Intense physical activity imposes substantial oxidative, metabolic, and immunological stress on the human body. It is often accompanied by reductions in plasma glutamine levels, making this amino acid conditionally essential. Glutamine plays a vital role in energy production, nitrogen transport, acid–base balance, antioxidant defense, and immune function. It is required in the biosynthesis of neurotransmitters, nucleotides, nicotinamide-derived coenzymes, glutathione, and hexosamines, making it a candidate for supporting exercise recovery. In addition, glutamine may support key mechanisms involved in muscle adaptation and recovery during exercise-induced stress by contributing to redox balance, energy sensing, anabolic signaling, intestinal barrier integrity, and immune function. This narrative review aims to synthesize biochemical mechanisms underlying glutamine effects relevant to exercise and evaluate preclinical and clinical findings on supplementation outcomes, with emphasis on timing strategies. Preclinical findings demonstrate that glutamine can modulate protein synthesis, reduce oxidative stress, improve intestinal integrity, and attenuate immune and inflammatory disturbances. Limited preclinical data suggest that post-exercise supplementation may better resolve muscle and organ damage. Clinical trials, however, report heterogeneous outcomes: several studies show improvements in markers of intestinal permeability and intestinal epithelial damage, oxidative stress, muscle damage, and inflammation, whereas others report minimal or no effect, including limited influence on performance outcomes. Variability in timing protocols, participant characteristics, and measured endpoints contributes to inconsistent findings. Overall, glutamine demonstrates several biologically plausible mechanisms that could support recovery and overall health in active individuals, athletes, and specific clinical populations. However, current evidence remains insufficient to determine clear supplementation benefits or define an optimal timing strategy. Future research using standardized protocols and integrated biochemical and functional endpoints is needed to clarify timing effects. Until such evidence emerges, recommendations should remain individualized, considering athlete-specific needs.
Lazarević et al. (Fri,) studied this question.