iet plays an important role in the public health, and suboptimal diet is estimated as the first leading cause of death and the third leading cause of disabilityadjusted life-years lost in the United States. 1 Among most populations worldwide, carbohydrate is the primary source of energy, providing 50% or more of daily energy, with lesser amounts from fat and protein. 2 Long-standing controversies have focused on the health consequences of dietary fat and carbohydrate. Some dietary guidelines have continued to recommend a low-fat diet (LFD) for prevention of chronic diseases, 3 although inconsistent associations have been reported between total fat consumption and health outcomes. [4][5][6][7] A low-carbohydrate diet (LCD) has become a popular strategy for weight loss and weight management in recent years, 8 but the long-term associations of carbohydrate-restricted diets with health outcomes remain controversial. 9,10 Beyond the quantity, evidence has indicated that quality and food sources of carbohydrate and fat play an important role in human diseases and health. 2,8,11 Consumption of carbohydrates from refined grains and added sugars has been adversely associated with health outcomes, whereas consumption of carbohydrates from whole grains, nonstarchy vegetables, and whole fruits appears to be beneficial. 2,12,13 Likewise, replacing saturated fat with unsaturated fat was associated with lower risk of heart disease and mortality. 14,15 During the past 2 decades, the overall macronutrient composition among US adults generally remained stable, but types of carbohydrate and fat intake IMPORTANCE It is crucial to incorporate quality and types of carbohydrate and fat when investigating the associations of low-fat and low-carbohydrate diets with mortality.OBJECTIVE To investigate the associations of low-carbohydrate and low-fat diets with total and cause-specific mortality among US adults.