Submitted to: Domestic Animal Endocrinology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 6, 2000
Publication Date: December 12, 2000
Interpretive Summary: Excess fat production in the modern broiler accounts for an annual loss to the poultry industry of 800 to 950 million dollars annually. The original source of this problem relates to selection genetic practices that emphasized rapid growth at the expense of other carcass characteristics. The literature is of limited value in determining methods to depress fat synthesis and allow lean tissue synthesis to remain at an elevated rate. A strain of male chicks noted for its rapid growth was used to study the effects of dietary protein, triiodothyronine and feeding regimens in controlling fat synthesis. Liver tissue when put into a culture system exhibited a high rate of lipid synthetic ability. When tissue was taken from chickens fed triiodothyronine or fasted for 24 hr, it exhibited a decrease in the ability to produce fat. Likewise, triiodothyronine attenuated some of the deleterious effects noted when chickens were switched from a starter to a low protein grower diet. Regimens described i this study may be of value if the slight growth-depressing effect of triiodothyronine can be overcome.
There are conflicting results concerning the role of the thyroid hormones in lipid metabolism. The experiments in this report were designed to examine the role of T3 in modifying responses obtained by shifting birds from moderate to low protein diets. Birds were grown from 7 to 28 days on a diet containing 18% protein. At this time, birds were switched to a diet containing 12% protein ñ T3. The switch was accomplished either immediatel or after a 24 hr fast. Measurements taken included in vitro lipogenesis (IVL), hepatic enzyme activities and plasma metabolites and thyroid hormones. Simply switching to birds to the low protein diet increased IVL, but rates were similar for three days following the switch. Feeding T3 in this same regimen resulted in lower, but again, constant rates of IVL. In contrast, although switching protein levels after a 24 hr fast increased IVL, the rate after two days of refeeding was nearly double that following one day. This accentuated response was somewhat attenuated by including T3 in the diet. Neither fasting nor refeeding altered plasma T3 relative to ad libitum values. Supplemental dietary T3 increased plasma T3 and results were not affected by feeding regimens. Plasma T4 was greatest in birds fasted for 24 hr and least in birds fed T3, suggesting that feeding regimens may regulate the conversion of T4 to T3. It is suggested from this study that some of the effects of alterations in dietary feeding regimens can be modulated by T3.