Indirect calorimetry: an indispensable tool to understand and predict obesity.

Obesity is a physiological condition of chronic positive energy balance. While the regulation of energy metabolism varies widely among individuals, identifying those who are metabolically prone to weight gain and intervening accordingly is a key challenge for reversing the course of the obesity epidemic. Indirect calorimetry is the most commonly used method to measure energy expenditure in the research setting. By measuring oxygen consumption and carbon dioxide production, indirect calorimetry provides minute-by-minute energy expenditure data that makes it the most valuable tool to distinguish the various components of energy expenditure, that is, sleeping and resting metabolic rate, thermic effect of food and the energy cost of activity. Importantly, such measures also provide information on energy substrate utilization. Here we summarized some of the research that revealed resting metabolic rate, spontaneous physical activity and respiratory quotient as key metabolic predictors of weight gain and obesity. Recent studies using indirect calorimetry in response to mid-term fasting or overfeeding have identified 'thrifty' and 'spendthrift' phenotypes in people who differ in propensity to weight gain. We propose the use of indirect calorimetry data as a basis for personalized interventions that may be efficacious in slowing down the rise of global obesity.