Endocrinology, Vol 115, 1239-1247, Copyright © 1984 by Endocrine Society
Aging and dietary modulation of rat skeleton and parathyroid hormone
DN Kalu, RH Hardin, R Cockerham and BP Yu
Studies were carried out on SPF F344 male rats to evaluate the effects of
aging and life-prolonging food restriction, without malnutrition, on rat
skeleton and circulating PTH. Six-week-old F344 rats were divided into five
groups. Group 1 rats were fed ad libitum a diet that contained 21% protein.
Group 2 rats were fed 60% of the mean food intake of group 1 rats from 6
weeks of age for the rest of their lives. Group 3 rats were fed 60% of the
ad libitum food intake until 6 months of age and then switched to ad
libitum feeding. Group 4 rats were fed ad libitum until 6 months of age,
and then switched to 60% of the ad libitum food intake. Group 5 rats were
fed ad libitum a diet that contained only 12.6% protein so that these
animals ingested the same amount of protein per day as the group 2 rats. In
group 1 animals, bone length, weight, density, and calcium content
increased rapidly with age and plateaued at about 12 months of age. There
was no evidence of bone loss in these animals until about 24 months of age,
but by 27 months, the animals had lost appreciable amounts of bone. The
circulating immunoreactive PTH levels of the animals increased with
advancing age, with a marked rise at 27 months. The age-related changes in
bone and serum PTH levels of rats in groups 3 and 5 were similar to those
of group 1 animals, except that a terminal increase in serum PTH did not
occur in group 5 rats. In the groups 2 and 4 animals which were food
restricted for the longest period, bone growth and maturation were slowed
down, but the animals did not experience senile bone loss or marked
terminal increase in circulating PTH. The salutary effects of food
restriction were, therefore, not due specifically to the restriction of
protein intake or to restricting food intake only during the period of
rapid growth.
