Many animals consume mixed diets that maximize their fitness by optimizing macronutrient intake. We tested whether brown bears (Ursus arctos), generalist omnivores that hibernate, regulated their diet to a common nutrient target, achieved a nutrient target related to fitness, and selected a nutrient target that differed between seasons and from other species with differing life histories. When given unlimited access to 2 or 3 highly digestible foods containing primarily protein, carbohydrate, or lipid, brown bears selected mixed diets in which protein provided 17% ± 4% SD of the metabolizable energy and 22% ± 6% of the dry matter. This dietary protein content maximized the rate of gain per unit of energy consumed, is similar to the level preferred by other omnivores, and is less than that preferred by obligate carnivores. Between seasons, bears selected similar dietary protein levels, although the proportion of lipid was higher during the fall than during the spring. Bears strongly preferred lipids over carbohydrates, as did other carnivores, but they used lipids and carbohydrates with equal efficiency to produce a dietary protein content that maximized mass gain per unit of energy intake. Thus, dietary sources of lipids and carbohydrates play an interchangeable and important role in determining the productivity of bears that goes beyond their role in providing energy.