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Obesity is a growing problem with many associated health risks and associated costs.1,2 During 1971 to 2000, the prevalence of obesity in the US increased from 14.5 % to 30.9 %.3 In the US, more than 37 % of adults and almost 17 % of youths were obese in 2009–10.4 Currently over 1.1 billion adults worldwide are overweight, and of these, 312 million are obese.5 Analysis of data from the US Longitudinal (CARDIA study)6 and cross-sectional (NHANES)7 studies to determine the distribution of weight gain over time, found that the average US adult is gaining 0.5–1 kg/year.8 This article will examine the concept of energy balance and its relevance in combating the obesity epidemic.
The Concept of Energy Balance
The concept of energy balance is based on the fundamental thermodynamic principle that energy cannot be destroyed, and can only be gained, lost, or stored by an organism. Energy balance is defined as the state achieved when the energy intake equals energy expenditure. This concept may be used to demonstrate how bodyweight will change over time in response to changes in energy intake and expenditure. When the body is in energy balance, bodyweight is stable.9,10 Humans take in energy through the intake of food and drink, and expend energy through the resting metabolic rate (RMR)—the thermic effect (TEF) of food and physical activity. The RMR is the energy expenditure required for maintaining normal body functions and homeostasis. The RMR is proportional to body mass, in particular fat-free mass. TEF refers to the energy required to absorb, digest, and metabolize the food consumed and typically accounts for 8–10 % of daily energy expenditure. The energy expended due to physical activity (EEact) accounts for energy that is expended in addition to the RMR and TEF, including voluntary exercise, shivering, postural control, and voluntary movement. It is calculated by multiplying the energy expenditure of an activity by the time spent performing it, and is the most variable component of energy expenditure.10 The more sedentary the individual is, the lower the effect of physical activity. This may be as low as 100 calories per day, whereas elite athletes may expend 3,000 calories per day of physical activity. The decline in energy expenditure that occurs with advancing age is mainly the results of declining lean body mass, which reduces TEF and EEact.
Disturbances in energy balance cause changes in body mass, although the timeframe over which this occurs varies between individuals and may explain the large interindividual response to weight-loss interventions. A positive energy balance, in which energy intake exceeds expenditure causes weight gain, with 60–80 % of the resulting weight gain being attributable to body fat.11 In negative energy balance, when energy
Figure 1: Continued Weight Gain in the Population According to the Concept of Energy Balance14
Figure 2: Relationship between Energy Balance and Physical Activity Level10