Overview

Several studies have reported that diabetic subjects have lower plasma vitamin C concentrations than non-diabetic subjects. Although urinary vitamin C loss in diabetic subjects was reported to be increased in two studies, these are difficult to interpret due to lack of controlled vitamin C intake, inadequate sampling, lack of control subjects, or methodology uncertainties in vitamin C assay and sample processing. Consequently, it is unclear whether diabetic subjects truly have both low plasma and high urine vitamin C concentrations. We propose that low plasma vitamin C concentrations in diabetic subjects are due in part to inappropriate renal loss of vitamin C in these subjects but not in healthy controls. We will study vitamin C concentrations in patients with type 1 and type 2 diabetes and in matched healthy research subjects. Vitamin C concentrations in plasma, neutrophils (as a proxy for tissue concentrations) and in urine will be measured in outpatients. In those willing to be admitted to the Clinical Center, we will measure 24-hour urinary excretion of vitamin C while on a vitamin C free diet, and creatinine clearance, a measure of glomerular filtration rate. On day 2 of the inpatient study, subjects will receive a single 200mg dose of oral vitamin C and we will measure vitamin C concentrations in frequent blood and urine samples to determine the renal threshold and relative bioavailability for vitamin C. Single nucleotide polymorphisms (SNPs) will be determined in genomic DNA responsible for the two proteins mediating sodium-dependent vitamin C transport, SVCT1 and SVCT2. If low plasma and high urine vitamin C concentrations are found in diabetic subjects, further studies will be needed to explore mechanisms and to determine recommended dietary allowances for this patient population.

Eligibility

• INCLUSION CRITERIA:

        We propose to study of five thousand male and female subjects between the ages of 18 and
        65. This will include healthy subjects and subjects with type 1 or type 2 diabetes. To be
        included in the study, study subjects should
          -  be in good general health
          -  have no significant illnesses that compromise clinical stability other than the
             complications of diabetes mellitus alone or in the context of metabolic syndrome.
             Subjects with ischemic heart disease and/or peripheral artery disease are eligible for
             arm 1 of the protocol.
          -  have serum creatinine < 2.5
          -  for healthy volunteers, be normotensive at the time of the study, with a blood
             pressure less than or equal to 140/90
          -  for diabetic subjects, blood pressure less than or equal to 170/95 as long as
             clinically stable and in usual state of health, for example, no chest pain, shortness
             of breath, headache, syncope or fatigue
        The aim of this study is to determine whether diabetic subjects lose vitamin C in the urine
        in their normal clinical condition (i.e. while on treatment) and not in the native
        untreated state of uncontrolled hyperglycemia. Therefore the patients will not discontinue
        medication.
        EXCLUSION CRITERIA (for arm 1):
        Exclusion criteria will include the following:
          -  significant organ malfunction leading to clinical instability including liver disease,
             pulmonary disease, stroke and anemia at investigator discretion
          -  serious or chronic illness or history of serious or chronic illness resulting in
             clinical instability other than complications of diabetes
          -  pregnancy
          -  alcohol abuse, drug addiction or the use of illegal drugs
          -  positive HIV or hepatitis (B or C) screening tests (subjects will be notified of these
             test results).
          -  presence of other concomitant conditions which in the judgment of the investigators
             can influence vitamin C metabolism or vitamin C renal handling
        EXCLUSION CRITERIA (for arms 2 and 3):
        Exclusion criteria will include the following:
          -  significant organ malfunction leading to clinical instability including liver disease,
             pulmonary disease, ischemic heart disease, heart failure, stroke, peripheral vascular
             disease, and anemia at investigator discretion
          -  other serious or chronic illness; history of serious or chronic illness; coronary
             artery disease, or peripheral vascular disease resulting in clinical instability
          -  pregnancy
          -  alcohol abuse, drug addiction or the use of illegal drugs
          -  positive HIV or hepatitis (B or C) screening tests (subjects will be notified of these
             test results).
          -  presence of other concomitant conditions which in the judgment of the investigators
             can influence vitamin C metabolism or vitamin C renal handling
        For inpatient subjects, an additional exclusion criterion is consumption during the
        hospitalization of any foods or beverages other than those in the vitamin C free diet.