Should mono- or poly-unsaturated fats replace saturated fat in the diet?

The effects of diets differing in saturated, monounsaturated and polyunsaturated fatty acid composition (SAFA, MUFA and PUFA, respectively) on plasma lipoproteins and factor VIIc were investigated in 28 middle-aged men and women with mild to moderate hyperlipidaemia. The subjects were stabilized on a diet with a total fat content fairly typical of New Zealand, containing approximately 40% energy as fat, before entering a randomized cross-over trial of diets high in PUFA (20% energy; SAFA and MUFA 10% each) or a high MUFA diet (20% energy; SAFA and PUFA 10% each). After 6-week periods on each diet the subjects returned to a high SAFA diet. Body weight and blood pressure remained unchanged during the study. Total and LDL cholesterol, HDL cholesterol and the HDL2 subfraction were significantly lower on both the MUFA and the PUFA diet than on SAFA. However, there were no statistically significant differences in lipoprotein concentrations on the MUFA and PUFA diet. Factor VIIc concentrations were similar on the three diets. The proportion of PUFA in a MUFA diet appears to be a major determinant of the relative lipoprotein response to such a diet. In order to avoid a reduction in HDL-C when replacing SAFA with MUFA it may be necessary to ensure that PUFA does not provide more than about 8% total energy. Thus careful planning is needed to identify the most appropriate foods to replace those rich in SAFA in diets designed to reduce the lipoprotein-mediated risk of coronary heart disease.