Vitamin E (RRR-a-tocopherol) is the most important lipid-soluble antioxidant, which protects lipoproteins and cell membrane lipids from oxidative damage. This ability is coupled to other antioxidant systems (vitamin C, glutathione, lipoic acid) that can recycle the vitamin E radical (19). In the absence of such systems, vitamin E can behave as an oxidant (20). Dietary vitamin E is transported to the liver and secreted from the liver within very-low-density lipoproteins (VLDL). It is distributed among VLDL and LDL during the transfer and metabolism of the lipoprotein lipids (21). Thermodynamic partitioning also permits some transfer into high-density lipoproteins (22). An important part of vitamin E is constituent of cell membranes where it protects the lipid moiety against peroxidation (23).
About one-half of the total plasma vitamin E is a constituent of circulating LDL. Interindividual variations in plasma vitamin E are closely related to those in LDL (24,25). The concentration of vitamin E per LDL particle is rather low (i.e., in the order of 5-9 molecules compared with 2200 molecules of cholesterol and 170 molecules of triglycerides) (26). Nevertheless, the level of vitamin E in LDL is an independent factor that influences susceptibility of LDL to oxidation. The lagtime of in vitro LDL oxidation was found to be related to the level of vitamin E in LDL when diabetic patients were supplemented with vitamin E (27,28). For persons with usual nutritional habits, the corresponding relationship was observed in two studies (29,30) but not in others (3,31,32).
There are several publications on vitamin E concentrations in the plasma of diabetic patients. The assumption that the oxidative stress in diabetes is due to deficient vitamin E in plasma could not be confirmed. In most studies, no statistically significant differences in the concentrations between diabetic and control persons were observed. Two studies exhibited even significantly higher levels in diabetes (Table 1). The weighted mean values (control subjects 26.4 ± 1.9 ^tmol/L, IDDM 24.4 ± 1.6 |imol/L, NIDDM 22.4 ± 2.0 |imol/L) are only marginally different for the plasma of control persons, IDDM, and non-IDDM (NIDDM).
Published data on vitamin E concentrations in LDL (Table 2) show very similar mean values for diabetic persons and control subjects (control subjects 7.5 mol/mol, IDDM 7.0 mol/mol, NIDDM 6.8 mol/mol). One study demon-
Antioxidant Vitamins in Diabetics
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