Results from animal studies indicate that the type of dietary fat consumed has a profound effect on insulin action in tissues. Feeding a semipurified diet high in polyunsaturated to saturated fatty acid (P/S) ratio to insulin-dependent diabetic rats as compared to feeding them a diet low in P/S ratio increased insulin binding and insulin action in adipocytes [1] and skeletal muscle [2,3]. Alteration in the fatty acid composition of plasma membrane phospholipid by dietary P/S ratio has been suggested to be a cause of these changes in insulin action. Beneficial effects of fish oil in preventing the development of insulin resistance in rats caused by a high intake of saturated fatty acids have also been reported [4,5].Individuals with insulin-dependent diabetes mellitus are prone to develop disorders in lipid and lipoprotein metabolism, of which hyperlipidaemia is the most common [6,7]. Elevated plasma triacylglycerol and reduced HDL cholesterol also increase the risk Diabetologia (1997) Summary This study was conducted to examine the effect of w 3 fatty acid supplementation on plasma lipid, cholesterol and lipoprotein fatty acid content of non-insulin-dependent diabetic individuals consuming a higher (0.65, n = 10) or lower (0.44, n = 18) ratio of dietary polyunsaturated to saturated fatty acid (P/ S). The participants were initially given an olive oil supplement (placebo) equivalent to 35 mg of 18:1 ⋅ kg body weight -1 ⋅ day -1 for 3 months. This was followed by two w 3 supplement periods in a randomized crossover. In these 3-month periods, participants were given a linseed oil supplement equivalent to 35 mg of 18:3w3 ⋅ kg body weight -1 ⋅ day -1 or a fish oil supplement equivalent to 35 mg of 20:5w3 + 22:6w3 ⋅ kg body weight -1 ⋅ day -1 . At the end of each supplement period, a blood sample was drawn from each participant for lipid, lipoprotein, insulin, glucagon and C-peptide analyses. At the end of each 3-month period a 7-day dietary record was completed to calculate dietary fat intake and P/S ratio.Results indicate that fish oil significantly reduced plasma triacylglycerol level (p < 0.05) and increased 20:5w3 and 22:6w3 content of all lipoprotein lipid classes. Linolenic acid supplementation had no effect on plasma triacylglycerol level, but it increased 18:3w3 content of lipoprotein cholesterol ester fractions (p < 0.05). A slight increase in 20:5w3, but not 22:6w3, content was noted in lipoprotein lipid classes as a result of 18:3w3 supplementation. LDL and HDL cholesterol, insulin, glucagon and C-peptide levels were not affected by either w 3 supplement. It is concluded that a modest intake of w 3 fatty acids, such as could be obtained from consuming fish regularly, will reduce plasma triglyceride level without affecting LDL or HDL cholesterol levels. [Diabetologia (1997) Corresponding author: Dr. M. T. Clandinin, Nutrition and Metabolism Research Group, Department of Agriculture, Food and Nutritional Science, University of Alberta, 4-10 Agriculture/Forestry Centre, Edmonton, Alberta T6G 2P5, Canada A...