With a moderate calorie restriction (from 10% below maintenance to approximately 1000 calories below maintenance) without exercise, there is inevitably a decline in resting energy expenditure and a loss of muscle. When exercise is added fat loss increases and the loss of muscle decreases. The drop in metabolic rate is also decreased. (22-25).
Resistance training alone, combined with a slight calorie restriction causes greater bodyfat loss and a maintenance/increase in lean body mass than just restricting calories alone (24,26). Essentially, the caloric deficit causes the fat loss and the weight training signals the body to keep the muscle so that only fat is lost. This is an important consideration. From a calorie burning perspective, aerobic exercise and caloric restriction are essentially identical.
Aerobic training alone, while increasing fat loss in some studies, does not generally increase muscle except in very inactive individuals (24) . Remember that adding muscle raises metabolic rate in the long term. Any caloric restriction should be accompanied by resistance training to prevent the loss of LBM and possibly to even increase it.
Aerobic exercise can increase fat loss and may be added if desired and if time allows. However, too much aerobic exercise can have the same effect as too few calories: lowering metabolic rate and slowing fat loss. A total caloric deficit of more than 1000 cal/day seems to be the threshold for slowing the metabolism (14).
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