title: 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.
abstract: 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 +/- 2 microM) levels. Thirty-two diabetic subjects with low plasma vitamin C (<40 microM) were subsequently enrolled in a randomized, double-blind, placebo-controlled study of vitamin C (800 mg/day for 4 wk). Insulin sensitivity (determined by glucose clamp) and forearm blood flow in response to ACh, sodium nitroprusside (SNP), or insulin (determined by plethysmography) were assessed before and after 4 wk of treatment. In the placebo group (n = 17 subjects), plasma vitamin C (22 +/- 3 microM), fasting glucose (159 +/- 12 mg/dl), insulin (19 +/- 7 microU/ml), and SI(Clamp) [2.06 +/- 0.29 x 10(-4) dl x kg(-1) x min(-1)/(microU/ml)] did not change significantly after placebo treatment. In the vitamin C group (n = 15 subjects), basal plasma vitamin C (23 +/- 2 microM) increased to 48 +/- 6 microM (P < 0.01) after treatment, but this was significantly less than that expected for healthy subjects (>80 microM). No significant changes in fasting glucose (156 +/- 11 mg/dl), insulin (14 +/- 2 microU/ml), SI(Clamp) [2.71 +/- 0.46 x 10(-4) dl x kg(-1) x min(-1)/(microU/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.
authors.affiliation: Diabetes Unit, National Center for Complementary and Alternative Medicine, NIH, Bldg. 10, Rm. 6C-205, 10 Center Dr. MSC 1632, Bethesda, MD 20892-1632, USA.
authors.forename: Hui
authors.initials: H
authors.lastname: Chen
authors.forename: Rajaram J
authors.initials: RJ
authors.lastname: Karne
authors.forename: Gail
authors.initials: G
authors.lastname: Hall
authors.forename: Umberto
authors.initials: U
authors.lastname: Campia
authors.forename: Julio A
authors.initials: JA
authors.lastname: Panza
authors.forename: Richard O
authors.initials: RO
authors.lastname: Cannon
authors.forename: Yaohui
authors.initials: Y
authors.lastname: Wang
authors.forename: Arie
authors.initials: A
authors.lastname: Katz
authors.forename: Mark
authors.initials: M
authors.lastname: Levine
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Quon
mesh_terms: D000284:Administration, Oral; D000328:Adult; D001205:Ascorbic Acid / Q000008:administration & dosage* / Q000097:blood*; D001794:Blood Pressure / Q000187:drug effects; D003924:Diabetes Mellitus, Type 2 / Q000097:blood / Q000188:drug therapy*; D004730:Endothelium, Vascular / Q000187:drug effects / Q000503:physiopathology*; D005542:Forearm / Q000098:blood supply; D015309:Glucose Clamp Technique; D006801:Humans; D007333:Insulin Resistance; D008875:Middle Aged; D010919:Placebos; D014664:Vasodilation / Q000187:drug effects
chemical_list: D010919:Placebos; D001205:Ascorbic Acid