It is a widely held belief amongst medical professionals and dietitians that weight gain is caused by small incremental accumulations of fat tissue due to a positive energy balance. This belief results in an associated belief that weight loss is possible through the creation of a negative energy balance induced by increased physical activity and reductions in energy intake. However, a voluminous literature does not support this contention, because the energy balance of the body is dynamic and adapts to food intake and physical activity through counterregulatory mechanisms that prevent loss of adipose tissue. In this regard, the hypothalamus can downregulate the thermic effect of food, the thermic effect of activity, the desire for physical activity, as well as increase appetite in order to maintain energy balance in spite of increase efforts to force weight loss. The down regulation of thyroid and anabolic hormones and increases in the catabolism of skeletal muscle also cause reductions in resting metabolic rate.
The fact that resting metabolic rate is reduced following forced calorie restriction and aerobic exercise regimens aimed at causing weight loss is problematic. Reductions in resting metabolic rate signify skeletal muscle loss and increase the risk of weight regain at future time points. In fact, the reductions in resting metabolic rate seen with traditional reducing diets and exercise regimens explain the weight regain seen in these sorts of protocols and defines their failure. Increasingly the reduced resting metabolic rates seen from traditional weight loss plans are being reported to be long-term phenomenon, that may be semi-permanently maintained to the health detriment of the individuals. For example in a recent study1, researcher assessed the long term reduction in thermogenesis seen following a traditional calorie counting weight loss protocol. Following a very low calorie diet for 8 weeks and a 44 week maintenance period, resting metabolic rate was assessed along with fat mass and body weight.
The results showed that resting metabolic rate fell significantly following weight loss and this low level was maintained unchanged throughout the 44 week follow-up period. In addition, fat mass and body weight fell initially during the weight loss phase, but then started to increase back to baseline levels. The fall in resting metabolic rate was associated with the fall in body weight, suggesting that some of the lost weight was muscle. When the authors estimated the predicted resting metabolic rate, there was an improvement in the figure over the 44 week maintenance period. Therefore while the actual resting metabolic rate remained depressed, the predicted figure returned close to normal. Predictions of resting metabolic rate may therefore not accurately reflect true measurements. The discrepancy between the predicted and measured resting metabolic rate was likely due to the fact that the weight regain was fat, not muscle, with the former not contributing significantly to resting metabolic rate and therefore levels remaining depressed. .
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