**Figure 9.** **Subgroup analysis of GRx levels stratified according to sex.** The test for subgroup differences showed a significant subgroup effect (*p* = 0.69), suggesting that sex did not modified the effect of MT intervention, compared with vehicle-treated groups. It is interesting to note that the pooled effect estimate for both subgroups favors MT intervention over the control intervention. The unit for GRx almost in all studies is nmol/mg, except Nogues MR et al. (nmol/mL). The prism represents the overall statistical results of the experimental data, squares represent the weight of each study, and horizontal lines represent the 95% CIs for each study. GRx, glutathione reductase; MT, melatonin; I, heterogeneity; CI, confidence interval; SD, standard deviation; IV, independent variable.