EFFECT OF AGE, CALCIUM SOURCE, AND RADIOLABELING METHOD ON WHOLE-BODYCA-47 RETENTION IN THE RAT

In a longitudinal study we determined the effect of animal age as well as Ca source and radiolabeling method on Ca bioavailability by measur ing whole body 47Ca retention (WBR) in male Sprague-Dawley rats. The W BR assay was performed without surgery or anesthesia, and the same gro ups of animals were studied at 8, 16, 20, and 32 wk of age. Rats were administered a 6-mg radiolabeled oral dose of Ca as Ca citrate malate (CCM) or intrinsically or extrinsically labeled CaCO3 or hydroxyapatit e (HAP). Fractional Ca retention was measured from the 72-h postdose W BR divided by WBR at time 0. WBR was significantly affected by Ca sour ce with CCM > CaCO3 > HAP at all ages (P < 0.001). The rank order and relative bioavailabilities of these Ca salts in the rat model agreed w ell with literature values for Ca absorption in adult humans. Although percent WBR decreased significantly with advancing age (P < 0.001), t he mean rate of decline (-3.4%/wk) was not affected by Ca source. Extr insic radiolabeling overestimated (approximately 20%) Ca bioavailabili ty when the rats were young. However, the magnitude of this effect dim inished with advancing animal age and was not significant across all a ges (repeated measures analysis of variance P = 0.10).