Use of non-absorbable markers in studies of human nutrient absorption.

Neutron activation analysis signifies a new era in the methodology of studies on nutrient utilization in humans. Yet the classical metabolic balance technique incorporating inert, non-absorbable markers will continue to be an invaluable tool for such studies. This review describes the use and abuse of a number of 'classical' inert markers, and describes the potential use of radioactive and naturally-occurring isotopes, and of heavy metals. The paucity of data critically appraising markers in infant balance trials is apparent. The comparative merits of conventional markers--carmine, polyethylene glycol 4000, chromium oxide--should be assessed. Indeed, it might be argued that the advancing knowledge on the use of naturally occurring isotopes in adults should make similar studies on infants a priority.