Effect of triiodothyronine on leucine kinetics metabolic rate, glucose concentration and insulin secretion rate during two weeks of fasting in obese women

We investigated the effect of the fasting-induced reduction in triiodothyronine (T₃) on metabolic changes that occur during fasting. In five obese women (T₃ group) the serum T₃ level during a 2200 kcal/day diet was 3.34 ± 0.23 nmol/l while taking 20 μg T₃ 8-hourly, 0.65 ± 0.04 after one week of fasting while not taking T₃, and 2.9 ± 0.3 after a second week of fasting while taking T₃ 20 μg 8-hourly. In three other obese women (control group) who did not take T₃, T₃ concentrations decreased during fasting and remained low at the end of both the first and second week of fasting. In the T₃ group, resting metabolic rate (RMR) (O₂ ml/min) decreased 11.7 percent at the end of one week of fasting, but then increased by 5.3 percent at the end of the second week of fasting, whereas in the control group RMR decreased by 13.1 percent after one week of fasting and a further fall of 13.3 percent occurred by the end of the second week of fasting. The RMR at the end of 2 weeks of fasting of the control group was significantly lower (P < 0.05) than predicted using the equation of Dore, whereas that of the T₃ group was as predicted using the same equation. The changes in leucine flux, leucine oxidation and non-oxidative portion of leucine flux were not different during fasting between the T₃ group and the control group. Plasma concentrations of glucose, insulin and C-peptide increased from the end of one week of fasting to the end of 2 weeks of fasting in the T₃ group, whereas they decreased in the control group. It is concluded that the changes in T₃ concentration during fasting affected RMR, glucose metabolism and insulin secretion but failed to influence leucine metabolism.