The objective of this work is the evaluation of how sex, age and the kind of followed treatment affect weight loss, as well as the development of an explanation to assess the evolution of weight loss dynamics in overweight men and women. The methods use include a study with hundred and nineteen overweight participants (18 – 50 years; Body Mass Index (BMI) >25 and <29.9 kg/m2), which were randomly assigned to four treatments, namely, strength training (S; n = 30), endurance training (E; n = 30), a combination of S and E (SE; n = 30) training, and a careful treatment including diet and physical recommendations (C; n = 29). Each of the different training groups exercised three times per week during twenty-four weeks, and their daily diet was also carefully controlled and restricted to a specific protocol during the testing period. Then, body weight changes in the different participants were evaluated every fifteen days. Based on this study, we developed and validated different sets of equations to accurately capture the weight loss dynamics. Results: There were no significant differences in terms of global body weight changes from the statistical viewpoint, neither concerning the carried out treatment nor individuals’ ages. However, significant differences on the weight loss tendency were found due to the participant sex. Conclusion: The effectiveness of the different possible treatments for weight loss varies by sex and, based on our experimental observations, a quadratic function fit provides the most accurate model to capture the specific weight loss dynamics.