Submitted to: Feedstuffs
Publication Type: Review Article
Publication Acceptance Date: April 3, 2000
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 tboth 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.
Male broiler chickens growing from 7 to 35 days were fed a diet containing 15% crude protein/kg diet supplemented with lysine to equal that in diets containing 17, 18.3 and 20% crude protein/kg diet. A second group of male broiler chickens growing over the same period were fed a diet containing 12% crude protein supplemented with lysine, arginine, tryptophan, threonine eand isoleucine equal to that in diets containing 14.4, 17.2 and 20% g crud protein. Growth was improved by lysine supplementation but not to the level attained by feeding 20% crude protein. Lysine, arginine, tryptophan, threonine and isoleucine supplementation of a low-protein diet also improved growth, but growth again fell short of that attained by feeding a diet containing 20% crude protein. Plasma insulin-like growth factor-I (IGF-I) and thyroxine (T4) concentrations increased and triiodothyronine decreased as the crude protein level increased from 15 to 20% diet. Supplemental lysine did not affect plasma levels of these hormones. Although dietary crude protein noticeably changed rates of in vitro lipogenesis, changing either the level of a single limiting amino acid or the levels of several limiting amino acids did not change lipogenesis. Indices of either metabolism or plasma hormones may be of limited utility is assessing dietary protein quality.