The pooled RR of incident MetS was 1.81 (95% CI: 1.49–2.14; P = 0.013, Figure 3) for the highest versus lowest category of ALT activities in eight data clusters from six studies (involving 28988 participants and 1936 cases); high heterogeneity was found in overall (I2 = 60.7%, P = 0.013) and IDF-defined studies (I2 = 76.7%, P = 0.005) while no statistical heterogeneity was observed in NCEP-ATPIII-defined studies (I2 = 0.0%, P = 0.410). This evidence concerns the gene GPT and metabolic syndrome.