Phenylalanine flux in adult men: estimates with different tracers and route of administration.

The kinetics of three different tracers of phenylalanine were studied when continuously infused together either by an intravenous (IV) or intragastric (IG) route in six young healthy men during a fasting state. During IV infusion, mean flux values were 39.2 +/- 1.8, 40 +/- 3, 41.8 +/- 3.6 mumol.kg-1.h-1 for L-[ring-2H5], [15N], and L-[1-13C]phenylalanine, respectively (differences not significant). Fluxes were higher (P less than .001) during IG than IV infusion, indicating a disappearance of tracer during the first pass through the splanchnic area. Also, higher fluxes were found for [ring-2H5]phenylalanine (74 +/- 6.23 mumol.kg-1.h-1) compared with [15N] and L-[1-13C]phenylalanine (54.24 +/- 4.7 and 61.15 +/- 5.3 mumol.kg-1.h-1) during IG infusion. Proton exchange might explain this difference, possibly limiting in vivo use of this label when the tracer is to be administered by the IG route.