Effects of Insufficient Levels of Dietary Protein on IGF-I and IGFBPs in Young Chickens

The influence of varying dietary proteins levels from 0% to 18%, which is the level of dietary protein requirement, on plasma IGF-I concentration and tissue IGFBP-2 and -4 gene expression was examined. Body weight gain was gradually lowered by decreasing dietary protein levels from 18% to 4.5%, and body weight was lost when chickens fed a protein-free diet. Plasma IGF-I concentration of chickens fed the 18% crude protein diet was the highest of all, and it was gradually decreased by reducing dietary protein concentrations. Gene expression of IGFBP-2 in the gizzard was elevated by decreasing dietary protein levels from 18% to 9%, and the further reduction in dietary protein content lowered gizzard IGFBP-2 gene expression. No significant change in hepatic IGFBP-4 gene expression was observed in chickens fed experimental diets with varying dietary protein levels. Multiple linear regression analysis revealed that the influence of the change in plasma IGF-I concentration and gizzard IGFBP-2 gene expression on body weight change of chickens fed varying amounts of dietary protein seemed to be independent because the interactive effect of both factors on body weight change was not significant.