AJP - Endocrinology and Metabolism, Vol 261, Issue 3 E382-E388, Copyright © 1991 by American Physiological Society
Maintenance of thyroid hormone production during exercise-induced weight loss
H. L. Katzeff and C. Selgrad
Department of Medicine, North Shore University Hospital, Manhasset, New York.
Calorie restriction reduces thyroxine (T4) and 3,5,3'-triiodothyronine (T3)
production, but the effects of exercise-induced weight loss on thyroid
hormone metabolism in rodents are unclear. We studied the effects of
chronic exercise on T4 and T3 metabolism comparing exercising (exercise)
rats pair fed to sedentary (control) rodents and to weight-matched underfed
sedentary animals (underfed; caloric intake 75% of ad libitum-fed
controls). The exercise group utilized voluntary running wheels (28 days),
and thyroid hormone metabolism was assessed using a three-compartment
kinetics model. The exercise and underfed groups were equivalent in weight,
but protein mass was greater in the exercise vs. underfed groups (30.4 +/-
0.5 vs. 27.9 +/- 0.5 g; P less than 0.05). During exercise, the T4 plasma
clearance rate (PCR) was decreased (-39.2%; P less than 0.01) and the T4
concentration in serum was increased (48.6%; P less than 0.01), resulting
in an unchanged T4 plasma appearance rate (PAR) vs. the control group. The
decrease in T4 PCR in the exercise group was associated with a lower
transport rate of T4 out of the slow pool (P less than 0.01). In the
underfed group there was a reduction in both T4 serum concentration and PAR
(-36%; P less than 0.01) compared with the control group, which was
associated with a decrease in the volume of distribution (-25%; P less than
0.01). T3 PAR decreased 38.7% (P less than 0.01) during underfeeding but
only 16.9% (P = not significant) during exercise vs. the control
group.(ABSTRACT TRUNCATED AT 250 WORDS)
