The importance of vitamin c in diabetes mellitus –

Nutritional Influences on Illness

Townsend Letter for Doctors and Patients ,  Dec, 2002   by Melvyn R. Werbach

Compared to apparently healthy people, diabetics appear to consume less vitamin C. (1) Even when vitamin C intake is in the range normally considered to be adequate, ascorbic acid levels are frequently depressed in both the plasma (2) and leukocytes; (3) in fact, most studies have found that diabetics have at least 30% lower circulating ascorbic acid levels than normals, (4) although this finding is at least partly due to their lower intake of the vitamin. (5) Results of studies employing animal models of diabetes suggest that these low levels, when not due to low intake, are caused by increased urinary excretion of the vitamin (6) and defective transport across cell membranes, (7) along with increased oxidation of ascorbic acid to dehydroascorbic acid (DHAA). (8)

Defective cell membrane transport of ascorbic acid into vascular epithelial cells (cells that line the inside of blood vessels) when blood sugar is elevated may promote the development of atherosclerosis and angiopathies, (9) while the accumulation of DHAA may not only promote the diabetic process (10) but also oxidative damage. (11) (The antioxidant glutathione reduces DHAA to vitamin C in red blood cells. (12) As glutathione is frequently depressed in diabetics, (12) inadequacy of this nutrient could be a cause of the elevated DHAA levels.)

While diabetes causes these disorders in ascorbic acid metabolism, a vitamin C deficiency may provoke disorders in glycoregulation resembling diabetes. Thus a vicious circle may occur which requires an increased supply of ascorbic acid in order to be terminated. (13)

Supplementation

There is substantial evidence that diabetics may require supplemental ascorbic acid in order to achieve tissue saturation and maximal physiologic function. (14) Vitamin C supplementation has been shown to improve glucose tolerance (15) and lipid profiles (16) in non-insulin-dependent diabetics, and to reduce cutaneous capillary fragility. (17) For example, in an open trial each gram of oral vitamin C taken daily permitted a 2 unit reduction in daily insulin requirements for a juvenile diabetic. (18)

There is some evidence that supplementation may reduce the nonenzymatic glycosylation of proteins, (19) a process that plays an important role in the development of diabetic complications, although its efficacy in this regard is still uncertain. (14) Moreover, after continued ascorbic acid administration, DHAA disappears. (12)

Finally, vitamin C supplementation may lower sorbitol levels in red blood cells independent of changes in diabetic control. (20) This is important because tissue sorbitol accumulation is associated with the development of diabetic complications including cataracts, retinopathy and neuropathy. Hyperglycemia causes the accumulation of sorbitol in tissues due to the action of the enzyme aldose reductase on glucose; intracellular sorbitol draws in water by osmosis while low molecular weight compounds are lost in an attempt to maintain osmotic equilibrium. (21)

Modified from Werbach MR with Moss J, Textbook of Nutritional Medicine. Tarzana, California, Third Line Press, Inc., 1999.

References

(1.) Singh RB et al. Dietary intake and plasma levels of antioxidant vitamins in health and disease: a hospital-based case-central study. J Nutr Environ Med 5:235-42, 1995

(2.) Sinclair AJ et al. Low plasma ascorbate levels in patients with type 2 diabetes mellitus consuming adequate dietary vitamin C. Diabetic Med 11:893-8, 1994

(3.) Cunningham JJ et al. Reduced mononuclear leukocyte ascorbic-acid content in adults with insulin-dependent diabetes mellitus consuming adequate dietary vitamin C. Metabolism 40:146-9, 1991

(4.) Will JC, Byers T. Does diabetes mellitus increase the requirement for vitamin C? Nutr Rev 54(7):193-202, 1996

(5.) will JC, Ford Es, Bowman BA. Serum vitamin C concentrations and diabetes: findings from the Third National Health and Nutrition Examination Survey, 1988-1994. Am J Clin Nutr 70(1):49-52, 1999

(6.) Zebrowski EJ, Bhatnagar PK. Urinary excretion pattern of ascorbic acid in streptozotocin diabetic and insulin treated rats. Pharm Res Commun 11(2):95-103, 1979

(7.) Cunningham JJ. Altered vitamin C transport in diabetes mellitus. Med Hypotheses 26:263-5, 1988

(8.) Som S et al. Ascorbic acid metabolism in diabetes mellitus. Metabolism 30(6):572-7, 1981

(9.) Kapeghian JC, Verlangieri AJ. The effects of glucose on ascorbic acid uptake in heart endothelial cells: Possible pathogenesis of diabetic angiopathies. Life Set 34(6):577-84, 1984