Study of the Effects of Vitamin C on Patients With Type 2 Diabetes

Diabetes is a disease characterized by decreased sensitivity to the action on insulin to promote sugar (glucose) use and blood vessel relaxation (vasodilation) in muscle. Insulin's ability to cause blood vessel relaxation is controlled, in part, by nitric oxide (NO).

Nitric oxide is a substance produced by the cells lining blood vessel walls (endothelium). Increased blood flow to the muscle accounts for increased sugar (glucose) to areas of the body. Therefore, if the cells of blood vessel walls (endothelium) are not functioning properly it may contribute to insulin resistance.

Injections of Vitamin C directly into the arteries have been shown to improve blood vessel reaction to nitric oxide in diabetic patients. Researchers believe this may be due to Vitamin C's ability to increase the levels of nitric oxide in blood vessels.

The goal of this study is to determine the effects of vitamin C on both insulin sensitivity and endothelium function of patients with type 2 diabetes. An additional goal of the study is to determine the effects of vitamin C on patients with vitamin C deficiency.

Patients participating in this study will undergo a series of testes to determine insulin sensitivity and blood vessel reactivity. Patients will be divided into two groups. One group will receive doses of oral vitamin C. The other group will receive doses of a placebo (inactive pill not containing vitamin C). Patients will receive the medications for four weeks and then be tested again for insulin sensitivity and blood vessel reactivity.

Researchers believe that doses of vitamin C in diabetics or patients with vitamin C deficiency will improve insulin sensitivity and function of endothelium. Results gathered form this study may provide information about vitamin C levels in diabetics and may lead to the development of new therapies to treat insulin resistance and endothelium dysfunction.

Diabetes is characterized by decreased sensitivity to the actions of insulin to promote both glucose utilization and vasodilation in skeletal muscle beds. Insulin's vasodilator action is mediated, in part, by endothelial-derived nitric oxide (NO). Increased blood flow accounts for approximately 25% of the increase in skeletal muscle glucose disposal mediated by insulin. Therefore, endothelial dysfunction may contribute to insulin resistance. Intraarterial administration of vitamin C improves NO-dependent vascular reactivity in diabetic subjects (but not normal subjects). This may be due to antioxidant properties of vitamin C that result in relative increases in the level of NO in the diabetic vasculature. In this exploratory protocol, our primary objective is to assess the effects of oral administration of vitamin C on both insulin sensitivity and endothelial function in subjects with type 2 diabetes. A secondary, peripheral objective, is to study these effects in vitamin C-deficient clinical research volunteers. Hyperinsulinemic euglycemic glucose clamp procedures and forearm blood flow measurements will be used to assess both insulin sensitivity and vascular reactivity in diabetic subjects and clinical research volunteers who have plasma vitamin C levels less than 30 microM. The subjects will then be given either placebo or oral vitamin C supplementation (800 mg/day) for four weeks and assessment of insulin sensitivity and vascular reactivity will be repeated. Plasma levels of vitamin C will be measured to confirm that subjects in the experimental group have an appropriate increase in vitamin C levels. We hypothesize that chronic oral administration of vitamin C to diabetic or clinical research volunteers who are deficient in vitamin C will improve insulin sensitivity and endothelial function. Our study will provide information about vitamin C levels in diabetic subjects and may suggest a potential therapy to significantly improve endothelial dysfunction and insulin resistance.