Abstract
The effects of a 50% reduction in normal food intake for a period of 10 weeks were measured on secretion of growth hormone (GH), thyroxine (T 4), and triiodothyronine (T 3) in 5 1 2 −6 1 2 - month-old, and 17 1 2 −18 1 2- month-old male rats. In full-fed controls, GH, T 3, and T 4 were lower in the old and middle-aged than in the young rats. By the 10th week of underfeeding, GH, T 3, and T 4 were reduced in all age groups, but the decrease in T 3 and T 4 in the middle-aged and old rats was greater than in the young rats. Pulses of GH ceased in all the underfed groups. Upon refeeding for 5 days, pulses of GH and levels of GH returned to full-fed control values in the young and middle-aged but not in the old rats. T 3 values in the young and middle-aged rats returned to full-fed control levels, but remained below control levels in the old rats. T[in4 values reached control levels in all age groups upon refeeding. The differences in the response to underfeeding and refeeding by the middle-aged and old rats as compared to the young rats may be due to their initially lower secretion of GH and thyroid hormones and to the age-related decrease in neuroendocrine function.
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