Abstract
Growing evidence suggests that dietary n-3 very long chain polyunsaturated fatty acids (eicosapentaenoic acid; EPA and docosahexaenoic acid; DHA) reduce the risk of coronary heart disease and stroke, alpha-linolenic acid (alpha-LNA) is the natural precursor of EPA and DHA and is an abundant and accessible source of dietary n-3 PUFA that can be further elongated and unsaturated in vivo. The overall aim of the project is to examine the conversion of alpha-LNA to its long chain metabolite, most importantly DHA. This aim was accomplished by a combination of a human dietary intervention study to assess accumulation of EPA and DHA from dietary alpha-LNA, and 13C-tracer studies of alpha-LNA & linoleic acid (LA) conversion to their long-chain metabolites. The dietary intervention trial was a 12-week parallel design in men expressing an atherogenic lipoprotein phenotype, a common source of lipid-mediated coronary heart disease risk. Diets were enriched with 18 g of alpha-LNA as flaxseed oil (n=21), with a high LA oil (n=17), or with fish-oil (6g/d n=19) as a positive control group. Thus the intention was to provide an increased intake of alpha-LNA with a low ratio of n-6 to n-3 PUFA (1:1 or less), minimizing competition between alpha-LNA and the abundant LA and, in theory, increasing the conversion of alpha-LNA to LC n-3 PUFA. The results from the dietary intervention indicate that, dietary alpha-LNA as flaxseed oil can increase n-3 membrane fatty acid contents, through a 3-fold increase in alpha-LNA (p <0.001)) and 2.5 fold increase in EPA (p<0.001) at week-12, decreasing in n-6:n-3 ratio (p =0.001), but not changing DHA level. In contrast the fish oil diet increased both EPA and DHA. Dietary alpha-LNA had 7% of the efficacy of preformed EPA from fish oil to increase membrane EPA levels. Subjects on the 13C tracer study were a sub-group of the intervention study, studied after 12-weeks on the high alpha-LNA (n=6) or high n-6 (n=5) diets. Subjects were given an oral mixture of 400 mg each of uniformly 13C labelled alpha-LNA and LA in a milk shake after an overnight fast. 13C enrichment was measured in fatty acids isolated from plasma at 1,2,3,7,10 and 14 days after the dose. Of the dose appearing in the plasma 35-45% was converted to EPA with no dietary effects. Some conversion to DHA did occur especially in the high n-6 group (3.9% of dose) compared with the flaxseed-oil group (mean value 0.8% of the dose; p < 0.05). In the single subject studied on the fish-oil diet there was a much lower conversion rate compared to the flaxseed-oil diet. The variability between subjects for percent conversion to DHA ranged from zero to 6.2% of the dose appearing in plasma. Taken together these results clearly establish the effectiveness of dietary alpha-LNA as a method of increasing the concentration of EPA, but not DHA, in membrane phospholipids, with up to 7% of the efficacy of preformed EPA. The increase in the EPA: AA ratio (eicosapentanoic: arachidonic acid) in membrane phospholipids with dietary alpha-LNA is likely to reduce the overall inflammatory environment with beneficial effects for long-term health.