Abnormalities of ascorbic acid metabolism and diabetic control: differences between diabetic patients and diabetic rats

Ascorbic acid is required in the synthesis of collagen and is also an important anti-oxidant. In a previous study, plasma ascorbic acid concentration was found to be decreased in diabetic patients but there was no relationship with blood glucose level. In the current study of diabetic patients, both plasma ascorbic acid and its urinary excretion correlated inversely with glycosylated hemoglobin level. Plasma ascorbic acid was also lower in diabetic rats but urinary ascorbic acid was elevated. The divergent trend in urinary ascorbic acid excretion observed in diabetic patients and diabetic rats may be due to difference in the ability of these two species to synthesize ascorbic acid. Difference in renal reabsorption of ascorbic acid may also be a relevant factor. The lower plasma and urinary ascorbic acid levels in diabetic patients with more severe hyperglycaemia indicates that this group of patients is particularly at risk of developing deficiency of this vitamin. As ascorbic acid has many important functions in the body, it may be necessary to supplement this vitamin in patients with chronically poorly controlled diabetes.

The effects of dietary n - 3 fatty acid in animal models of type 1 and type 2 diabetes

We studied the incorporation of dietary n - 3 fatty acids from marine oils into red cell membranes, using animal models of type 1 diabetes (streptozotocin-treated Wistar rats) and type 2 diabetes (gold-thioglucose-injected CBA/T6 mice). In type 1 diabetic rats, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) were higher following marine oil supplementation, and the increase in EPA was significantly greater than in non-diabetic animals (3.4 +/- 1.4% vs. 0.8 +/- 1.6%). Marine oil supplementation also resulted in higher levels of EPA and DHA in mice, but the increases were quantitatively similar with and without type 2 diabetes. Improvement in glycosylated haemoglobin following n - 3 fatty acid feeding was only found in type 2 diabetes (6.5 +/- 2.9% vs. 9.5 +/- 1.2%). This was associated with a higher plasma insulin concentration (170 +/- 40 vs. 136 +/- 41 mU/ml). The theory that n - 3 fatty acids improve insulin sensitivity would have predicted a decrease in plasma insulin levels. Our results suggest that n - 3 fatty acids may modify insulin secretion.

The effect of improved control on blood filtration properties and non-enzymatic glycosylation of erythrocyte proteins in type 2 diabetes

Abnormal blood flow in the microcirculation has been reported in diabetes and may be important in the pathogenesis of diabetic complications. The mechanism of this is not understood but non-enzymatic glycosylation of erythrocyte membrane or haemoglobin causing reduced erythrocyte deformability and a secondary change in filtration properties of blood have been suggested as possible factors. The relationship of non-enzymatic glycosylation of erythrocyte membrane and glycosylated haemoglobin to filtration time of blood was investigated during stabilization of diabetes with sulphonylurea therapy. Over an 8-month period, glycosylated haemoglobin, non-enzymatic glycosylation of erythrocyte membrane, and filtration time fell by 41%, 71%, and 53% of the initial value, respectively, but the rate of decline was slower for filtration time which did not change significantly until the last month. Due to the different time-course of improvement, no relationship was found between filtration time and glycosylated haemoglobin or non-enzymatic glycosylation of erythrocyte membrane whereas glycosylated haemoglobin and non-enzymatic glycosylation of erythrocyte membrane correlated significantly (r = 0.49, p less than 0.001). These results suggest that the abnormal filtration property of blood in diabetes is not a direct consequence of non-enzymatic glycosylation, and suggest that the erythrocytes made in the hyperglycaemic milieu are abnormally rigid. The filtration properties of blood are only improved when new generations of erythrocytes enter the circulation.

Ascorbic acid metabolism and polyol pathway in diabetes

It has been reported previously that the plasma concentration of ascorbic acid (AA) is reduced in streptozocin-induced diabetic rats and can be normalized by treatment with the aldose reductase inhibitor tolrestat. This study was designed to investigate further the relationship between the polyol pathway and AA metabolism in diabetic rats. Disturbance of AA metabolism was demonstrable after 1 wk of diabetes. Dietary myo-inositol supplementation was effective in normalizing plasma AA levels, as was treatment with tolrestat. In untreated diabetes, despite low plasma AA concentration, there was increased urinary excretion of AA that was reversed by treatment with either tolrestat or myo-inositol. In contrast, AA supplementation normalized plasma AA concentrations while further increasing urinary AA excretion. The abnormality of AA metabolism was less severe in galactose-fed rats, which had normal plasma AA levels and only minor increases in urinary AA excretion. These studies demonstrated a disturbance in the regulation of plasma and urinary AA concentration in experimental diabetes and confirmed the relationship of AA with the polyol pathway. Because AA has many important biological functions, abnormalities of AA metabolism could be important in the pathogenesis of some diabetic complications. The interaction of the polyol and AA pathways suggests that this could be another site of action for aldose reductase inhibitors.

Deficiency of ascorbic acid in experimental diabetes. Relationship with collagen and polyol pathway abnormalities

The plasma and tissue concentration of ascorbic acid (AA) is reduced in diabetes. This study was designed to investigate the mechanism and significance of this phenomenon. The low plasma AA concentration of diabetic rats can be normalized by dietary AA supplement (20-40 mg/day), a dosage approximately equal to the maximal synthetic rate of this substance in the rats. Treatment of diabetic rats with this regime prevented the decrease in activity of granulation tissue prolyl hydroxylase (PRLase), an AA-dependent enzyme required for maintaining the normal properties of collagen. The decreased plasma AA concentration and granulation tissue PRLase activity in diabetes can also be normalized by the aldose reductase inhibitor tolrestat. We conclude that in diabetic animals there is a true deficiency of AA that may be responsible for some of the changes of collagen observed in diabetes. Treatment with AA or an aldose reductase inhibitor may prevent some of the diabetic complications with underlying collagen abnormalities.