OBESITY AND THE ADIPOCYTE Assessment of human energy balance

The ability to make accurate in vivo assessments of human energy balance lies at the core of much obesity research. It is now gaining even greater importance as it becomes necessary to integrate new genetic and molecular findings with the social and environmental conditions in which their influences are expressed. The energy balance equation has three primary components (energy intake, energy expenditure and energy stored) which can be measured using a range of techniques as shown in Table 1. It is critical to select appropriate methods for a given experimental setting. None of the techniques are perfect but some of the imperfections can be minimised by simultaneous application of two or more methods, especially for body composition (Jebb & Elia 1995). Where this is not possible it is essential that investigators face up to the inherent limitations of the methodology. For instance, we have demonstrated that overweight and obese subjects tend to provide seriously biased under-estimates of habitual food intake and this problem also extends to post-obese subjects. Figure 1 shows the under-reporting of energy intake in relation to measured energy expenditure, corrected for changes in body weight during the measurement period (Black et al. 1993). On average the obese subjects underestimated intake by "36% and the post-obese by "27%. We conclude that it is virtually impossible to obtain reliable estimates of energy intake in naturalistic behavioural studies. The discovery that obese people consume much more energy than previously acknowledged has had a profound influence on investigations of human obesity by steering the focus away from a search for energy-sparing metabolic defects and refocussing it on the dysregulation of food intake (Prentice et al. 1989). The doubly-labelled water method, which derives accurate estimates of total free-living energy expenditure