Thyroid function, energy balance, body composition and organ growth in protein-deficient chicks

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'-triiodothyronine) 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 alpha-glycerol-3-phosphate dehydrogenase (alpha-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 alpha-GP shuttle activity, although sensitive indicators of protein deficiencies, are not good predictors of altered thermogenic activity in protein-deficient chicks.