Trans isomers of alpha-linolenic acid and platelet build-up in humans | Isomeres trans de l'acide alpha-linolenique et agregation plaquettaire chez l'homme

The major trans isomers of alpha-linolenic acid (9c,12c,15t-;9t,12c,15c-18:3 and 9t,12c,15t-18:3) are found in refined and frying oils, and are then ingested by humans. Among these isomers, it has been shown that 9c,12c,15t-18:3 isomer can be desaturated and elongated into a delta17-trans isomer of eicosapentaenoic acid (5c,8c,11c,14c,17t-20:5 or 17t-20:5). It is present in tissues of rats fed heated oils. It is also present in human platelet lipids, suggesting that similar routes could exist in humans. Previous studies on human platelets showed that the 17t-20:5 can alter aggregation as compared to its cis homologue, eicosapentaenoic acid, when it is added by an exogenous route or incorporated into platelet lipids. Moreover, it may compete with arachidonic acid metabolism. A recent study was carried out on rat platelets triggered with arachidonic acid. This study confirmed the antiaggregatory effect of 17t-20:5 and showed that the fatty acids containing a trans double bond at the delta11 position (11t- and 11t,17t-20:5) were more antiaggregant than EPA. Furthemmore, in opposition to EPA, 17t-20:5 was also able to induce platelet aggregation. It was metabolised into three compounds, among which two were formed through the cyclooxygenase pathway and one through the 12-lipoxygenase pathway. Platelet aggregation was studied in human involved in an European contract. In this study, a randomised double blind dietary intervention trial was carried out with healthy male volunteers (n = 88) from three European centres, Clermont-Ferrand (France, n = 31), Edinburgh (Scotland, n = 26) and Maastricht (Netherlands, n = 31). After 6 weeks of a diet without food containing all trans fatty acid, subjects either continued for 6 weeks with a low trans diet or a diet in which trans alpha-linolenic add provided 0.6% of energy. Platelet aggregation was measured three times during the dietary intervention (0,6 and 12 weeks). As expected, this study showed that 15t-18:3 was converted into 17t-20:5 by humans and the 17t-20:5 was the only EPA isomer incorporated in platelet phospholipids. But this incorporation was not sufficient to alter platelet activity and metabolism of arachidonic acid into thromboxane A2. In conclusion, a 6 week-consumption of 0.6% of total energy as trans isomers of alpha-linolenic acid did not modify the activity of human platelet, when aggregation was induced by collagen or by a thromboxane A2 analogue