Effect of dietary alpha-linolenic acid on thrombotic risk factors in vegetarian men

Vegetarians have lower platelet and plasma concentrations of n-3 polyunsaturated fatty acids (PUFAs) than do omnivores. We recently showed that male vegetarians have higher platelet aggregability than do omnivores. We investigated whether male vegetarians (n = 17) who consumed an increased amount of dietary  -linolenic acid (ALA) showed any changes in their tissue profile of PUFAs, plasma thromboxane concentrations, platelet aggregability, or hemostatic factors. During the study, all subjects maintained their habitual vegetarian diets except that a proportion of dietary fat was replaced with vegetable oils and margarines that were provided. Initially, all subjects consumed a low-ALA diet (containing safflower oil and safflower oil�based margarine) for 14 d; they then consumed either a moderate-ALA diet (containing canola oil and canola oil�based margarine) or a high-ALA diet (containing linseed oil and linseed oil�based margarine) for 28 d. Blood samples were collected at day 0 (baseline), day 14, and day 42. Eicosapentaenoic acid, docosapentaenoic acid, total n-3 PUFAs, and the ratio of n-3 to n-6 PUFAs were significantly increased (P < 0.05), whereas the ratio of arachidonic acid to eicosapentaenoic acid was decreased (P < 0.05), in platelet phospholipids, plasma phospholipids, and triacylglycerols after either the moderate-ALA or high-ALA diet compared with the low-ALA diet. No significant differences were observed in thrombotic risk factors.  ALA from vegetable oils (canola and linseed) has a beneficial effect on n-3 PUFA concentrations of platelet phospholipids and plasma lipids in vegetarian males. (Author�s abstract)
Omnivores can obtain their 20- and 22-carbon n-3 long-chain PUFAs either from dietary ALA or directly by consumption of fish, eggs, or animal products. Lactoovovegetarians can gain a limited amount of 20- and 22-carbon n-3 long-chain PUFAs from milk, dairy products, and eggs. Because animals can convert ALA to 20- and 22-carbon n-3 long-chain PUFAs and plants cannot, there are no 20- or 22-carbon n-3 long-chain PUFAs in plant-based vegan diets. A diet with a low ratio of n-3 to n-6 PUFAs (n-3:n-6) can cause a reduced tissue n-3:n-6 [ie, increased ratio of arachidonic acid (AA; 20:4n-6) to eicosapentaenoic acid (EPA; 20:5n-3)], which may promote production of thromboxane A2 (a potent platelet aggregating agent), leading to increased thrombosis tendency. The aim of the present study was to examine the effect of dietary ALA on atherosclerotic and thrombotic risk factors. The following question was asked: Do male vegetarians who daily consume an increased dietary ALA intake compared with their habitual diet exhibit a decreased ratio of AA to EPA in tissue, decreased plasma 11-dehydro-TXB2 concentrations, and reduced in vitro platelet aggregation? Because the formation of 22:6n-3 from 22:5n-3 also requires a  6 desaturase, it is likely that none of the intermediate n-3 PUFAs (18:3 to 22:5) will be effective tissue sources of 22:6n-3 compared with dietary 22:6n-3. The results of this study indicate that canola and linseed oils have a similar effect on the fatty acid profile of platelet phospholipids and plasma lipids. The use of ALA from vegetable oils (canola and linseed) as a dietary fat for daily food preparation may have a beneficial effect on increasing the n-3 PUFA content of platelet phospholipids and plasma lipids in vegetarian populations. Because such small amounts of EPA and other long-chain n-3 PUFAs are produced after ALA-rich diets, it appears that the 2 main sources of n-3 PUFAs (plants and fish) do not have equivalent biological effects in humans. (Editor�s comments)