Determination of Fractional Absorption of Dietary Calcium in Humans

Abstract

Four dual-isotopic label methods for determining true fractional absorption of dietary calcium were compared in 23 subjects. The ratio of the integrals of oral label in a 24-h pooled urine to intravenous label in the same urine is called α_24h and was taken as the standard against which the others were compared. α_Spot is the ratio of the fraction of oral label to the fraction of intravenous label in a single urine specimen; α_Lag is the ratio of the level of oral label in blood 4 h after administration to the level of intravenous label in blood 2 h after administration. α_Dec is obtained by deconvoluting response to the intravenous label from the response to the oral tracer. Results were as follows: α_24h = 0.273 ± 0.124, α_Dec = 0.300 ± 0.101 (n = 14), α_Spot = 0.359 ± 0.179, and α_Lag = 0.271 ± 0.103. The Bland-Altman approach for comparison of methods was used to show that results for α_Spot and α_Lag can be expected, with a 95% confidence limit, to differ from the value of α_24h by 60 and 69%, respectively. The results for α_Dec were shown to be not only indistinguishable from α_24h but identical from a theoretical perspective.