Unfavourable effects of trans monounsaturated fatty acid (trans-C18:1) isomers on health variables have been reported. Reports on their actual intake, however, are scarce, because of the absence in many nutrient databases of values for trans-C18:1, and the wide variation in the level of trans fatty acids between different brands of the same product. We therefore examined whether the intake of trans-C18:1 is reflected by trans-C18:1 concentrations in serum triacylglycerols or platelet phospholipids. Thirtyeight men received two diets in random order. During the first experimental period twenty men consumed a Western-type control diet for six weeks, and eighteen men consumed a modified diet in which 70% of the fat was replaced by palm oil. After a wash-out period of 3 weeks, regimens were crossed over (second experimental period). The proportion of total fatty acids from trans-C18:1 in the diet decreased from 4·7 (SEM 0·27) during the control to 2·1 (SEM 0·16) on the modified diet (P < 0·001). Trans-C18:1 in serum triacylglycerols decreased from 3·5 (SEM 0·13) to 2·8 (SEM 0·11)% (P < 0·001), and in platelet phospholipids from 1·0 (SEM 0·06) to 0·7 (SEM 0·4)% (P < 0·001). After the first experimental period trans-C18:1 in the diet correlated with trans-C18:1 in serum triacylglycerols (r 0·41; P = 0·014), and platelet phospholipids (r 0·52; P = 0·001). Also, differences in the intake between the two periods correlated with changes in the proportion of trans-C18:1 in serum triacylglycerols (r 0·56; P= 0·001) and platelet phospholipids (r 0·58; P < 0·001). These results suggest that analyses of blood lipid fractions can be used to estimate the intake, and to monitor changes in the intake, of trans-C18:1.
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