Isocaloric substitution of plant-based for animal-based protein and physical fitness in adolescents: a cross-sectional study from the EHDLA study

Background Plant and animal protein sources have been associated with health outcomes, but evidence on physical fitness within isocaloric substitution frameworks remains limited. This study examined whether modeled isocaloric substitutions between plant- and animal-derived protein sources were associated with physical fitness in adolescents. Methods This cross-sectional analysis included adolescents aged 12–17 years from Spain. Source-specific protein variables were estimated from a 45-item food frequency questionnaire using reported consumption frequencies, standard portion sizes, and food-composition values. Physical fitness was evaluated using the Assessing the Levels of Physical Activity and Fitness (ALPHA-Fit). Using robust linear regression models, the association between a theoretical 5%-energy plant-for-animal protein contrast and physical fitness was estimated, while adjusting for age, sex, socioeconomic status, sleep duration, physical activity, sedentary behavior, body mass index, total energy intake, and energy from other macronutrients. Results The analytical sample comprised 790 adolescents (55.1% girls; median age: 14.0 years). Median energy intake was 2,585.9 kcal⋅day –1 . No associations were observed between the modeled 5% of total-energy substitution of animal protein by plant protein and physical fitness outcomes (all p > 0.05). In food-level models, the theoretical substitution of egg-derived protein by plant protein was associated with lower standing long jump performance (standardized beta coefficient [β] = −0.55, 95% confidence interval [CI]: −1.06 to −0.06) and slower 4 × 10 m performance (β = 0.58, 95% CI: 0.12 to 1.04). A similar association with slower 4 × 10 m performance was observed for the modeled substitution of oily-fish-derived protein by plant protein (β = 0.33, 95% CI: 0.01 to 0.65). The theoretical substitution of root-vegetable-derived protein by animal protein was inversely associated with overall physical fitness (β = −1.23, 95% CI: −2.17 to −0.29), whereas the modeled substitution of cereal-derived protein by animal protein was associated with faster 4 × 10 m performance (β = −0.53, 95% CI: −0.89 to −0.17). Conclusion In adolescents, modeled isocaloric substitutions between plant- and animal-derived protein sources were not associated with overall physical fitness, although food-level analyses showed heterogeneous associations, suggesting the relevance of the broader food matrix.