Vitamin C and Red Blood Cell Function in Diabetes: A Study on Glycemic Control and Supplementation

Diabetes type two is a debilitating disease that leads to chronic morbidity such as accelerated microvascular disease. Accelerated microvascular disease may produce blindness, end stage renal disease, myocardial infarction, stroke, and limb ischemia. Strategies to prevent or delay microvascular disease have the potential to improve the lives of millions and prevent catastrophic illness. The major focus of prevention of microvascular disease in diabetes has been on the endothelium and its role in protection of blood vessels. An unexpected means to prevent microvascular disease in diabetes may be coupled to the function of vitamin C in red blood cells (RBCs) of diabetic subjects. Based on new and emerging data, vitamin C concentrations in RBCs may be inversely related to glucose concentrations found in diabetes. Based on animal data, we hypothesize that RBCs with low vitamin C levels may have decreased deformability, leading to slower flow in capillaries and microvascular hypoxia, the hallmark of diabetic microangiopathy. Low vitamin C concentrations in RBCs of diabetic subjects may be able to be increased, by using vitamin C supplements. Findings in animals may not accurately reflect effects in humans because of species differences in mechanisms of vitamin C entry into RBCs. Therefore, clinical research is essential to characterize vitamin C physiology in RBCs of diabetic subjects. In this protocol we will investigate physiology of vitamin C in RBCs of diabetic subjects as a function of glycemia, without vitamin C supplementation (stage 1) and with vitamin C supplementation (stage 2). We will screen type II diabetic subjects on insulin and/or oral hypoglycemic medication(s) and select those with hemoglobin A1C concentrations of <= 12%. To investigate how response to the nutritional interventions in individuals with diabetes varies from normal, nondiabetic controls will also be recruited and studied. Selected subjects will be hospitalized twice, each time for approximately one week. The primary objective of the first hospitalization (stage 1) will be to evaluate the effect of hyperglycemia on vitamin C RBC physiology regardless of baseline vitamin C concentrations (without any vitamin C supplementation). The second hospitalization (stage 2) investigates the effect (if any) of vitamin C supplementation to changes in RBC physiology during periods of normal (euglycemic) and elevated (hyperglycemic) glucose concentrations. As inpatients, subjects will have two venous sampling periods each of approximately 24 hours. On admission, subjects may be fitted with continuous glucose monitors (CGMs), subjects will be transitioned to an individualized inpatient diabetes regimen determined by investigators, based on pre-admission diabetes regimen and glycemic control. For participants with diabetes, the inpatient diabetes regimen will be titrated to achieve euglycemia (fasting and pre-meal glucoses <140mg/dl) prior to the first sampling period (euglycemic sampling). The first sampling period will be performed under conditions of euglycemic control for 24 hours. The second sampling period will be performed under controlled hyperglycemia induced by decreasing doses of the diabetes regimen and providing a high carbohydrate load diet (70-75% carbohydrate). Correction-scale insulin will be provided for glucoses >350-400mg/dl. Hyperglycemia will not exceed 9 hours and will be reversed by reinstituting insulin. For nondiabetic controls, an oral glucose tolerance test (75 grams dextrose) will be administered on admission. Controls will receive the same metabolic diets and undergo the sampling schedule as the cohort with diabetes. During the two sampling periods, samples will be withdrawn via venous catheter for RBC deformability, vitamin C concentrations and other related research studies. Following completion of stage 1, subjects considered for participation in stage 2 will be provided a prescription for vitamin C 500mg twice daily. Given that vitamin C and vitamin E are related antioxidants, and that both vitamins appear to be associated with RBC rigidity, diabetic subjects may also be given a prescription for 400 international units (IU) of vitamin E (RRR alpha tocopherol) daily. Subjects will continue vitamin C and E supplementation for a minimum of 8 weeks depending on RBC vitamin C concentrations. To evaluate any effect of vitamin E supplementation, plasma and RBC vitamin E levels may be measured concurrently with vitamin C levels during various phases of stages 1 and 2. All subjects will be seen as outpatients at biweekly or monthly intervals with regular measurement of plasma and/or RBC vitamin C concentrations. Target RBC vitamin C concentration >30uM is required prior to stage 2 inpatient sampling studies. Vitamins C and E supplementation will be discontinued upon inpatient admission for stage 2. Risk of both vitamin supplements are minimal as both supplementation doses are safe. Outcomes are to measure RBC rigidity and vitamin concentrations before and after supplementation. After a minimum of 8 weeks (depending on RBC vitamin C levels), subjects will be hospitalized again, and sampling repeated as described. In this manner, each subject serves as his/her own control, and deformability of red blood cells can be determined in relation to glycemia and to vitamin C concentrations in RBCs and plasma. Subjects will be required to consume standardized meals during inpatient stays. All meals will be prepared by the NIH Clinical Center Metabolic Kitchen. To avoid obscuring plasma vitamin C changes that may result from hyperglycemia, dietary vitamin C content will be approximately 30-35 mg per meal. Additionally, to avoid confounding vitamin E measurements, diets will provide approximately 6 mg alpha tocopherol per day. Standardized meals at the 2nd inpatient admission will be provided to match what was consumed by the subject at their 1st inpatient admission.