Urinary Vitamin C Loss in Diabetic Subjects

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.