Acute diseases are characterized by a catabolic state, resulting in a negative nitrogen balance and muscle wasting. Increasing protein intake often proves to have little effect in limiting muscle protein loss. This suggests a qualitative inadequacy of the usual nutritional supports to meet the amino acid requirements of the critically ill patient. Therefore, it can be assumed that the additional intake of limiting amino acids would allow the sparing of muscle proteins. The aim of this review is to examine whether metabolic and kinetics studies using labelled amino acids can help identify the pathways activated in injury and their specific amino acid requirements. The kinetics of cysteine, arginine and glutamine, which are mainly cited as conditionally indispensable in stress situations, are presented. Moreover, amino acids can act as mediators or signal molecules and modulate numerous functions. The optimal conditions allowing the best expression of these activities are discussed.