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

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Abstract
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-α-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.

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