentrations and enrichments. Fractional absorption was calculated from d3- to d6-α-T areas under the curve, from a novel mathematical model, and from the balance method (oral d3-α-T minus fecal d3-α-T excreted).

RESULTS

Estimated α-T fractional absorption during the 40% fat intervention was 55% ± 3% (mean ± SEM; n = 10), which was 9% less than during the 0% fat intervention (64% ± 3%, n = 10; P < 0.02). Fasting had no apparent effect (56% ± 3%, n = 7), except it slowed plasma oral d3-α-T appearance. Both balance data and model outcomes confirmed that the DLM fat did not potentiate d3-α-T absorption. During the IV emulsion clearance, HDL rapidly acquired d6-α-T (21 ± 2 nmol/L plasma per minute). During the first 8 h postdosing, triglyceride-rich lipoproteins (TRLs) were preferentially d3-α-T enriched relative to LDL or HDL, showing the TRL precursor role.

CONCLUSIONS

Quantitatively, α-T absorption is not limited by fat absence or by fasting. However, α-T leaves the intestine by a process that is prolonged during fasting and potentiated by eating, suggesting that α-T absorption is highly dependent on chylomicron assembly processes. This trial was registered at clinicaltrials.gov as NCT00862433.

Vitamin C transporter Slc23a1 links renal reabsorption, vitamin C tissue accumulation, and perinatal survival in mice

Levels of the necessary nutrient vitamin C (ascorbate) are tightly regulated by intestinal absorption, tissue accumulation, and renal reabsorption and excretion. Ascorbate levels are controlled in part by regulation of transport through at least 2 sodium-dependent transporters: Slc23a1 and Slc23a2 (also known as Svct1 and Svct2, respectively). Previous work indicates that Slc23a2 is essential for viability in mice, but the roles of Slc23a1 for viability and in adult physiology have not been determined. To investigate the contributions of Slc23a1 to plasma and tissue ascorbate concentrations in vivo, we generated Slc23a1-/- mice. Compared with wild-type mice, Slc23a1-/- mice increased ascorbate fractional excretion up to 18-fold. Hepatic portal ascorbate accumulation was nearly abolished, whereas intestinal absorption was marginally affected. Both heterozygous and knockout pups born to Slc23a1-/- dams exhibited approximately 45% perinatal mortality, and this was associated with lower plasma ascorbate concentrations in dams and pups. Perinatal mortality of Slc23a1-/- pups born to Slc23a1-/- dams was prevented by ascorbate supplementation during pregnancy. Taken together, these data indicate that ascorbate provided by the dam influenced perinatal survival. Although Slc23a1-/- mice lost as much as 70% of their ascorbate body stores in urine daily, we observed an unanticipated compensatory increase in ascorbate synthesis. These findings indicate a key role for Slc23a1 in renal ascorbate absorption and perinatal survival and reveal regulation of vitamin C biosynthesis in mice.

Cognitive dysfunction in patients treated for pituitary tumours

Assessment of neuropsychological status and mood was carried out on 36 patients who had been treated for pituitary tumour and on 36 healthy controls. Impairments in memory and executive function were exhibited by the patients even when other known causes of cognitive dysfunction had been excluded. There was no difference in mood between the two groups, and the deficits in cognitive dysfunction were not related to mood disturbance or to the effects of radiotherapy. The defects were however, related to the presence of surgery, although not to the type of surgery. Reasons for the cognitive dysfunction are unclear but are likely to be multifactorial possibly including the effects of neurosurgery and/or hormone imbalance resulting from pituitary surgery.