Indirect calorimetry and nutritional problems in clinical practice

Indirect calorimetry is a simple and affordable tool for measuring energy e xpenditure and for quantifying the utilization of macronutrients. Its use i s becoming increasingly widespread, but it is necessary to know its methodo logical features and its theoretical and practical limitations. Indirect ca lorimetry measures the rate of resting energy expenditure (REE), the major component of the total daily energy expenditure. Thus, indirect calorimetry reliably estimates the individual energy needs. Coupling the measurement o f body composition to that of REE expands the diagnostic potential of indir ect calorimetry. Once the lean and fat compartments have been measured, it is possible to establish on the basis of REE whether an individual is hyper - or hypometabolic. The evaluation of substrate oxidation by indirect calor imetry is subject to more severe theoretical constraints, because certain m etabolic assumptions must be made. The clinical applications are practicall y unlimited. In the critically ill, a major goal is to maintain energy bala nce during the hypermetabolic response following trauma. The REE measuremen t is valuable from the diagnostic standpoint, because it recognizes discrep ancies from the expected time-course of hypermetabolism, for example signal ing a potentially catastrophic hypometabolic response. REE is also indispen sable for providing correct nutritional support because both hyper - and un dernutrition lead to increased mortality. In young or elderly patients, in whom energy consumption may be very different from that predicted from equa tions based on anthropometric measures, indirect calorimetry is particularl y useful.