Chronic effect of leucine supplementation on strength performance: a systematic review and meta-analysis

Leucine is an amino acid that stands out with its capacity to increase muscle protein synthesis and activate the mTOR signaling pathway. This meta-analysis systematically evaluates the chronic effects of leucine and its metabolites on strength performance. In the literature search conducted in accordance with the PRISMA guideline, 366 studies were examined in Web of Science, PubMed, Academic Search Ultimate, and Scopus databases, and 20 studies that met the eligibility criteria were included in the meta-analysis. The findings show that leucine supplementation provides a significant effect on upper body maximum strength (SMD = 0.24; 95% CI: 0.008–0.48; I² = 25.1%). This low level of heterogeneity supports the reliability of the analyses. In contrast, small but non-significant effects were found in the analyses of lower-body strength (SMD = 0.17; 95% CI: -0.26 to 0.61), whole-body strength (SMD = 0.17; 95% CI: -0.24 to 0.59), hypertrophy (SMD = 0.07; 95% CI: -0.17 to 0.32), and power performance (SMD = 0.37; 95% CI: -0.05 to 0.79). The high heterogeneity rate in the lower body strength analyses (I² = 68.8%) was related to methodological variation and differences in the participant groups. In contrast, low heterogeneity levels were obtained in parameters such as hypertrophy (I² = 9.08%) and power performance (I² = 17.6%). The results show that the effects of leucine supplementation depend on factors such as individual differences, training level, dosage and metabolite type. Leucine provided more pronounced effects, especially in studies involving untrained individuals. In the future, it is recommended that these factors be examined in more detail by subgroup analyses and methodologically consistent protocols be adopted.