Dysprosium as a nonabsorbable marker for studies of mineral absorption with stable isotope tracers in human subjects

Two studies were conducted to determine if dysprosium (Dy) could be used as a quantitative fecal marker for studies of zinc-70 (70Zn), copper-65 (65Cu) and magnesium-26 (26Mg) absorption in humans. In the first experiment, Dy excretion was shown to be complete (104 +/- 9%; mean +/- SD, n = 6) and the kinetics of fecal Dy excretion closely paralleled that of 70Zn but not 65Cu. Because of the similarity in 70Zn and Dy excretion kinetics, a method for estimating 70Zn absorption was developed which used 70Zn and Dy data from only the first two stools passed after isotope administration. Average estimates of 70Zn absorption based on the two-stool (partial pool) vs total pool (5-day composite) method were not different (28.0 +/- 5.2 vs 24.4 +/- 4.1%, respectively; means +/- SEM; p > 0.10). In the second study, the same questions was addressed relative to 26Mg absorption. 26Mg and Dy also exhibited nearly identical excretion patterns. Average estimates of 26Mg absorption based on the partial pool vs total pool method were comparable yielding mean values of 22.7 +/- 3.4 vs 23.2 +/- 2.5% and 26.1 +/- 1.8 vs 24.3 +/- 1.8%, respectively, from magnesium-26 oxide (26MgO) and 26Mg glycinate. Advantages and limitations of the partial pool method for estimating mineral absorption are discussed.