Supplemental triiodothyronine, feeding regimens, and metabolic responses by the broiler chicken

There are conflicting results concerning the role of the thyroid hormones i n lipid metabolism. The experiments in this report were designed to examine the role of T-3 in modifying responses obtained by shifting birds from mod erate to low protein diets. Birds were grown from 7 to 28 d on a diet conta ining 18% protein. At this time, birds were switched to a diet containing 1 2% protein +/- T-3. The switch was accomplished either immediately or after a 24 hr fast. Measurements taken included in vitro lipogenesis (IVL), hepa tic enzyme activities and plasma metabolites and thyroid hormones. Simply s witching to birds to the low protein diet increased IVL, but rates were sim ilar for three days following the switch. Feeding T-3 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 t wo days of refeeding was nearly double that following one day. This accentu ated response was somewhat attenuated by including T-3 in the diet. Neither fasting nor refeeding altered plasma T-3 relative to ad libitum values. Su pplemental dietary T-3 increased plasma T-3 and results were not affected b y feeding regimens. Plasma T-4 was greatest in birds fasted for 24 hr and l east in birds fed T-3, suggesting that feeding regimens may regulate the co nversion of T-4 to T-3. It is suggested from this study that some of the ef fects of alterations in dietary feeding regimens can be modulated by T-3. ( C) 2000 Elsevier Science Inc. All rights reserved.