Plasma ascorbate concentrations and blood cell dehydroascorbate transport in patients with diabetes mellitus

Oxidized LDL and antioxidants

Several studies have shown that the oxidative modification of low-density lipoprotein (LDL) may promote fatty-streak formation, the early lesion of atherosclerosis. Epidemiologic data suggest that decreased levels of micronutrient antioxidants are associated with an increased frequency of cardiovascular disease. Micronutrients that can act as antioxidants--ascorbic acid, alpha-tocopherol, and beta-carotene--have been shown to prevent LDL oxidation in vitro and retard the progression of atherosclerosis in animal models. In addition, supplementation of human subjects with these compounds have been shown to increase the resistance of their LDL to oxidation without producing side effects. Thus, these antioxidant micronutrients have the potential to become an additional treatment modality in the therapeutic arsenal against atherosclerosis.

Origin of the microangiopathic changes in diabetes

The mechanism of development of microangiopathy is incompletely understood, but relates to a number of ultrastructural, biochemical and haemostatic processes. These include capillary basement membrane thickening, non-enzymatic glycosylation, possibly increased free radical activity, increased flux through the polyol pathway and haemostatic abnormalities. The central feature appears to be hyperglycaemia, which is causally related to the above processes and culminates in tissue ischaemia. This article will briefly describe these processes and will discuss possible pathogenic interactions which may lead to the development of the pathological lesion.

The role of endothelium in the pathogenesis of diabetic microangiopathy

Damage caused to the vessel wall by diverse mechanisms may lead to diabetic microangiopathy. Consequently, research work is more and more focusing on the pathophysiology of vascular cells, with particular emphasis on endothelium. This paper reviews the present knowledge on the alterations of small vessel endothelium in diabetes. The most important risk factors for diabetic microangiopathy are the duration of disease and the degree of metabolic control maintained throughout the years. However, genetic factors may also contribute. These are examined first, followed by the presumed roles played by increased protein glycation and the production of Advanced Glycosylation End Products, the "polyol pathway" and free radical generation. Endothelium is a widespread, extremely active organ which regulates complex physiologic functions and its structure and function are discussed in the second section of this review. The third part deals with how diabetes can affect endothelium and describes observations on endothelial metabolism in vitro as well as morphologic and functional alterations in the patients. Unfortunately, the mechanisms leading to progressive degeneration of the microcirculation and organ damage in diabetic patients remain largely unaccounted for.

Effect of vitamin C on sorbitol in the lens of guinea-pigs made diabetic with streptozotocin

Marginally vitamin C-deficient guinea-pigs treated with the diabetogenic agent streptozotocin were compared with those liberally supplied with vitamin C, for functional indices of vitamin C status, particularly in the eye lens. Weanling male Dunkin-Hartley guinea-pigs were fed on diets with 0.1 g vitamin C/kg (marginally deficient), or 5 g/kg (liberally supplied), and some received intraperitoneal streptozotocin (two doses of 150 mg/kg body-weight). About half the streptozotocin-treated animals had high urinary glucose following an oral glucose dose; these animals also grew more slowly than the others. At 4 months after streptozotocin the animals were killed for measurement of tissue vitamin C, glucose and sorbitol. Streptozotocin moderately increased the concentration of glucose in plasma, lens and aqueous humour. Lens sorbitol levels increased only in the group exposed to streptozotocin plus marginal vitamin C. There was a significant (P less than 0.02) positive correlation between urinary glucose and lens sorbitol levels overall. Liberal vitamin C intake may thus counteract the effect of streptozotocin diabetes on lens sorbitol, suggesting a new function of vitamin C, possibly related to cataractogenesis and to the biochemical lesions associated with diabetes.

Is hyperglycemia risky enough to justify the increased risk of hypoglycemia linked with tight diabetes control?

There is an ongoing debate about the possible disadvantages of human insulin use with respect to a possibly lower awareness of hypoglycemia than is associated with animal insulin usage. Participants in this debate have not, however, discussed a major contributory factor to this life-threatening acute complication of diabetes, the pressure on patients to achieve normal levels of blood glucose. This pressure stems from the view that hyperglycemia is the major causative factor in the long-term diabetic complications. However, the evidence that supranormal levels of tissue and plasma glucose contribute to the diabetic tissue damage is not as strong as the arguments on behalf of this position. Indeed, elevated glycemia may be no more than a crude index of other, unknown metabolic derangements which may be causative agents in diabetes-associated tissue damage. Intensive efforts to "normalize" glycemia lack experimental and clinical justification, distract attention from other possible mechanisms, and may impose an unnecessary risk on the insulin-dependent diabetic population since intensive "normalization" of glycemia lowers hypoglycemia awareness, and thus increases risk of hypoglycemia, irrespective of the type of insulin used.

Pathogenesis of diabetic microangiopathy: an overview

Major susceptibility factors for diabetic microangiopathy include duration of disease and probably quality of metabolic control. The mechanism of development of microangiopathy is incompletely understood but appears to involve functional abnormalities within the microcirculation, enhanced glucose metabolism, hemostatic abnormality, and genetic susceptibility. This article reviews the factors believed to be involved in pathogenesis and attempts to draw these together by suggesting a sequence of pathogenic interactions that could result in the microvascular changes seen in susceptible target organs. Possibilities for therapeutic intervention based on these pathogenic mechanisms are discussed. A small pilot trial of an oral hypoglycemic agent, gliclazide, is reported, providing evidence for a specific action of this drug on thromboxane synthesis and platelet aggregation. This is independent of glycemic control and may in part be mediated by a fall in lipid peroxides.

Disturbed handling of ascorbic acid in diabetic patients with and without microangiopathy during high dose ascorbate supplementation

Abnormalities of ascorbic acid metabolism have been reported in experimentally-induced diabetes and in diabetic patients. Ascorbate is a powerful antioxidant, a cofactor in collagen biosynthesis, and affects platelet activation, prostaglandin synthesis and the polyol pathway. This suggests a possible close interrelationship between ascorbic acid metabolism and pathways known to be influenced by diabetes. We determined serum ascorbic acid and its metabolite, dehydroascorbic acid, as indices of antioxidant status, and the ratio, dehydroascorbate/ascorbate, as an index of oxidative stress, in 20 matched diabetic patients with and 20 without microangiopathy and in 22 age-matched control subjects. Each study subject then took ascorbic acid, 1 g daily orally, for six weeks with repeat measurements taken at three and six weeks. At baseline, patients with microangiopathy had lower ascorbic acid concentrations than those without microangiopathy and control subjects (42.1 +/- 19.3 vs 55.6 +/- 20.0, p less than 0.01, vs 82.9 +/- 30.9 mumol/l, p less than 0.001) and elevated dehydroascorbate/ascorbate ratios (0.87 +/- 0.46 vs 0.61 +/- 0.26, p less than 0.01, vs 0.38 +/- 0.14, p less than 0.001). At three weeks, ascorbate concentrations rose in all groups (p less than 0.0001) and was maintained in control subjects (151.5 +/- 56.3 mumol/l), but fell in both diabetic groups by six weeks (p less than 0.01). Dehydroascorbate/ascorbate ratios fell in all groups at three weeks (p less than 0.0001) but rose again in the diabetic groups by six weeks (p less than 0.001) and was unchanged in the control subjects. Dehydroascorbate concentrations rose significantly from baseline in all groups by six weeks of ascorbic acid supplementation (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Reduced mononuclear leukocyte ascorbic acid content in adults with insulin-dependent diabetes mellitus consuming adequate dietary vitamin C

Several recent studies suggest that vitamin C (ascorbic acid [AA]) status may be altered in insulin-dependent diabetes mellitus (IDDM). We measured the AA content of mononuclear leukocytes (MN-AA) as an indicator of tissue vitamin C status in adults with IDDM and nondiabetic adults matched for age and sex. Dietary vitamin C intake and plasma AA were analyzed to ensure that vitamin C availability was adequate. Dietary vitamin C intakes were above recommendations and were not different between the groups. MN-AA was reduced by 33% on average (P less than .05) in adults with IDDM (1.75 microgram/mg total protein [TP]) when compared with nondiabetics (2.60 micrograms/mg TP). When MN-AA is indexed to the dietary vitamin C intake (MN-AA/100 mg diet C), the storage deficit in adults with IDDM averages 50% (P less than .05). This observation suggests an impaired tissue AA storage in adults with IDDM and supports the theory that intracellular scurvy contributes to the chronic degenerative complications of the disease.

Vitamin C and vitamin E status in the spontaneously diabetic BB rat before the onset of diabetes

Ascorbic acid (AA), dehydroascorbic acid (DHAA), and vitamin E were measured in tissues and plasma of 30 control and 30 spontaneously diabetic BioBreeding rats (BBdp) during development and before the onset of diabetes. At weaning, rats were fed an AIN-76 semisynthetic diet for 30, 64, or 113 days, after which plasma and tissues from 10 rats of each group were collected and analysed for AA, DHAA, and vitamin E. AA and DHAA levels were significantly increased in plasma and spleen of the diabetes-prone rats compared with those of the control group at 30 and 64 days, but the difference disappeared by 113 days. No differences were observed in liver, adrenals, thymus, and pancreas at any of the time periods. However, lower levels of vitamin E were observed in adrenal gland, thymus, and pancreas of the diabetes-prone rats. It is concluded that BBdp rats have an altered metabolism of AA, DHAA, and vitamin E, before the onset of diabetes. These changes could be due to genetic and physiological factors operating during development of this rat strain.

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.

Altered vitamin C transport in diabetes mellitus

Tissues sequester vitamin C in concentrations exceeding that present in plasma. The transmembrane transport mechanisms have been shown to be influenced by the concentration of glucose in vitro. On this basis an impairment of tissue vitamin C status may be present in diabetes mellitus. Recent evidence in support of this hypothesis, first proposed by Mann in 1974, is reviewed.

Antioxidants in sickle cell disease: the in vitro effects of ascorbic acid

The authors examined the ability of antioxidants to prevent in vitro oxidant damage to the sickle red blood cell (RBC). One millimolar ascorbic acid and alpha-mercaptopropionylglycine significantly (p less than 0.005) protected against RBC Heinz body formation during incubation with acetylphenylhydrazine, while cysteine, cysteamine, and methionine did not. The effect of ascorbic acid was concentration dependent with concentrations as low as 0.1 mM having significant antioxidant effects. Ascorbic acid protected the RBC against hydrogen peroxide induced hemolysis as well (p less than 0.05). Ascorbic acid had a significant stimulatory effect on the rate of glucose oxidation by the pentose phosphate shunt (PPS), especially in the sickle RBC. Ascorbic acid did not protect the RBC from a patient with chronic hemolytic anemia due to G6PDTorrance from Heinz body formation, suggesting that an intact PPS is necessary for ascorbic acid to express its antioxidant properties. These data suggest that clinical trials should be undertaken to examine the efficacy of ascorbic acid in the treatment of SCD.