**Abstract**

*Trans* fatty acids (TFA) are found naturally in ruminant foods (R-TFA) by biohydrogenation in ruminant animals or industrially produced oils (IP-TFA) by partial hydrogenation of vegetable or fish oils. The intake of TFA mainly IP-TFA is associated with an elevated risk of coronary heart disease (CHD), while some prospective cohort studies showed that R-TFA were associated with a lower risk for sudden cardiac death (SCD). Our case-control study showed that *trans*-C18:2 isomers (IP-TFA) were significantly higher, and palmitelaidic acid (R-TFA) levels were lower in patients with acute coronary syndrome (ACS) compared with healthy men. Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) have different effects on cardiometabolic risk factors. Delta-5 desaturase (D5D) is a key enzyme in the conversion of linoleic acid and alpha-linoleneic acid to arachidonic acid (AA) and EPA, respectively. Previous studies reported that low D5D estimated from the ratio of AA to dihomo-gamma linolenic acid predicts the incident cardiovascular disease. In our cross-sectional study with 436 men with ACS, various atherogenic lipid markers such as small dense LDL cholesterol and malondialdehyde-modified LDL were significantly inversely associated with D5D activity. We found that the EPA/AA may be a superior risk marker than DHA/AA in terms of correlation with atherogenic lipid profiles.

**Keywords:** trans fatty acid, omega-3 fatty acids, delta-5 desaturase, coronary heart disease
