Background & Aims Recently, the validity of mass balance model (MBM) was questioned based on two feeding studies. Thus, we simulated both of these feeding trials. Methods MBM describes the temporal evolution of body weight and body composition under a wide variety of feeding experiments. This computational study simulated, utilizing MBM, the underfeeding trial by Hall et al. (Cell Metab. 2015;22:427-36) and the overfeeding trial by Horton et al. (Am J Clin Nutr. 1995;62:19-29). Results Our simulation results indicate that data from both of these feeding trials perfectly match MBM-based predictions, i.e., MBM gives a remarkably accurate description of experimental data. Conclusions It is becoming increasingly clear that our model (MBM) is perfectly able to predict body weight and body composition fluctuations under a wide variety of feeding experiments.