Flow Models of Feeding

Did Predicted collagen Measured collagen Predicted carbonate Measured carbonare 

In a steady state, there can be no net isofractionation in the excreted components (by definition) the mass-weighted sum of all the 5 values for the excreted material must equal that of the diet. It so happens that what is excreted very often is dominated by one component. 

For humans, 80-90% of ingested nitrogen is excreted as urea. Minor contributions include ammonia and creatinine (in urine), nitrogen compounds in 

Any Body Tissue in Direct Equilibrium with the Major Excretum Will Reflect the Isotopic Composition of the Diet as a Whole 

Any body tissue in direct equilibrium with the major excretum should reflect the isotopic composition of the diet as a whole. This seems to be the case for bioapatite carbonate, which is thought to be in equilibrium with plasma bicarbonate, which itself is in equilibrium with respired CO2. In fact the Ambrose and Norr (1993) and Tieszen and Fagre (1993) data sets (among others) show clearly that the bioapatite carbonate differs from total diet (or respired CO ) by an amount approximating to the equilibrium isotopic fractionation in the system (Mook 1989)  

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