Letter to the Editor

I read with interest the article published in your Journal by Block et al.1 They showed a direct correlation of blood cell levels of trans, trans-linoleic acid with acute coronary syndromes. Strictly, trans, trans isomer of linoleic acid is 9-trans,12-trans octadecadienoic acid (9t,12t-18:2). However, I have my doubts about the presence of this particular fatty acid in blood cells. Several years ago, Ratnayake et al2 showed that 9-cis,13-trans-octadecadienoic acid (9c,13t-18:2) is the major trans isomer of linoleic acid in partially hydrogenated oils2 and in some processed foods.3 The 9t,12t-18:2 isomer is a minor component in partially hydrogenated oils.2, 3 In gas chromatography analysis, the peak for 9t,12t-18:2 overlaps or closely elutes with that for 9c,13t-18:2.2, 4 Because reference standard for 9t,12t-18:2 is readily available but not that of 9c,13t-18:2, many analysts mistakenly identify the 9c,13t-18:2 isomer as 9t,12t-18:2.5 On the 100-m SP-2560 capillary column, 9t,12t-18:2 can be resolved from 9c,13t-18:2, provided the column temperature is operated isothermally at 180°C. However, very often overlaps of these 2 trans isomers occur when the SP-2560 column is operated using a temperature program as used by Block et al.1

Several years ago, we examined the tissue levels of 18:2 trans isomers in rats fed partially hydrogenated canola oil that provided 1.9% and 0.9% of total fat as 9c,13t-18:2 and 9t,12t-18:2, respectively.6 9t,12t-18:2 was not detected in any of the tissue samples, whereas 9c,13t-18:2 was found in all tissues. This would indicate that 9t,12t-18:2 is more rapidly β oxidized than 9c,13t-18:2. Thus, it is reasonable to assume that the 9t,12t-18:2 is also less abundant in human red cells than 9c,13t-18:2.

It is very likely that the fatty acid identified as the trans, trans isomer of linoleic acid by Block et al1 is 9c,13t-18:2. It is important to clarify the correct identification of these 2 isomers because the 9t,12t-18:2 is present to a lesser extent in foods than 9c,13t-18:2.2, 3