To compare the effects on serum lipoproteins of three isocaloric diets with reduced total fat and saturated fatty acid (SFA) contents but with different proportions of monounsaturated (MUFA) and polyunsaturated fatty acids (PUFA).

A low-fat diet (LF) provided 20 en% fat, 7.9% SFA, 7.8% MUFA, 3.0% PUFA; a high-PUFA diet (HP) 26 en% fat, 7.5% SFA, 8.2% MUFA, 8.1% PUFA; and a high-MUFA diet (HM) 26 en% fat, 7.3% SFA, 14.1% MUFA, 3.2% PUFA. Diets were consumed for 8 weeks in a parallel design, after 2 weeks on a habitual diet with 33–34 en% fat, 13–14% SFA, 12% MUFA, 6% PUFA, and followed by an 8-week period on habitual diet. Compliance to diet was monitored by repeated food records and weekly visits to a nutritionist.

45 free-living, middle-aged couples who were randomly allocated into the three diet groups; 43 men and 44 women completed the study.

During the diet periods, a small but significant reduction in body weight of 0.4–1.0kg was observed in all groups. Similar and significantreductions of mean weight-adjusted serum total cholesterol (4–8%), low-density lipoprotein (LDL) cholesterol (7–11%), and high-densitylipoprotein (HDL) cholesterol (8–11%) were observed during the diets. HDL2-cholesterol and apoprotein (apo) A-I levels were reduced whereas HDL3-cholesterol and apoA-II increased. ApoB was significantly decreased during the HM diet only. Serum triglycerides increased significantly during diets LF (25%, P< 0.01) and HP (19%, P< 0.05) but not during diet HM (5%, NS).

Reduction in the intake of total fat and saturated fatty acids reduced serum LDL-cholesterol and HDL2-cholesterol concentrations irrespective of the relative proportions of MUFA and PUFA in the diets. The results suggest that there might be some advantage in increasing the proportion of MUFA in low-fat diets, since the HM diet rich in MUFA reduced apoB and slightly attenuated the increase in serum triglycerides that is commonly associated with dietary fat reduction.