Effect of Recombinant Human Igf-I on Growth, Food Conversion and Fat Distribution in Relation to Hormone Plasma Levels in Broiler Chicks (original) (raw)

1991

Abstract

Insulin-like Growth Factor-I (IGF-I) is known to be a growth promoting factor in mammals. This effect also includes a shift from lipid to protein synthesis and IGF-I therefore has been proposed to function as a lean growth factor. In broilers selection has resulted in fast growing strains which however often show an undesired fat accumulation. In the following study, the effect of IGF-I on body weight, fat deposition and distribution, and on circulating levels of IGF-I, growth hormone (GH), tyroxine (T4) and triiodotyronine (T3) was examined. Different doses of IGF-I (0, 0.03, 0.1 and 0.3 mg/kg BW.day) were administered by mini-osmotic pumps in 40 broiler chicks of a commercial strain between 4 and 6 weeks of age since fat deposition is maximal during this period. Blood samples were taken prior to and 1 and 2 weeks after implantation. Plasma IGF-I, GH, T3 and T4 levels were measured by RIA. At the end of the experimental period all birds were killed, abdominal fat measured, leg and breast muscles dissected and fat extracted. Body weight and relative growth were not affected by IGF-I treatment. However, while fat content in breast and thigh muscles remain unchanged, % abdominal fat was decreased significantly (P < 0.01) with 0.3 mg IGF-I treatment. Plasma IGF-I levels increased, as expected, in a dose-dependent way, while GH did not change with treatment but decreased with increasing age in all groups. A transient decrease in plasma T4 and increase in T3 was observed, at least with the higher IGF-doses. After 2 weeks the effect of IGF on circulating thyroid hormone levels disappeared. Although no effect of IGF-I on body weight or growth rate in commercial broiler chicks was observed, a decrease in abdominal fat indicate a repartition of nutrients available for net energy accretion. The observation that only the abdominal fat depot was affected confirms many other reports indicating a higher flexibility in fat content for depot fat compared to intra-and intermuscular fat. Since T3 is extremely effective in reducing abdominal fat in chickens, the increased T3 levels following IGF-I administration may also be, partly or totally, responsible for the abdominal fat reduction. Further research into causes of the T3-IGF-I interaction and its consequences on animal growth are needed to elucidate this.

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