Dietary trans fatty acids increase conjugated linoleic acid levels in human serum

Conjugated linoleic acid (CLA), fatty acids with 18 carbon atoms and two conjugated cis/trans double bonds, have shown anticarcinogenic effects in experimental studies. We determined the proportion of CLA (the sum of cis-9, trans-11 and trans-9, cis-11 CLA) of total fatty acids in the diets and serum samples of healthy subjects who consumed for 5 weeks a diet high in saturated fatty acids mainly from dairy fat, followed by 5 weeks on a diet high (8.7% of energy, en%) in trans fatty acids from partially hydrogenated vegetable oil (40 subjects) or a similar diet high in stearic acid (9.3 en%, 40 subjects). All diets contained equal amounts of fat and cis-monounsaturated and cis-polyunsaturated fatty acids. The fatty acid compositions of the pooled diets and fasting serum samples drawn at the end of the diet periods were analyzed by gas chromatography, and CLA was identified by comparison with a standard of C18:2 conjugated dienes. The proportions of CLA in the dairy fat, trans fatty acid, and stearic acid diets were 0.37, 0.04, and 0.10% of total methylated fatty acids, respectively. The corresponding mean (SD) proportions in serum were 0.33 (0.07)% after the dairy fat diet, higher, 0.43 (0.12)%, P < 0.001, after the trans fatty acid diet, and lower, 0.17 (0.06)%, P < 0.001, after the stearic acid diet. The difference between dairy fat and stearic acid diets was explained by different dietary intakes but increased amounts of CLA not present in the diet were incorporated into serum lipids during the trans fatty acid diet. CLA in human tissues is partly derived from the diet but part of it may be formed by conversion from dietary trans fatty acids.