Abstract
Dietary long chain n-3 fatty acids in fish-oil have proven efficacy in reducing cardiovascular risk associated with an atherogenic lipoprotein phenotype (ALP) and in reducing CHD mortality. However, the acquisition of these health benefits is seriously limited by low habitual intakes of oily fish. Since the shorter chain fatty acid alpha-linolenic acid (ALA) can be converted, to a variable extent, in vivo to its longer chain counterparts, in theory, it should have the capacity to exert fish oil like effects on cardiovascular risk. To test this hypothesis, a pilot study was designed to assess the practical issues of delivering 16g of ALA per day to a group of normal healthy (n=9) volunteers. Outcomes were used in a larger study designed to examine the relative effects of diets enriched with ALA in flaxseed oil, and fish oil on plasma lipids, lipoproteins and selected haemostatic variables in subjects with an ALP. Normal, healthy male subjects (n=57) with an ALP were randomly assigned to one of three diets for 12 weeks; a diet enriched with flaxseed oil (high ALA n=21), a "control" diet enriched with sunflower oil (high linolenic acid n-17) and the "control" diet supplemented with fish oil capsules (3g EPA+DHA n=19). Evidence for dietary compliance was provided by 7-day records of food intakes and increases in the concentration of n-3 PUFA in erythrocyte membrane phospholipids. The pilot study provided valuable information on the delivery of ALA into the study diet, which improved accuracy of dietary dose, portability and stability of the oil and aided dietary compliance in the principal study. The flaxseed, fish oil and "control" diets achieved intake ratios of n-6:n-3 of 0.4, 5.2 and 30.0 respectively. There was no overall difference in any measured variable between the 3 diets (6 & 12 week post diet) or between the flaxseed and fish-oil groups compared to control. Total plasma cholesterol decreased relative to baseline values, within all 3 test diets (pre versus post-diet). Plasma TAG was significantly decreased after the fish oil diet, relative to baseline (-23%. P<0.001). The change in plasma TAG was inversely associated with the level of DHA (C22:6 n-3) in erythrocyte membrane fatty acids at 12 weeks (r2 = 48% p=0.001). LDL subclasses showed a significant reduction towards larger, lighter particles after fish-oil (small, dense LDL-3 -22% p=0.003). There was no change in the concentrations of plasma fibrinogen, factor VII, or in the plasma activity of PAI-1 on any diet or endothelial function as measured by flow-mediated dilatation on a subset on each diet. In conclusion, the fish-oil diet induced predictable changes in plasma lipids and lipoproteins that are associated with lower CHD risk. The flaxseed-oil diet did not reproduce these effects even in the presence of low intakes of dietary n-6 fatty acids.