Here, we infer the minimal fundamental macronutrient niche of brown bear using population diet estimates as indicators of realized macronutrient niches. To that end, we collected and synthesized data from the literature to estimate the proportions of macronutrients in the diets of brown bear populations, following the approach recently applied to a small carnivore (Martes martes; Remonti, Balestrieri, Raubenheimer, & Saino, 2016) and invasive omnivore (Sus scrofa; Senior, Grueber, Machovsky‐Capuska, Simpson, & Raubenheimer, 2016). We then applied a compositional statistical paradigm (Aitchison, 1982) to our data analysis of macronutrient proportions. We hypothesized that the brown bear's broad diet could be associated with the following functional adaptations of omnivory: The Nutrient Balancing Hypothesis predicts that a wide diet serves to increase the range of food options that can be combined to achieve a balanced macronutrient intake. This hypothesis predicts that the macronutrient composition of bear diets from different ecological and geographic populations is similar despite consuming different foods (i.e., a narrow fundamental macronutrient niche). This type of nutrient balancing has been observed in primates (Raubenheimer, Machovsky‐Capuska, Chapman, & Rothman, 2015; Rothman et al., 2007), badger (Meles meles; Remonti, Balestrieri, & Prigioni, 2011), and pine marten (Martes martes; Remonti et al., 2016).The Nutritional Generalism Hypothesis postulates that a broad diet combined with the ability to tolerate a wide range of dietary macronutrient intakes enables a species to occupy a diverse range of habitats. This hypothesis predicts variation in brown bear diet compositions among populations and a wide fundamental macronutrient niche. This type of nutrient balancing has been observed in gannets (Morus spp.; Tait, Raubenheimer, Stockin, Merriman, & Machovsky‐Capuska, 2014) and wild boar (Senior et al., 2016).The Seasonal Variation Hypothesis predicts that the proportion of macronutrients in the diets of brown bear will vary seasonally (Coogan et al., 2014). It is well known that the protein content of bear diets declines over the active season in several ecosystems (López‐Alfaro, Coogan, Robbins, Fortin, & Nielsen, 2015). However, the multivariate relationship between seasonal macronutrient proportions has been less well established (but see Coogan et al., 2014 and Costello et al., 2016). This hypothesis is nonmutually exclusive with either of the above hypotheses.The Prehibernation Optimal Diet Hypothesis predicts that the macronutrient composition of brown bear diets will be closer to the self‐selected optimal ratio for mass gain of captive bears during the prehibernation hyperphagic season, because selective pressure during this period will be the highest (i.e., behavioral and physiological adaptation).