The amount of daily physical activity is very difficult to quantify, particularly over the prolonged period of time (years) involved in the development of obesity.
Typically an increase in weight of 10 kg occurs over a period of perhaps 2 years. This indicates that the individual has a positive energy balance of some 3040 kcal/day, or the equivalent of 1-2% of total energy expenditure. This corresponds to an extra energy intake of less than half a sandwich per day or, on the output side, a walk of 15-30 min/day. In the short term these are such small changes in energy balance that they could perhaps correspond to differences in minor unconscious movements ('fidgeting') . No method exists that can measure such differences over a sufficient period of time. The available methods, such as measurements of V02 , step registration, Holter monitoring or activity history, are not very helpful at this low level of long-term change in energy balance. The problem is equally difficult for both energy intake and energy output, and such technical problems make obesity research very complicated. Free living energy expenditure can be measured with reasonable accuracy by the double-labeled water technique, and by subtracting resting metabolic rate from these readings a good estimate of the energetic cost of physical activity can be achieved. However, this method is very costly, which prohibits its use in population studies.
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