Increasing Age Impairs the Thyroid Hormone Response to Overfeeding 1

Abstract

Abstract

Aging in both man and rodent is associated with increases in body weight and body fat. In young adult rats, mixed calorie overfeeding increases triiodothyronine production and decreases weight gain efficiency compared with chow diets. This study was undertaken to determine whether 4- and 14-month-old adult rats have similar responses in thyroxine (T₄) and triiodothyronine (T₃) metabolism during mixed calorie overfeeding. Four- and 14-month-old male Sprague-Dawley rats were each separated into two groups (n = 14): control (CHOW) versus chow + mixed calorie (CAFE) overfeeding for 28 days. The 4-month-old CAFE-fed rats ingested 662 ± 54 extra kcal over 28 days and gained 147 ± 7 vs 113 ± 5 g (P < 0.01) for their age-matched CHOW group. The 14-month-old CAFE group ingested 309 ± 45 extra kcal and gained 58 ± 6 g weight vs −12 ± 5 g for their age-matched CHOW group (P < 0.01). Serum T₄ concentrations were unchanged during overfeeding or aging. The serum T₃ concentration was increased 24% in the 4-month-old CAFE group compared with the age-matched CHOW group (P < 0.05), but there was no difference in the serum T₃ concentration between the 14-month-old CAFE and CHOW groups. The metabolic clearance and production rates of T₃ and T₄ were decreased in the 14-month-old vs 4-month-old groups (P < 0.01). The T₃ metabolic clearance rate was increased in the CAFE versus CHOW group in the 4-month-old groups (137.3 ± 11.2 vs 103.0 ± 10.1 ml/kg/hr, P < 0.01) but unchanged in the 14-month-old CAFE and CHOW groups (47.6 ± 6.6 vs 53.4 ± 5.3 ml/kg/hr, not significant). Liver type I iodothyronine 5′-deiodinase activity increased during overfeeding in the young (63.6%, P < 0.02) but not in the older rat group.