Absorption of dietary zinc in man: comparison of intrinsic and extrinsic labels using a triple stable isotope method

Abstract

Determination of true absorption of dietary minerals in human subjects using the “extrinsic-tag” approach with stable isotopes requires establishment of its validity. The present study was conducted to test the quantitative aspects of absorption of an extrinsic-tag of zinc labeled with ⁷⁰Zn as compared with an “intrinsic-tag” of the mineral as ⁶⁸Zn-labeled chicken meat given simultaneously to healthy male subjects. Three diet periods were used in which diet modulation with respect to zinc daily intake and nature of protein source (chicken/soy protein isolate) was also examined. Absorption was measured via quantitative fecal isotope balance of ⁶⁴Zn, ⁶⁸Zn, and ⁷⁰Zn. For the three diet periods 1 (protein: chicken; Zn intake: 10 to 11 mg/day), 2 (protein: chicken/soy protein isolate, 50/50; Zn intake: 10 to 11 mg/day), and 3 (protein: chicken; Zn intake: 7 mg/day), fractional absorption of the extrinsic tag (mean ± 1 SEM) was 0.46 ± 0.06, 0.46 ± 0.06, and 0.66 ± 0.04, respectively. The comparable values for intrinsic ⁶⁸Zn were 0.57 ± 0.06, 0.57 ± 0.06, and 0.72 ± 0.04. There was a highly significant correlation (r = 0.91), between zinc absorption from the two labels. However, absorption of intrinsic ⁶⁸Zn was significantly higher (p < 0.02) during all periods than that for the extrinsic ⁷⁰Zn. A 50% replacement of protein from chicken meat with the soy protein isolate did not alter fractional absorption of zinc from either tag. The ratio of fractional absorption of the extrinsic/intrinsic tag (mean ± 1 SEM) was 0.79 ± 0.06, 0.79 ± 0.04, and 0.92 ± 0.03 for periods 1, 2, and 3, respectively.