Effect of dietary fish oil supplementation on peroxidation of serum lipids in patients with non-insulin dependent diabetes mellitus

Lipid peroxidation may be important in the development of cardiovascular disease, a common cause of mortality and morbidity in non-insulin dependent diabetes mellitus (NIDDM). We assessed the degree of lipid peroxidation by measuring plasma malondialdehyde, as thiobarbituric acid reacting substances (TBARS), in 23 non-insulin dependent diabetic patients. Plasma levels of lipid standardised alpha-tocopherol (vitamin E), lipid content of whole plasma and lipoprotein fractions, glycosylated haemoglobin, glycosylated low density lipoprotein (LDL) and fasting blood glucose were also measured. On completion of the baseline studies patients randomly received either fish oil or matching olive oil capsules in a double blind crossover fashion for 6 weeks followed by a 6 week washout period and a final 6 week treatment phase. Studies, identical to the initial baseline studies, were performed at the end of the active treatment periods at 6 and 18 weeks. Treatment with olive oil did not change levels of TBARS, vitamin E or indices of glycaemic control compared with baseline. Total cholesterol and triglyceride (TG) content of plasma and lipoprotein fractions were not significantly altered. Treatment with fish oil resulted in elevation of TBARS (P<0.001) and reduction of vitamin E (P < 0.01) compared with baseline and olive oil treatment. Plasma cholesterol was unchanged. A reduction in plasma TG compared with baseline occurred but failed to reach significance (P = 0.07). Changes in apo B containing lipoproteins induced by fish oil failed to reach significance. No significant changes were observed in the concentration or composition of high density lipoprotein (HDL). Fish oil treatment showed no change in glycaemic control as assessed by glycosylated haemoglobin and LDL although a rise in fasting blood glucose just failed to reach significance (P= 0.06). Lipid peroxidation in NIDDM can be exacerbated by dietary fish oil. This potentially adverse reaction may limit the therapeutic use of fish oils in such patients.