Glutamate (Glu) and glutamine (Gln) are considered non-essential amino acids. However, the capacity to synthesise these amino acids endogenously may not meet the requirements, deeming them conditionally essential. This may be due to several reasons, including production-related stressors such as handling, crowding, or the replacement of high-quality protein sources in fish diets. When endogenous synthesis is insufficient, protein synthesis, immunity, antioxidant defence, cell signalling, gene expression and intestinal integrity may be compromised, which can affect overall health and performance.

This study aimed to evaluate existing knowledge on the dietary supplementation of Glu and Gln. For this, a comprehensive literature search was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines, resulting in a dataset of 32 papers covering 19 different species. A meta-analysis was conducted to systematise the available data, better understand the effects of Gln and Glu on fish growth and identify knowledge gaps. The inclusion criteria were that at least two dietary treatments (control and supplemented diet) had been evaluated in peer-reviewed papers. Studies without growth results or in vitro, or with other supplementation, were excluded.

To enable cross-study comparison, the percentage of maximum weight gain (%WGMax) was used to normalise heterogeneous datasets. Most studies were conducted on juveniles and freshwater species, with Gln supplementation found to be more frequently investigated than Glu. Supplementation levels and trial durations varied widely, although most experiments lasted up to 60 days. Overall, the dietary inclusion of Gln or Glu showed consistent potential to improve growth performance, feed utilisation, and intestinal health. However, defining optimal supplementation levels remains challenging due to variability in study design, diet composition, trophic level, and developmental stage.