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Hypocalcemia, Hyperparathyroidism and Bone Resorption in Rats Induced by Dietary Phosphate¹

Department of Food Science, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801

Adult rats "deep labeled" with ⁴⁵Ca were meal-fed either a control diet (0.6% Ca, 0.3% P) or a high phosphate diet (0.6% Ca, 1.2% P). Blood samples taken at hourly intervals after a 1-hour meal revealed a significantly lower plasma Ca concentration (P < 0.001) in the high P group, although the values for both groups fell within the "normal" physiological range. This depression in plasma Ca was associated with an increase in ⁴⁵Ca excretion which previously has been shown to represent a net increase in bone resorption. Switching the diets resulted in a prompt reversal of plasma Ca levels and rates of ⁴⁵Ca excretion. The uptake of 1-¹⁴C--aminoisobutyric acid by the parathyroid gland was significantly greater (P < 0.02) in the high phosphate group, indicating a stimulation of parathormone synthesis. After feeding the high P diet for several days the plasma Ca titer was found to be chronically depressed, even at the end of the 11-hour fast between meals. These results show that a high P intake depresses plasma Ca concentration sufficiently within the range generally considered normal to cause a stimulation of parathyroid hormone secretion and an increase in bone resorption. The significance of these observations for human nutrition is discussed.

KEY WORDS: • calcium • phosphorus • parathyroid

¹ Supported by funds from the Illinois Agricultural Experiment Station, research grant number 102 from the National Dairy Council, and NIH grant AM 17169.

² To whom reprint requests should be addressed.

Manuscript received 4 April 1974.

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