UCP1-expressing brown adipocytes convert nutrient energy into heat, thus linking thermal regulation to systemic metabolism. Despite rising interest in the potential beneficial effect of UCP1⁺ cells, their precise contribution to systemic metabolism remains unexplored. To address this question, we used the UCP1-DTR mouse model in conditions of sustained, highly efficient UCP1⁺ cell ablation. We observed that the induced massive loss of UCP1⁺ cells in adult mice caused no deleterious metabolic effects, but rather led to lower body weight and improved glucose tolerance at room temperature (RT, 22-24°C). In contrast, upon cold challenge (6°C), UCP1⁺ cell-ablated mice displayed severe cold intolerance associated with altered glucose and fatty acid metabolism. In these conditions, glucagon played a critical role, attempting to provide sufficient energy to cope with hypothermia, yet failing to compensate for the loss of UCP1⁺ cells. Finally, assessment of UCP1⁺ cell regeneration showed that UCP1⁺ cells partially recover over time, but restoration of cold tolerance was only achieved after chronic exposure to cold. Together, these results challenge the importance of UCP1⁺ cells for metabolism at RT, while underlying their critical importance in thermogenic, metabolic and hormonal control upon acute cold.