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Comparison of in Vitro and in Vivo Measurements of Dietary Ca Exchangeability and Bioavailability^1,2,

Division of Nutritional Sciences, Cornell University, Ithaca, NY 14853

An in vitro method for measuring dietary mineral exchangeability (miscibility with an extrinsic isotopic tracer) was tested by comparison with in vivo measurements in rats. Collards, spinach and soybeans, intrinsically labeled with ⁴⁵Ca, were fed to rats together with extrinsically added ⁴⁷Ca. Absorption of both tracers was determined by measuring their concentrations in the femur 2 days after consumption of the labeled test meals. The same intrinsically and extrinsically labeled foods were digested in vitro for estimation of Ca exchangeability and solubility after peptic-pancreatic digestion. Corresponding in vivo and in vitro estimates of exchangeability agreed closely for the three foods tested. Solubility after in vitro peptic-pancreatic digestion (potential bioavailability) showed discrepancies. In vitro values were somewhat higher for collards than in vivo fractional absorption. The reverse was true for Ca in soybeans and spinach. Thus, the in vitro procedure appears adequate for measuring intrinsic dietary Ca exchangeability but requires modification if it is to be a reliable substitute for in vivo measurements of Ca bioavailability.

KEY WORDS: • bioavailability • exchangeability • extrinsic isotope • mineral

¹ Supported in part by National Institutes of Health Grant 18569 and U.S. Department of Agriculture. Cooperative Agreement 58-320A4-9-91.

² Part of this work was presented at the American Institute of Nutrition meeting, New Orleans, April, 1982, Fed. Proc. 41, 771 (1982). (abs.)

Manuscript received 11 August 1982.