Effects of diets enriched in saturated (Palmitic), monounsaturated (Oleic), or trans (Elaidic) fatty acids on insulin sensitivity and substrate oxidation in healthy adults

OBJECTIVE - Diets high in total and saturated fat are associated with insulin resistance. This study examined the effects of feeding monounsaturated, saturated, and trans fatty acids on insulin action in healthy adults. RESEARCH DESIGN AND METHODS - A randomized, double-blind, crossover study was conducted comparing three controlled 4-week diets (57% carbohydrate, 28% fat, and 15% protein) enriched with different fatty acids in 25 healthy men and women. The monounsaturated fat diet (M) had 9% of energy as C18:1cis (oleic acid). The saturated fat diet (S) had 9% of energy as palmitic acid, and the trans fatty acid diet (T) had 9% as C18:1trans. Body weight was kept constant throughout the study. After each diet period, insulin pulsatile secretion, insulin sensitivity index (S₁) by the minimal model method, serum lipids, and fat oxidation by indirect calorimetry were measured. RESULTS - Mean S₁ for the M, S, and T diets was 3.44 ± 0.26, 3.20 ± 0.26, and 3.40 ± 0.26 × 10^-4 min^-1·μU ^-1·ml^-1, respectively (NS). S₁ decreased by 24% on the S versus M diet in overweight subjects but was unchanged in lean subjects (NS). Insulin secretion was unaffected by diet, whereas total and HDL cholesterol increased significantly on the S diet. Subjects oxidized the least fat on the M diet (26.0 ± 1.5 g/day) and the most fat on the T diet (31.4 ± 1.5 g/day) (P = 0.02). CONCLUSIONS - Dietary fatty acid composition significantly influenced fat oxidation but did not impact insulin sensitivity or secretion in lean individuals. Overweight individuals were more susceptible to developing insulin resistance on high-saturated fat diets.