Submitted to: Growth Development and Aging
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 25, 1999
Publication Date: N/A
Interpretive Summary: Excess fat production by the modern broiler chicken presents a two-fold problem. The consumer has health concerns about the link between cardiovascular disease and dietary fat. The producer would like to produce more lean meat rather than fat which is condemned at the processing plant. Adding dietary thyroid hormones to the broiler diet will depress the ability of the chicken to synthesize fat from dietary carbohydrate and protein sources. Historically this shift in metabolism has resulted in dietary fat being merely shunted to replace that synthesized from other feed ingredients. We have found that altering the thyroid axis in the broiler will cause permanent changes in fat synthesis and storage, such that dietary fat will not be shunted to body fat stores. The present study demonstrates that dietary protein levels do not measurably affect the response of broilers to thyroid hormones. This study further indicates that both dietary protein levels and thyroid hormones change avian lipid metabolism. This finding is opposite to that noted in mammals. Cost-benefit data must be derived for each producer before a blanket recommendation can be made for altering diets to accommodate metabolic modifiers.
The purpose of this experiment was to study the effects of dietary protein on metabolic adaptations in birds given T3 at 28 d of age. Knowledge about the role of dietary protein in adaptations to perturbations of thyroid metabolism is lacking. Male broiler chickens were fed diets containing either 120 or 300 g crude protein/kg from 7 to 28 d. At this time, one-half of each group was given that diet supplemented with 1 mg/ kg T3. Birds were sampled at 0, 2, 5, 7, 9 and 12 d following the switch. Measurements taken at these intervals included in vitro lipogenesis (IVL), growth and feed consumption, hepatic enzyme activities and plasma thyroid hormones and metabolites. As expected, IVL was greater at 28 d in birds fed the lower level of protein. Moreover, when T3 was added to either diet, IVL decreased by 50% after 5 d. Plasma IGF-I and T4 were greater while T3 and GH were less when birds were fed the higher level of crude protein. Plasma T3 increased and T4 decreased 3 d following the addition of T3 to diets containing either level of crude protein. Pancreatic amylase was greater and norepinephrine less in birds fed the higher level of crude protein, lending some support for neural control of pancreatic activities. Most metabolic changes in response to feeding T3 occurred within 2 to 5 d, suggesting that changes in intermediary metabolism preceded morphological changes. In addition, dietary protein levels may not affect adaptive responses to T3.