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The prevalence of obesity in children, as in adults, is increasing dramatically. The extent to which this is due to reduced energy expenditure, increased energy intake, or both, is unclear at present. This in part reflects the limitations of existing models of the pre-obese state. In childhood acute lymphoblastic leukaemia (ALL), patients typically gain weight excessively during and after 2 years of therapy, and are at high risk of becoming obese. Previous studies have failed to identify the cause of obesity in these patients. We have tested the hypothesis that excess weight gain in ALL is due to reduced total energy expenditure (TEE), measured using the doubly-labelled water method, and have identified risk factors for excess weight gain in ALL. Pre-obese children with ALL in the dynamic phase of weight gain are less physically active than their peers, with a reduced TEE of approximately 1.2 (95 % CI 0.2, 2.2) MJ/d. While other factors might contribute to excess weight gain, lifestyle (i.e. reduced habitual physical activity) plays a central role in ALL. Several considerations suggest that ALL might be a useful model of the pre-obese state: lifestyle is critical to development of obesity in ALL; ALL is relatively common; approximately 70 % of patients survive; patients are readily accessible during the 2 years of therapy and beyond.

Recent methodological advances have led to a tremendous improvement in our ability to measure energy expenditure, body composition and fat distribution in children. The availability of new and improved measurement techniques has greatly enhanced the scope of research studies in children. The key findings from the present review are as follows: total energy expenditure in young children is approximately 25% lower than current recommendations for energy intake and revised recommendations are necessary; reduced energy expenditure, however, does not necessarily explain the greater prevalence of obesity in the population as a whole or in sub-groups at greater risk of obesity; qualitative aspects of physical activity (e.g. time, intensity) may be more important than the energy expenditure of physical activity in the regulation of body composition; for body composition assessment, DXA is emerging as a technique which can substantially improve the accuracy and standardization in children; body fat begins to accumulate in the obese; waist:hip ratio or waist circumference are inadequate markers of intra-abdominal adipose tissue in children and adolescents; finally, the early accumulation of fat in the intra-abdominal region is significantly related to the development of adverse health effects, including dyslipidaemia and glucose intolerance.