The energy balance model (EBM) and its operational form, calories-in-calories-out (CICO), have dominated obesity research for nearly a century. While these frameworks have delivered valuable public-health insights, they rest on indirect mass-to-energy conversions and persistent misconceptions about thermodynamic principles. Here I demonstrate that a first-principles mass balance model (MBM) provides a conceptually simpler, mathematically consistent, and mechanistically superior alternative. By tracking macronutrient mass directly in grams – without intermediate energy conversions – the MBM aligns analysis with physiological reality and delivers 40–65% lower propagated uncertainty than conventional energy-balance approaches. I clarify that calories cannot be eaten or oxidized, that E = mc² is irrelevant to human metabolism, and that the First Law of Thermodynamics concerns only energy, not mass. I further show that both the carbohydrate-insulin model and the recently proposed protein partitioning model remain anchored in the same energy-accounting framework the MBM transcends. Extending the analysis to practical domains, I demonstrate why the body senses mass restriction, not calorie restriction; why exercise induces weight loss only when it produces a net negative mass balance; how the MBM explains adaptive thermogenesis as an emergent property of mass-clearance down-regulation rather than an unexplained residual; and what mass-balance principles demand of pharmacotherapy and supplementation. Together, these arguments establish the MBM as a more parsimonious, mechanistically faithful, and clinically actionable paradigm for human body weight regulation.