Trans isomers of alpha-linolenic acid and platelet build-up in humans

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 ore then ingested by humans. Among these isomers, it has been shown that 9c, 12 c, 15t-18:3 isomer can be desaturated and elongated into a Delta 17-trans i somer 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 aggr egation 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 showe d that the fatty acids containing a trans double bond at the Delta 11 posit ion (11t- and 11t, 17t-20:5) were more antiaggregant than EPA. Furthermore, in opposition to EPA, 17t-20:5 was also able to induce platelet aggregatio n. It was metabolised into three compounds, among which two were formed thr ough 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 ca rried out with healthy mole volunteers (n = 88) from three European centres , Clermont-Ferrand (France, n = 31), Edinburgh (Scotland, n = 26) and Maast richt (Netherlands, n = 31). After 6 weeks of a diet without food containin g all trans fatty acid, subjects either continued for 6 week with a low tra ns diet ora diet in which trans cx-linolenic acid provided 0.6% of energy. Platelet aggregation was measured three times during the dietary interventi on (0, 6 et 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 p hospholipids. But this incorporation was not sufficient to alter platelet a ctivity and metabolism of arachidonic acid into thromboxane A(2). In conclu sion, a 6 week-consumption of 0.6% of total energy as trans isomers of alph a-linolenic acid did not modify the activity of human platelet, when aggreg ation was induced by collagen or by a thromboxane A(2) analogue.