Excess fat accumulation is a key feature of overweight and obesity that is mainly driven by nutrient overload and insufficient physical activity. White adipose tissue displays lipid overload and hypertrophy accompanied by macrophages infiltration, hypoxia, inflammation and excess production of reactive oxygen species (ROS). An inflammatory response and ROS production are also evident in other metabolism regulating tissues and organs such as skeletal muscle, liver, pancreas and hypothalamus, contributing to a chronic inflammatory state, redox status disturbances and metabolic complications. There is overwhelming evidence showing that adults with overweight/obesity exhibit lower glutathione (GSH) levels in blood erythrocytes, skeletal muscle cells and subcutaneous and visceral adipose tissue cells. GSH, a tripeptide consisting of the amino acids glutamate, cysteine and glycine, is the most abundant thiol-containing antioxidant in the human body and has been, recently, characterized as a novel therapeutic target for the treatment of numerous chronic diseases, due to its potent intracellular redox buffering capacity. Interestingly, lower GSH levels have been associated with diet-induced weight loss resistance, while enhancement of GSH levels through N-acetylcysteine (NAC) supplementation reduces markers of oxidative stress, inflammation, insulin resistance, hypertension, endothelia dysfunction and improves vitamin D metabolism. NAC is a thiol donor that elicits antioxidant effects by (i) directly scavenging ROS and (ii) providing reduced cysteine through deacetylation, which supports the biosynthesis of endogenous GSH via the activity of γ-glutamylcysteine synthase. The aim of this study is to investigate whether NAC supplementation can enhance the exercise training-induced improvements on physical fitness and metabolic health in adult men and women with overweight/obesity.