In this study we compared the effects of variations in dietary fat and carbohydrate (CHO) content on concentrations of triglyceride-rich lipoproteins in 8, healthy, nondiabetic volunteers. The diets contained, as a percentage of total calories, either 60% CHO, 25% fat, and 15% protein, or 40% CHO, 45% fat, and 15% protein. They were consumed in random order for 2 weeks, with a 2-week washout period in between. Measurements were obtained at the end of each dietary period of plasma triglyceride, cholesterol, low-density lipoprotein (LDL) cholesterol, high-density lipoprotein (HDL) cholesterol, remnant lipoprotein (RLP) cholesterol, and RLP triglyceride concentrations, both after an overnight fast and throughout an 8-hour period (8 A.M. to 4 P.M.) in response to breakfast and lunch. The 60% CHO diet resulted in higher (mean +/- SEM) fasting plasma triglycerides (206 +/- 50 vs 113 +/- 19 mg/dl, p = 0.03), RLP cholesterol (15 +/- 6 vs 6 +/- 1 mg/dl, p = 0.005), RLP triglyceride (56 +/- 25 vs 16 +/- 3 mg/dl, p = 0.003), and lower HDL cholesterol (39 +/- 3 vs 44 +/- 3 mg/dl, p = 0.003) concentrations, without any change in LDL cholesterol concentration. Furthermore, the changes in plasma triglyceride, RLP cholesterol, and RLP triglyceride persisted throughout the day in response to breakfast and lunch. These results indicate that the effects of lowfat diets on lipoprotein metabolism are not limited to higher fasting plasma triglyceride and lower HDL cholesterol concentrations, but also include a persistent elevation in RLPs. Given the atherogenic potential of these changes in lipoprotein metabolism, it seems appropriate to question the wisdom of recommending that all Americans should replace dietary saturated fat with CHO.