collectively the findings presented here demonstrate that there are discrepant findings regarding the role of energy expenditure in the cause of obesity.this discrepancy could potentially be explained by a number of additional factors. for example, differences or changes in energy expenditure and energy intake could occur at distinct critical periods of development (such as in early infancy or adolescence), and may result in energy imbalance. in addition, there could be individual differences and susceptibility to the impact of altered energy expenditure on the regulation of energy balance, as demonstrated in studies such as those of bouchard et al, in which twins were challenged with underfeeding or overfeeding. the impact of energy expenditure on the cause of obesity could vary among different subgroups of the population (e.g.,male versus female,active versus inactive, different ethnic groups) and could also have differential effects within individuals at different stages of development.it is conceivable that susceptible individuals fail to compensate for periodic fluctuations in energy expenditure. although a 14 day measure of energy expenditure by doubly labeled water is considered a long-term measure, this time period is actually short when compared with the time scale for the development of obesity, which can be a slow and gradual process. for example, in a previously cited longitudinal study comparing children of two obese parents versus children of two nonobese parents, the difference in the rate of change in fat mass relative to fat-free mass was less than 1 kg of fat per year, or less than 3 g of excess fat gain per day. this rate is equivalent to a continual daily energy imbalance of 25 kcal/d (approximately 2% of total daily energy flux). from a methodologic standpoint, even the most sophisticated of current techniques would be unable to identify this energy imbalance as a defect in energy expenditure components (or as an excess in energy intake,relative to needs).