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A Comparison of Intrinsic and Extrinsic Tracer Methods for Estimating Calcium Bioavailability to Rats from Dairy Foods¹

Maciej S. Buchowski, Karen C. Sowizral, Frederick W. Lengemann^*, Darrell Van Campen and Dennis D. Miller^,2

Department and Institute of Food Science, New York State College of Agriculture and Life Sciences, Cornell University ^* New York State College of Veterinary Medicine, Cornell University U.S. Plant, Soil and Nutrition Laboratory, Agricultural Research Service, United States Department of Agriculture, Ithaca, NY 14853

Dairy products doubly labeled with ⁴⁵Ca and ⁴⁷Ca were used to evaluate an extrinsic labeling procedure for calcium bioavailability determination. Nonfat milk, yogurt, and fresh cheese curd were prepared from caprine milk that was intrinsically labeled with ⁴⁵Ca. The products were then labeled extrinsically with ⁴⁷Ca and administered to rats by gavage. The ⁴⁷Ca to ⁴⁵Ca ratio in bone and teeth averaged about 1.00 with either milk, yogurt, or CaCl₂, but the ratio was about 1.04 when dosed with cheese curd. Ca absorption, determined by whole-body counting of ⁴⁷Ca, was lower (P < 0.05) in cheese curd (59%) than in either milk (69%), yogurt (72%), or CaCl₂ (72%). Expressed as percent of dose, the absorption of ⁴⁷Ca was highly correlated with bone ⁴⁷Ca (r = 0.973) and with bone ⁴⁵Ca (r = 0.946). Correlation between tibia ⁴⁷Ca and tibia ⁴⁵Ca was r = 0.923. For the dairy products tested, our results indicated that extrinsic ⁴⁷Ca was absorbed similarly to intrinsic ⁴⁵Ca. Moreover, the percent of radioactive dose retained in bone appears to be a valid indicator of relative bioavailability of food Ca.

KEY WORDS: • calcium bioavailability • labeling • dairy products

¹ Supported by U.S. Department of Agriculture Grant No. 87-CRCR-1-2315.

² To whom reprint requests should be addressed: Dept. of Food Science, Stocking Hall, Cornell University, Ithaca, NY 14853.

Manuscript received 11 August 1988. Revision accepted 14 November 1988.