• doubly labeled water;
  • test weighing;
  • breast milk;
  • validity;
  • human milk

Accurate assessment of infant feeding is needed for clinical practice and research. We identified 32 studies that evaluated the validity of direct observation, test weighing, or doubly labeled water methods. Correlations with validation standards were highest for doubly labeled water and test weighing, and lowest for observation. Cost and availability of isotope may limit the doubly labeled water method to research studies, whereas observation may be useful for clinical practice. Test weighing could be applied to either setting, but it may be practical to sample less frequently over 24 hours. Validity results and intended use of the measurement should be considered when selecting a method.