High-dose oral vitamin C partially replenishes vitamin C levels in patients with Type 2 diabetes and low vitamin C levels but does not improve endothelial dysfunction or insulin resistance

Endothelial dysfunction is a hallmark of Type 2 diabetes related to hyperglycemia and oxidative stress. Nitric oxide-dependent vasodilator actions of insulin may augment glucose disposal. Thus endothelial dysfunction may worsen insulin resistance. Intra-arterial administration of vitamin C improves endothelial dysfunction in diabetes. In the present study, we investigated effects of high-dose oral vitamin C to alter endothelial dysfunction and insulin resistance in Type 2 diabetes. Plasma vitamin C levels in 109 diabetic subjects were lower than healthy (36 [+ or -] 2 [micro]M) levels. Thirty-two diabetic subjects with low plasma vitamin C (80 [micro]M). No significant changes in fasting glucose (156 [+ or -] 11 mg/dl), insulin (14 [+ or -] 2 [micro]U/ml), [SI.sub.clamp] [2.71 [+ or -] 0.46 x [10.sup.-4] dl x [kg.sup.-1] x [min.sup.-1]/([micro]U/ml)], or forearm blood flow in response to ACh, SNP, or insulin were observed after vitamin C treatment. We conclude that high-dose oral vitamin C therapy, resulting in incomplete replenishment of vitamin C levels, is ineffective at improving endothelial dysfunction and insulin resistance in Type 2 diabetes. hypertension; hyperglycemia; hypercholesterolemia; insulin sensitivity; sodium nitroprusside