Tracer techniques are attractive for the in vivo study of different aspects of nutrient assimilation and metabolism. Stable isotopes are the preferred tracers in studies involving human subjects, mainly because of safety reasons. Their widespread use in recent years has been stimulated by improvements both in the increased availability and diversity of stable isotope labelled compounds and in analytical (that is, mass spectrometric) methods for their quantitative analysis.1

Labelled amino acids have been used to study protein metabolism in vivo in a large number of studies mainly directed towards the measurement of protein synthesis and breakdown rates. Generally speaking, constant infusions of labelled amino acids such as L-[1-¹³C]-leucine are used.2 ,3 To study protein assimilation or protein metabolism during feeding, however, a representative oral tracer—that is, labelled protein, is needed.

Only a few techniques have hitherto been described for the production of stable isotope labelled proteins. Limitations and drawbacks inherent to those techniques have prevented their widespread application (small yield, low enrichment level of protein, inadequate labelling pattern), but now two proteins common in the normal diet are available in labelled forms—that is, milk and egg proteins labelled with L-[1-¹³C]-leucine.

These proteins have been recently produced by “Laboratoire de Nutrition Humaine” (Clermont-Ferrand, France) and by “Laboratorium Digestie-Absorptie” (Leuven, Belgium) (abstract 1).4 ,5In both cases the labelling technique is well described so that it can be reproduced by other scientists. Each technique has advantages for different reasons: production of egg proteins has a high tracer recovery, and infusion of lactating cows produces two types of proteins (whey protein and casein) with different physicochemical properties. Labelled proteins can be used to evaluate protein assimilation in various diseases in adults and children (pancreatic disease, abstract 2),6 to monitor the beneficial or detrimental …