Mechanisms of protein gain during protein feeding have been investigated using a combination of oral and intravenous labeled leucine in healthy young men. The oral labeled leucine was administered as a free oral tracer ([13C]- or [2H3]leucine) added to unlabeled whey protein or as whey protein intrinsically labeled with L-[1-13C]leucine. When the oral tracer was free leucine, it appeared in the plasma more rapidly than the unlabeled leucine derived from the whey protein, and this resulted in an artifactual 88% decrease of protein breakdown. When the oral tracer was protein bound, protein breakdown did not change significantly after the meal. In contrast, nonoxidative leucine disposal (i.e., protein synthesis) was stimulated by 63% by the meal. In conclusion, 1) an intrinsically labeled protein is more appropriate than an oral free tracer to study postprandial leucine kinetics under non-steady-state conditions and 2) protein gain after a single whey protein meal results solely from an increased protein synthesis with no modification of protein breakdown.