Flow-mediated vasodilation is not impaired when HDL-cholesterol is loweredby substituting carbohydrates for monounsaturated fat

Low-fat diets, in which carbohydrates replace some of the fat, decrease ser um cholesterol. This decrease is due to decreases in LDL-cholesterol but in part to possibly harmful decreases in HDL-cholesterol. High-oil diets, in which oils rich in monounsaturated fat replace some of the saturated fat, d ecrease serum cholesterol mainly through LDL-cholesterol. We used these two diets to investigate whether a change in HDL-cholesterol would change flow -mediated vasodilation, a marker of endothelial function. We fed thirty-two healthy volunteers two controlled diets in a 2x3.5 weeks' randomised cross -over design to eliminate variation in changes due to differences between s ubjects. The low-fat diet contained 59.7 % energy (en%) as carbohydrates an d 25.7 en% as fat (7.8 en% as monounsaturates); the oil-rich diet contained 37.8 en% as carbohydrates and 44.4 en% as fat (19.3 en% as monounsaturates ). Average (sd) serum HDL-cholesterol after the low-fat diet was 0.21 (sd 0 .12) mmol/l (8.1 mg/dl) lower than after the oil-rich diet. Serum triacylgl ycerols were 0.22 (sd 0.28) mmol/l (19.5 mg/dl) higher after the low-fat di et than after the oil-rich diet. Serum LDL and homocysteine concentrations remained stable. Flow-mediated vasodilation was 4.8 (SD 2.9) after the low- fat diet and 4.1 (SD 2.7) after the oil-rich diet (difference 0.7 %; 95 % C I -0.6, 1.9). Thus, although the low-fat diet produced a lower HDL-choleste rol than the high-oil diet, flow-mediated vasodilation, an early marker of cardiovascular disease, was not impaired.