In this paper, I analyze the constraints that limit biomass from becoming an alternative, sustainable and efficient energy source, at least in relation to the current metabolism of developed countries. In order to be termed sustainable, the use of an energy source should be technically feasible, economically affordable and environmentally and socially viable, considering society as a whole. Above all, it should meet society’s “metabolic needs,” a fundamental issue that is overlooked in the mainstream biofuels narrative. The EROI (Energy Return on Investment) of biofuels reaches a few units, while the EROI of fossil fuels is 20–30 or higher and has a power density (W/m2) thousands of times higher than the best biofuels, such as sugarcane in Brazil. When metabolic approaches are used it becomes clear that biomass cannot represent an energy carrier able to meet the metabolism of industrialized societies. For our industrial society to rely on “sustainable biofuels” for an important fraction of its energy, most of the agricultural and non-agricultural land would need to be used for crops, and at the same time a radical cut to our pattern of energy consumption would need to be implemented, whilst also achieving a significant population reduction.