The association between increased physical activity and a lower body mass index is well established in the literature, but the cause and effect of the association are not clear. In addition, physical activity can give an indication of energy expenditure, but the relationship is not straightforward as two individuals can expend different amount of energy on the same task. It is a general misconception that obese individuals have low resting metabolic rates, because a number of studies performed in calorimetry chambers have shown that obese subject actually have higher 24 hour energy expenditure that lean counterparts. However, in a calorimetry chamber, normal physical tasks are not able to be performed and so it is of more relevance to assess subjects under free living conditions. However, even under free living conditions it is reported that obese subjects do not have lower energy expenditure that lean subjects.
For example, researchers1 have investigated the total energy expenditure of 12 normal weight and 26 obese subjects under free-living conditions. The 12 normal weight women were within 9% of the recommended ideal weight. The 26 overweight women were between 19 and 54kg over idea weight and had body fat of 36 to 48% with a waist to hip ration of between 0.73 and 1.02. Overall, the mean energy expenditure of the overweight women was 11.2MJ/d more energy than the normal weight women, with 6.47MJ/d of this amount explained by an increase in basal metabolic rate and the rest explained by an increase in above basal energy expenditure. In both groups of women, the total energy expenditure was 1.7 times the basal expenditure. Following adjustment for weight and lean body mass there was no significant difference between energy expenditure.
Overall, this study supports others research by demonstrating that obese individuals have energy expenditures that are significantly higher than normal weight counterparts before correction for weight and lean body mass. The authors estimated that for every kg of bodyweight increase an additional 80kJ/d in energy expenditure would occur. Half of this was as a result of an increase in lean body mass, part of which is accounted for by increased skeletal muscle mass. Because the thermic effect of food is not increased in obese individuals, the rest of the energy expenditure is likely derived from an increased excursion during physical activity. Therefore for a particular task, the overweight individual may expend more calories due to the increase weight being carried and higher levels of lean mass. This study challenges the assumption that obese individuals perform less physical activity when considering energy expended.
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