Dietary trans α-linolenic acid does not inhibit Δ5-and Δ6-desaturation of linoleic acid in man

Dietary trans monoenes have been associated with an increased risk of heart disease in some studies and this has caused much concern. Trans polyenes are also present in the diet, for example, trans alpha -linolenic acid is formed during the deodorisation of alpha -linolenic acid-rich oils such as rapeseed oil. One would expect the intake of trans alpha -linolenic acid to be on the increase since the consumption of rapeseed oil in the western diet is increasing. There are no data on trans alpha -linolenic acid consumption and its effects. We therefore carried out a comprehensive study to examine whether trans isomers of this polyunsaturated fatty acid increased the risk of coronary heart disease. Since inhibition of Delta6-desaturase had also been linked to heart disease, the effect of trans alpha -linolenic acid on the conversion of [U-C-13]-labelled linoleic acid to dihomo-gamma -linolenic and arachidonic acid was studied in 7 healthy men recruited from the staff and students of the University of Edinburgh. Thirty percent of the habitual fat was replaced using a trans 'free'- or 'high' trans alpha -linolenic acid fat. After at least 6 weeks on the experimental diets, the men received 3-oleyl, 1,2-[U-C-13]-linoleyl glycerol (15 mg twice daily for ten days). The fatty acid composition of plasma phospholipids and the incorporation of C-13-label into n-6 fatty acids were determined at day 8, 9 and 10 and after a 6-week washout period by gas chromatography-combustion-isotope ratio mass spectrometry. Trans alpha -linolenic acid of plasma phospholipids increased from 0.04 +/- 0.01 to 0.17 +/- 0.02 and cis alpha -linolenic acid decreased from 0.42 +/- 0.07 to 0.29 +/- 0.08 g/100 g of fatty acids on the high trans diet. The composition of the other plasma phospholipid fatty acids did not change. The enrichment of phosphatidyl C-13-linoleic acid reached a plateau at day 10 and the average of the last 3 days did not differ between the low and high trans period. Both dihomo-gamma -linolenic and arachidonic acid in phospholipids were enriched in C-13, both in absolute and relative terms (with respect to C-13-linoleic acid). The enrichment was slightly and significantly higher during the high trans period (P < 0.05). Our data suggest that a diet rich in trans <alpha>-linolenic acid (0.6% of energy) does not inhibit the conversion of linoleic acid to dihomo-gamma -linolenic and arachidonic acid in healthy middle-aged men consuming a diet rich in linoleic acid.