Dietary protein effects on the Broiler's adaptation to triiodothyronine (T-3)

We have shown that low-protein diets increase lipogenesis and decrease feed intake. Carbohydrates are secondary to protein level under conditions of suboptimal protein nutrition. Metabolic responses are controlled by protein, rather than energy under these conditions. Male broilers were fed diets containing either 120 or 300 g crude protein/kg from 7 to 28 d. One-half of each group was given diets supplemented with 1 mg/ kg T-3. Birds wore 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. Pancreatic norepinephrine was measured to ascertain if central nervous system regulation could account for metabolic changes As expected. IVL was greater at 28 d in birds fed the lower level of protein. Moreover, when T-3 was added to either diet. IVL decreased by 50% after 5 d. Plasma IGF-I and T-4 were greater while T-3 and GH were less when birds were fed the higher level of crude protein. Plasma T-3 increased and T-4 decreased 3 d following the addition of T-3 regardless of dietary protein level. 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. Metabolic changes in response to feeding T-3 occurred within 2 to 5 d, suggesting that metabolic changes preceded morphological changes. Dietary protein levels do not alter short-term responses to T-3.