Replacement of dietary fats: Effects on serum lipids and plasma fatty acid composition with special emphasis on the metabolism of essential fatty acids: Dissertation

The effects of fat substitution on plasma fatty acid composition, serum lipid levels and plasma lipid classes were investigated in two studies comprising a total of 100 and 48 subjects, respectively. The analytical methods included gas chromatography (GC) of total plasma and plasma phospholipid (PL) fatty acids, including octadecenoic trans-isomers. A high performance liquid chromatographic method using evaporative light-scattering detection (HPLC-ELSD) was applied for the quantitation of lipid classes. Substitute fats included canola-type, ordinary or cold-pressed rapeseed oils containing ca 11% alfa-linolenic (alfa-LLA) and 23% linoleic (LA) acids, a test margarine, and olive and soybean oils. The average daily doses in the groups during the six-week substitutions, designed to replace butter or margarine on bread, ranged from 14 to 23 g (15-22% of total fat intake). The results demonstrate a preference for n-3 polyunsaturated fatty acid (PUFA) metabolism from alfa-LLA to longer-chain n-3 PUFAs over LA and n-6 PUFA metabolism. This was most completely evident in plasma PL, when butter was replaced by rapeseed oil, as a simultaneous fall in saturated fatty acid (SaFA) and serum LDL cholesterol levels. The effect of monounsaturated oleic acid (n-9 MUFA), the main fatty acid in rapeseed oil (60%), remained neutral showing no increase in PL. The changes in PL followed the order of competition between the unsaturated fatty acid families: (n-3) > (n-6) > (n-9). The increase in n-3 PUFAs was predominant at three weeks, while that in n-6 PUFAs was highest at six weeks, without suppressing n-3 PUFAs. This delay is in line with higher desaturase selectivity for alfa-LLA conversion to long-chain n-3 PUFAs, and with their suppressive effect on n-6 PUFAs. The test margarine (3% alfa-LLA; 28% LA) lacked an n-3 PUFA effect, and an increase in PL LA (n-6) was seen already after the first three weeks. Replacement of margarines by rapeseed oil first reduced both PL SaFAs and n-6 PUFAs, but simultaneously raised n-3 PUFAs and MUFAs. Oleic acid in dietary fat is thus a good counterpart with alfa-LLA. The amount of LA in the diet is in the key position during competition, since the rise in n-6 PUFAs at six weeks suppressed both n-3 PUFAs and MUFAs. Olive oil, instead of raising PUFAs, reduced LA levels in margarine users, which is desirable if the LA intake is high. However, due to the low alfa-LLA (