THE FOOD-INDUCED STIMULATION OF BONE-RESORPTION IN THE RAT, ASSESSED BY THE URINARY [H-3] TETRACYCLINE EXCRETION, IS MEDIATED BY PARATHYROID-HORMONE

Using the urinary excretion of[H-3]-tetracycline from prelabeled rats to monitor bone resorption, we have previously shown that food intake is associated with a rapid and large increase in bone resorption. This increase is blunted when the daily intake is fractionated into 4 port ions instead of being given at once. Food fractionation also leads to a large increase in bone mass. In order to establish whether the thyro parathyroid gland is involved in this effect, thyroparathyroidectomize d (TPTX) rats were studied. The food-induced increase in bone resorpti on was absent in TPTX rats. Therefore, the pattern of parathyroid horm one (PTH) was investigated during the development of the food-induced bone resorption, and under food fractionation. Rats were trained to ea t their daily high Ca food in less than two hours. Thereafter they wer e given food portions of 5 or 20 grams, respectively. PTH in serum was measured at 0, 1, 2, 3, and 4 hours after food intake. In rats given the large food portions, a conspicous increase of PTH was found. In co ntrast, serum PTH of rats fed the small food portion did only change t o an insignificant extent. This study in rats shows that the ingestion of a large meal induces a transient increase of PTH. The present resu lts can therefore explain the formerly observed acute increase in bone resorption following food intake and its blunting by food fractionati on. It is not known, whether such a mechanism occurs also in humans. I f so, these results would provide the rational basis to decrease bone resorption by food fractionation.