The assessment of energy expenditure has been of interest in the investigation of human nutrition for 100 years.<sup>2 Atwater WO, Benedict FG: Experiments on the metabolism of matter and energy in the human body. US Department of Agriculture, Office of Experiment Stations, Bulletin No. 136:1, 1903

Cliccare qui per andare alla sezione Riferimenti</sup> In fact, much of the early work in measurement of the basal metabolic rate (BMR) was conducted early in this century and even before.<sup>1 Amatruda J.M., Statt M.C., Welle S.L. Total and resting energy expenditure in obese women reduced to ideal body weight J Clin Invest 1993 ;  92 : 1236

Cliccare qui per andare alla sezione Riferimenti, 7 Benedict F.G., Milner R.D.   Experiments on the metabolism of matter and energy in the human body, 1903–1904 : US Department of Agriculture, Office of Experiment Stations (1917).  Bulletin No. 175.

Cliccare qui per andare alla sezione Riferimenti, 29 Harris J.A.   A biometric study of basal metabolism in man Washington, DC: Carnegie Institute (1919).  Publication 279.

Cliccare qui per andare alla sezione Riferimenti, 57 Rubner M. Die Gesetze des Energieverbrauchsbei der Ernahrun Lepzig Franz Deuticke 1902 ;

Cliccare qui per andare alla sezione Riferimenti</sup> However, many unanswered questions remain regarding the role that different components of energy expenditure have in energy imbalance, and reduced energy expenditure has long been suggested as one factor leading to weight gain.

Total daily energy expenditure (TDEE) can be partitioned among resting metabolic rate (RMR), the thermic effect of food (TEF), and physical activity (Figure 1). Any or all of these components of energy expenditure can be reduced, leading to obesity. At energy balance, energy intake equals energy expenditure, and any imbalance between the two leads to a gain or loss of body stores. This must be so because human energy metabolism does not violate the first law of thermodynamics that states that energy is neither created nor destroyed.

Many techniques are available to measure the different components of energy expenditure. They differ in instrumental and technical requirements as well as the accuracy and precision of the measurements. The methods discussed in this article are listed in Table 1. Measurements of TDEE, or 24-hour energy expenditure, have been made with whole-room calorimeters, portable calorimeters, factorial methods employing activity diaries, intake/balance methods based on dietary intake and changes in body composition, and doubly labeled water. RMR can be measured in the whole-room calorimeters or with more portable closed- or open-circuit systems, whereas the TEF is most often measured with open-circuit metabolic carts. Physical activity is the second largest component of TDEE and is the most variable. Ravussin and co-workers<sup>51 Ravussin E., Lillioja S., Anderson T.E. , e al. Determinants of 24-hour energy expenditure in man: Methods and results using a respiratory chamber J Clin Invest 1986 ;  78 : 1568-1578  [cross-ref]

Cliccare qui per andare alla sezione Riferimenti</sup> reported that the variability in spontaneous physical activity in subjects confined in a respiratory chamber was 100 to 800 kcal/day. Variability in free-living subjects would be expected to be even larger. Because of the magnitude of physical activity, it is a likely component of energy expenditure involved in the etiology of obesity. However, physical activity has been the most difficult to measure accurately in free-living subjects. Various monitors such as heart rate or motion sensors have been used, as well as activity diaries. With the advent of the DLW method, this component of energy can be better assessed. When combined with measures of RMR and TEF (or an estimate of TEF), the energy expended in physical activity can be accurately measured.