Vitamin C has several actions that provide a basis for its use in diabetes. It has been reported to lower erythrocyte sorbitol concentrations (important for preventing © 2007 Elsevier Australia
complications in type 1 diabetes), improve endothelial function (important for slowing atherosclerosis) and reduce blood pressure (Beckman et al 2001, Cunningham 1998). Plasma vitamin C levels seem to play a role in the modulation of insulin activity in aged healthy or diabetic subjects (Paolisso et al 1994) and are inversely related to those of glycosylated haemoglobin. Additionally, free radical production has been reported to be increased in patients with diabetes mellitus as a result of hyperglycaemia, which directly induces oxidative stress (Ceriello et al 1998). Blood glucose Studies testing the effects of supplemental vitamin C on plasma glucose levels directly are few at this stage. Although one early study demonstrated that an oral dose of 1500 mg vitamin C reduces plasma glucose levels in patients with type 2 diabetes (Sandhya & Das 1981), there are no further published studies that confirm this result.
Endothelial function The results of studies investigating the role of vitamin C on endothelial function in diabetes have attracted recent interest.
A double-blind, placebo-controlled study demonstrated that chronic oral vitamin C supplementation (500 mg/day) in type 2 diabetes significantly lowered arterial blood pressure and improved arterial stiffness compared with placebo (Mullan et al 2002). After 1 month's treatment, SBPfell from 142.1 to 132.3 mmHg, mean pressure from 104.7 to 97.8 mmHg, DBP from 83.9 to 79.5 mmHg and peripheral pulse pressure from 58.2 to 52.7 mmHg, whereas placebo had no effect.
A randomised study of women with a history of gestational diabetes showed that ascorbic acid supplementation resulted in a significant improvement of endothelium-dependent flow-mediated dilatation, with no effect seen for placebo (Lekakis et al 2000).
The mechanism of action appears to involve several steps, such as reduction in LDL oxidation, enhanced endothelial NO synthase activity and NO bioavailability, and reduced insulin resistance, which can cause endothelium-dependent, NO-mediated vasodilation.
Eye health Diabetes mellitus is associated with a number of ocular complications that can eventually lead to blindness. Vitamin C is found in high concentration in the eye and is thought to be important for protection against free radicals. This may have special significance for people with diabetes mellitus, as most studies have found circulating vitamin C levels at least 30% lower than people without the disease (Peponis et al 2002). One study demonstrated that a combination of oral vitamins C (1000 mg/day) and E (400 lU/day) improved tear film stability, tear secretion, and health of the ocular surface in diabetic patients. Vitamin C 1289
Due to the safety, cost effectiveness and generally encouraging results, a strategy of adding 200-600 mg of vitamin C to a healthy diet is worth considering for individuals with type 1 or 2 diabetes.
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