orced energy restriction causes muscle loss in obese individuals. This is related to the fact that obesity is a disease characterised by insulin resistance which prevents the correct oxidation of fatty acids and glucose as fuel sources. Reducing calorie intake below that which is required for maintenance results in catabolism of skeletal muscle tissue for energy, which ultimately results in a loss of fat free mass. This in turn reduces the resting metabolic rate (RMR) and increases the likelihood of weight regain. In addition to reducing food intake, proponents of forced energy restriction often recommend that exercise is performed to create a larger negative energy balance to further decrease body weight. Resistance training has been shown to increase fat free mass and may be a useful tool to reduce obesity. However, more catabolic exercise such as cycling, might not be beneficial to weight loss even if performed at high intensity.
For example, researchers1 have investigated the effects of calorie restriction in addition to high or low intensity exercise in 27 obese women. The subjects were required to consume a 1200 kcal/d diet in combination with either a low or high intensity exercise regimen at 80-90 or 40-50 % VO2 max, respectively. Exercise was performed for 3 days per week using a cycle ergometer. The high intensity exercise sessions lasted for 25 min and the low intensity exercise sessions lasted for 50 min, in order to keep total work output as similar as possible. The results showed that irrespective of which exercise regimen the subjects undertook, fat free mass decreased by 10 % from between baseline to post-test period. This suggests that in addition to the 7 % fat mass and 16 % reduction in skin fold measurements, the RMR of the subjects decreased significantly with both high and low intensity cycling.
The loss of 10 % of fat free mass in the subjects is concerning, but is not unusual for a forced energy restriction regimens. The RMR of the subjects would almost certainly have been modified downwards following this period of negative energy balance, requiring less energy to maintain their metabolic requirements. This would have increased the risk of weight regain during any subsequent maintenance phase. The high failure rate of forced energy restriction diets is caused primarily because of this decrease in RMR. Interestingly, the energy expenditure of the high intensity group was double that of the low intensity group during exercise. However, both groups lost very similar amounts of weight suggesting that the energy consumed during exercise in inconsequential to weight loss. As a whole these results show the problem with forced energy restriction diets, which is a dramatic fall in fat free mass that reduces resting metabolic rate.
That forced energy restriction is not required to lose body weight has been demonstrated using low carbohydrate diets (here), a Mediterranean diet (here) and by lowering the glycaemic index of meals (here). These dietary changes are effective because they improve insulin sensitivity which increases the ability of skeletal muscle to oxidise fatty acids. This prevents the catabolism of muscle tissue and maintains a high RMR. Forced calorie restriction has been shown to cause short-term weight loss, but can cause over half the weight to be lost as lean tissue (here). If forced energy restriction was an effective weight loss strategy, very low calorie diets would be more beneficial than low calorie diets. However, extreme calorie restriction causes no additional benefits over moderate calorie restriction (here). That adding exercise to a forced energy restrictive diet is no more effective than the diet alone (here) suggests that the traditional theory of obesity is flawed and requires reappraisal.