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Thyroid Function, Energy Balance, Body Composition and Organ Growth in Protein-Deficient Chicks¹

Bioresearch Laboratory, Department of Animal Sciences ^* Department of Human Nutrition and Foods, University of Vermont, Burlington, VT 05405

Protein-deficient diets (17, 10, 6.5 or 3% protein) and a 24% control diet were fed to growing chicks. A control group was pair-fed daily with each deficient group. Energy intake was lower in the 6.5 and 3% protein groups than in the other groups. However, weight gain, bone growth and feed conversion efficiency were lower with 10% protein or less. Relative thyroid weights were unaffected by dietary protein. Plasma T₃ (3,5,3'-trilodothyronine) levels were significantly higher in all deficient groups, whereas plasma T₄ (thyroxine) was lower. Plasma rT₃ (reverse T₃) was unaffected by the protein deficiencies, suggesting that enhanced conversion of T₄ to T₃ rather than to rT₃ had occurred. Hepatic -glycerol-3-phosphate dehydrogenase (-GP) shuttle activity increased markedly in protein-deficient chicks. Efficiency of energy utilization was unaltered in chicks fed 17 or 10% protein but was higher in chicks fed 6.5 and 3% protein than in controls. All deficient chicks had more fat and less protein and water in the tissues. The lower feed conversion efficiency therefore represents almost entirely a shift in body composition toward fat and does not reflect a loss of energy as heat. We conclude that elevations in plasma T₃ and in thyroid-controlled -GP shuttle activity, although sensitive indicators of protein deficiencies, are not good predictors of altered thermogenic activity in protein-deficient chicks.

KEY WORDS: • protein deficiency • thyroid hormones • thermogenesis • -glycerophosphate • energy utilization • body composition

¹ Research supported by the Vermont Agricultural Experiment Station, University of Vermont, Burlington, Hatch Project VT 353, and by the University of Vermont Research Advisory Council No. 80-02.

² To whom reprint requests should be sent.

Manuscript received 2 December 1986. Revision accepted 26 May 1987.