PRINCIPLES OF TRACER METHODOLOGY 



15 



which is easily measured by chemical analysis of the feces. If the plasma 

 calcium is labeled by injection with Ca^^ and has a specific activity of Sp, 

 then from relationships already discussed it follows that 



^ 

 Sf 



E 



100 - . 1 + ^ 



(1-7) 



where Sf is the specific activity of the feces. From these two equations 

 can be calculated the value for A, which is the ''true" availability, and 

 the value for E, which is the contribution from the body stores to the'fecal 

 calcium excretion. It is clear that any dietary calcium that is absorbed 



A units of 

 absorbed Qa 



Labeled 

 Plasma Ca 



Ingested 

 100 units Qa 



Fecal Excretion 

 ^ E units of endogenous Ca 

 ■^ 100 -A units of unobsorbed Ca 



Gastrointestinal Tract 



Fecal calcium = 100 -A+E 



Spec/ fie activity of fecal calcium 

 Specific activity of plasma calcium 



■^^^■^..1^ ut^iiviiy ui fjiubiiiu cuicium 100 - A +E 

 Fig. 1-1. Schematic diagram of principle of isotope-dilution method for determination 

 of endogenous fecal calcium. [From C. L. Comar, R. A. Monroe, W. J. Visek, and 

 Sam L. Hansard, Comparison of Two Isotope Methods for Determination of Endogenous 

 Fecal Calcium, J. Nutrition, 50: 459-467 (1953).] 



reaches the plasma pool, becomes labeled, and is measured as endogenous 

 calcium. 



In practice, the tagged calcium is injected intravenously daily for 10 

 to 15 days to produce uniform mixing in the plasma and steady-state con- 

 ditions in the movement of endogenous calcium into the feces. It is also 

 possible to get valid results after a single intramuscular injection with 

 samples taken at several days postinjection. For example, a 113-kg calf 

 was found to have an average daily calcium intake of 16.7 g and a fecal 

 excretion of 15.9 g. After daily injections of Ca^^ the ratio of the specific 

 activity of feces to that of plasma averaged 0.227. It was calculated that 

 the animal was excreting 3.6 g of endogenous fecal calcium daily and that 

 the ''true" absorption A was 26.6 per cent as compared with the apparent 

 absorption of 5 per cent. This isotope-dilution procedure would be valid 

 for any labeled substance, provided that the specific activity of the sul)- 

 stance being secreted into the intestinal tract from the body were the same 

 as that of the plasma. 



Application to Anahjtical Chemistry. The isotope-dilution method has 

 a general application to chemical analysis. In principle, if a labeled form 



