THE HISTORY AND THEORY OF THE DOUBLY LABELED WATER TECHNIQUE

Scientists have been measuring energy expenditure by using gas exchang e for the past 200 y. This technique is based on earlier work in the 1 660s. Gas exchange in respirometers provides accurate and repeatable m easures of resting metabolic rate. However, it is impossible to duplic ate in a respirometry chamber the diversity of human behaviors that in fluence energy expenditure. The doubly labeled water technique is an i sotope-based method that measures the energy expenditure of unencumber ed subjects from the divergence in enrichments of 2 isotopic labels in body water-1 of hydrogen and 1 of oxygen. The method was invented in the 1950s and applied to small animals only until the early 1980s, mos tly because of the expense. Since 1982, when the first study in humans was published, its use has expanded enormously. Although there is som e debate over the precise calculation protocols that should be used, t he differences between alternative calculations result in relatively m inor effects on total energy expenditure estimates (approximate to 6%) . Validation studies show that for groups of subjects the method works well, but that precision is still relatively poor (8-9%) and conseque ntly the method is not yet sufficiently refined to provide estimates o f individual energy expenditures.