For a century, the study of human body weight regulation has been dominated by the Energy Balance Model (EBM) and challenged by the Carbohydrate-Insulin Model (CIM). Both frameworks, however, rely on the indirect conversion of mass to energy, introducing propagated uncertainties and theoretical stalemates. The Eagle Project proposes a paradigm shift: direct mass accounting. By conducting feeding experiments in state-of-the-art whole-room metabolic chambers capable of continuous measurement for up to seven days, we will quantify every gram of mass entering and leaving the human body (food, beverages, O₂, CO₂, urine, feces, sweat). This extended duration eliminates the need for mathematical extrapolations. Furthermore, the project will integrate comprehensive, high-frequency blood sampling to map the full endocrine and metabolic response against physical mass fluxes. Committed to radical transparency, all raw data will be published in near real-time via preprints, free of paywalls and embargoes. The Eagle Project is not merely an experiment – it is a global, collaborative effort to resolve the metabolic stalemate and establish a new, physically rigorous foundation for nutritional science.