Quality improvement of fish feed by using insulin-like growth factor 1 as a potential indicator of amino acid deficiency in yellowtail

We developed a new method for improving the quality of fish feed, by using insulin-like growth factor 1 (IGF1) as a potential indicator of dietary amino acid deficiency in yellowtail (Seriola quinqueradiata). First, 7 dispensable amino acid solutions (alanine, asparagine, cysteine, glutamine, glycine, serine and proline) and 0.9% NaCl (control) solution were injected into the dorsal muscle of fish using a syringe. Twenty-four hours post-administration, white muscle tissue was collected from the fish opposite to the injection site and assayed for igf1 mRNA expression. Only fish injected with proline showed a significant increase in igf1 mRNA expression levels. Subsequently, a feeding trial was conducted with 3 different diets-the control diet; the control diet supplemented with 1% proline; and the control diets supplemented with 3% proline. Juvenile yellowtails were fed exclusively with 1 of the 3 diets until apparent satiation. The body weight and feed efficiency were significantly higher in fish fed with the 3% proline-supplemented diet than fish fed with the control diet. Further, after 3 weeks, the expression levels of igf1 in white muscle and liver were significantly higher in fish fed with the 3% proline-supplemented diet than in fish fed with the control diet. Our results suggest that igf1 mRNA expression in the white muscle tissue of yellowtails serves as a reliable indicator of amino acid deficiency. Additionally, the results of our feeding trial suggest that proline is required for juvenile yellowtail growth. Moreover, white muscle igf1 may represent a valuable tool for improving the quality of fish feed.