The New Biophysics of Weight Change

The energy balance model posits that body mass stability is achieved when the daily average absorbed energy intake equals the daily average expended energy. However, this work presents a mathematical analysis demonstrating that this explanation is incorrect. Precise calorimetry data from two major randomized controlled trials support this conclusion by showing significant inconsistencies with the principles of this paradigm. As an alternative, a mass balance approach is introduced to model body weight fluctuations. This new framework is computationally formulated based on daily records from a 367-day weight reduction self-experiment conducted by the author. Remarkably, the analysis revealed that body mass elimination conforms to Torricelli’s Law. This unexpected finding has an analytical origin and accurately predicted the weight loss evolution in studies on prolonged fasting. Both the new biophysical paradigm and the energy balance model were given the self-experiment initial conditions and then challenged to predict the actual body composition trajectory. The results are outstanding: the mass balance model precisely predicts the self-experiment using only simple measurements and without parameter fitting. In contrast, the energy balance model produced an inadequate description of the self-experiment even after fitting procedures. Therefore, the mass balance approach accurately correlates changes in body composition with alterations in food intake without requiring complex and expensive instrumentation. This can lead to more effective strategies for addressing overweightness by focusing on the true variables driving weight change, rather than relying on erroneous principles. Consequently, adopting a mass balance paradigm for weight management will undoubtedly advance our understanding of obesity. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.