Quantifying the contribution of dietary protein to whole body protein kinetics: examination of the intrinsically labeled proteins method

Intrinsically labeled dietary proteins have been used to trace various aspects of digestion and absorption, including quantifying the contribution of dietary protein to observed postprandial amino acid and protein kinetics in human subjects. Quantification of the rate of appearance in peripheral blood of an unlabeled (tracee) amino acid originating from an intrinsically labeled protein (exogenous R_a) requires the assumption that there is no dilution of the isotope enrichment of the protein-bound amino acid in the gastrointestinal tract or across the splanchnic bed. It must also be assumed that the effective volume of distribution into which the tracer and tracee appear can be reasonably estimated by a single value and that any recycling of the tracer is minimal and thus does not affect calculated rates. We have assessed these assumptions quantitatively using values from published studies. We conclude that the use of intrinsically labeled proteins as currently described to quantify exogenous R_a systematically underestimates the true value. When used with the tracer-determined rates of amino acid kinetics, underestimation of exogenous R_a from the intrinsically labeled protein method likely translates to incorrect conclusions regarding protein breakdown, including the effect of a protein meal and the anabolic impact of the speed of digestion and absorption of amino acids. Estimation of exogenous R_a from the bioavailability of ingested protein has some advantages as compared with the intrinsically labeled protein method. We therefore conclude that the bioavailability method for estimating exogenous R_a is preferable to the intrinsically labeled protein method.