The effect of replacing dietary saturated fat with polyunsaturated or monounsaturated fat on plasma lipids in free-living young adults

Objective: To examine, in free-living adults eating self-selected diets, the effects on plasma cholesterol of substituting saturated fat rich foods with either n-6 polyunsaturated or monounsaturated fat rich foods while at the same time adhering to a total fat intake of 30-33% of dietary energy. Design: Two randomised crossover trials. Setting: General community. Subjects: Volunteer sample of healthy free-living nutrition students at the University of Otago. Trial 1, n = 29, and trial II, n = 42. Interventions: In trials I and II participants were asked to follow for 2 1/2 weeks a diet high in saturated fat yet with a total fat content that conformed to nutrition recommendations (30-33% energy). During the 2 1/2 week comparison diet, saturated fat rich foods were replaced with foods rich in n-6 polyunsaturated rats (trial 1) whereas in trial II the replacement foods were rich in monounsaturated fats. Participants were asked to maintain a total fat intake of 30-33% of energy on all diets. Main outcome measures: Energy and nutrient intakes, plasma triglyceride ratty acids, and plasma cholesterol. Results: When replacing saturated fat with either n-6 polyunsaturated fat or monounsaturated fat, total rat intakes decreased by 2.9% energy and 5.1% energy, respectively. Replacing saturated fat with n-6 polyunsaturated rat (trial 1) lowered plasma total cholesterol by 19% [from 4.87 (0.88) to 3.94 (0.92) mmol/l, mean (s.d.)], low density lipoprotein cholesterol by 22% [from 2.87 (0.75) to 2.24 (0.67) mmol/l], and high density lipoprotein cholesterol by 14% [from 1.39 (0.36) to 1.19 (0.34) mmol/l], whereas replacing saturated rat with monounsaturated rat (trial II) decreased total cholesterol by 12%, low density lipoprotein cholesterol by 15%, and high density lipoprotein cholesterol by 4%, respectively. The change in the ratio of total to high density lipoprotein cholesterol was similar during trial I and trial II. Conclusions: Young adults are very responsive to dietary-induced changes in plasma cholesterol even when an isocaloric replacement of saturated fat with n-6 polyunsaturated or monounsaturated fiat is not achieved. Replacing saturated fat with either n-6 polyunsaturated or monounsaturated fat is equally efficacious at reducing the total to high density lipoprotein cholesterol ratio.