DETERMINATION OF FRACTIONAL ABSORPTION OF DIETARY CALCIUM IN HUMANS

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 alpha(24h) and was taken as the standard against which the others were compared. alpha(Spot) is the ratio of the fraction of oral label to the fraction of intravenous label in a single urine specimen; alpha(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. alpha(Dec) is obtained by deconvoluting response to the intravenous label from the response to the oral tracer. Results were as follows: alpha(24h) = 0.273 +/- 0.124, alpha(Dec) = 0.300 +/- 0.101 (n = 14), alpha(Spot) = 0.359 +/- 0.179, and alpha(Lag) = 0.271 +/- 0.103. The Bland-Altman approach for comparison of methods was used to show that results for alpha(Spot) and alpha(Lag) can be expected, with a 95% confidence limit, to differ from the value of alpha(24h) by 60 and 69%, respectively. The results for alpha(Dec) were shown to be not only indistinguishable from alpha(24h) but identical from a theoretical perspective.