The use of doubly-labeled water measured energy expenditure as a biomarker of dietary energy intake has been extensive and continues. The doubly-labeled water is an important research biomarker for studies designed to understand errors in the dietary assessment instruments. The doubly-labeled water method was pioneered and validated in small animals by Nathan Lifson and coworkers over 60 years ago. One of the primary advantages of the doubly-labeled water method is that it essentially converts body water into a metabolic recorder. The doubly-labeled water method has been validated against both respiratory gas analysis and measured dietary intake plus change in energy balance. The literature utilizing doubly-labeled water as a biomarker of usual energy intake demonstrates that self-report-based estimates of energy intake have generally been underestimates of energy intake and that the degree of underestimation can be large; particularly among individuals with Body Mass Index (BMI) exceeding 30 kg/m2.