During weight loss there is a decline in resting energy expenditure (REE). This lowered energy expenditure is due to the loss of body mass, to a lesser extent the thermic effect of food (which refers to the extra calories burned after a meal), and to a decline of the metabolic rate within the body tissues. The latter is considered to reflect a metabolic adaptation to conserve energy stores, generally referred to as ‘adaptive thermogenesis’. The degree of contribution by adaptive thermogenesis is debated but has important challenges for those who strive to lose weight.
An article in the November, 2012, of Obesity conducted a meta-analysis covering almost three thousand subjects. In the analysis a statistical model predicted energy expenditure prior to weight loss based on fat mass (FM) and fat-free mass (FFM). When the post weight loss FM and FFM data was included into the model, the energy expenditure was accurately predicted. Thus, the body composition was a strong predictor of energy expenditure and this does not support the concept of a greater than expected decrease in energy expenditure than that expected from the weight loss.
In another article, however, in the December, 2012, of Obesity conducted a prospective weight loss program utilizing calorie restriction, but different diet composition. In this study, the statistical model predicted energy expenditure based upon body mass index (BMI), without reference to the body mass composition. The authors found that the REE was lower than the model predicted on BMI at six months post weight loss, but the REE recovered to predicted after twenty-four months even while the weight loss is maintained.
The studies based their models in two major ways. The first article utilized body composition (FM and FFM) in its model whereas the other used only body size (BMI). The studies also differed in the latency from weight loss until the REE was modeled. These different methodologies probably explain the different conclusions. Taken together, there is probably an apparent adaptive thermogenesis associated with weight loss which returns to baseline after a sufficient period of time. For those trying to maintain weight loss, this suggests that weight maintenance should become easier in time.
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