(1) Background: Insufficient physical activity in adolescents remains an important issue for health promotion. Given the current relevance of understanding the adoption and maintenance of moderate and vigorous physical activity (MVPA), the aim of this study was to analyze, in a sample of adolescents, the role of grit personality as an antecedent of healthy eating and healthy weight (HEW) self-efficacy and its implications for the practice of MVPA. (2) Methods: Participants were 987 adolescents (597 girls, 390 boys) aged between 15 and 19 years from Mexico and Spain. The Spanish versions of the grit personality scale, the healthy eating and weight self-efficacy scale and the global physical activity questionnaire were used to measure the variables of interest. (3) Results: Mediated regression analysis showed that grit personality was not directly related to MVPA practice. However, the results indicate the significant relationship between grit personality and HEW self-efficacy, as well as the positive and significant relationship of this self-efficacy on MVPA practice. HEW self-efficacy totally mediated the relationship between grit personality and MVPA in both boys and girls. (4) Conclusions: These results suggest that having a grit personality (i.e., having interest and perseverance) is not enough for adolescents to be physically active, but that perceiving oneself as effective in having a healthy diet and healthy weight may be the key for adolescents to move more. At the intervention level, we suggest targeting an enhancement of young people's competence to eat healthily and regulate their weight as a strategy to enhance the performance of more MVPA, with a possible transfer between healthy behaviors (spill over).
Keywords: adolescents; grit personality; healthy eating; healthy weight; physical activity; self-efficacy.