Forced calorie restrictive diets cause weight loss, but much of this weight is skeletal muscle.
Most trying to lose weight believe that they must use forced calorie restrictive diets to be successful. This is based on the tired old ‘eat-too-much, do-too-little’ hypothesis of weight gain. Most such restrictive diets are successful short term and weight loss is seen on nearly all restrictive diets for an initial period of a few weeks. However, while most assume that this weight loss is beneficial, forced calorie restrictive diets cause detrimental changes that greatly increase the risk of weight regain in the future. The reason for this is that much of the weight lost on such diets will be skeletal muscle. Because total skeletal muscle mass determines the resting metabolic rates of the individuals, forced calorie restrictive diets significantly lower resting metabolic rate and this means that fewer calories must be ingested to maintain body weight. If forced calorie restriction proceeds and intake is decreased to compensate for the reduced energy expenditure, greater quantities of skeletal muscle are sacrificed for the production of energy, further lowering resting metabolic rate.
Such reductions in metabolic rate are well documented in the scientific literature. For example, in one study researchers measured the body composition of a group of overweight women1. They then assessed the energy expenditure of the women on a 100 % non-restrictive diet. Following this the women were placed on a 1000 kcal forced calorie restrictive diet that was comprised of the same foods and same macronutrient profile (14 % protein, 40 % fat, 46 % carbohydrate) as the original diet. After 1 week the subjects had lost weight, but their resting energy expenditures had dropped significantly. This was because all of the weight loss had come from lean body mass and the subjects had lost no body fat. Following 8 weeks on the restrictive diet the subjects had lost a mean 10 kgs of weight, of which less than half was body fat. At the end of the forced calorie restrictive diet phase the resting metabolic rate had fallen further. Refeeding for 1 week at 100 % of calories caused an increase in body weight, but did not cause increase the resting metabolic rate.
Following this experiment the subjects has lost body fat and lean tissue. However, they had also significantly reduced resting metabolic rates on account of the 5 kg of lean tissue that they lost. The subjects were not followed long-term, but other studies suggest that if they had, most would have returned to their original body weights, but with less lean tissue and more body fat. This would result in a lower requirement for energy each day to maintain metabolism. Such metabolic damage from forced calorie restrictive diets is the norm, not an exception. Termed ‘yo-yo dieting’, the downward spiral of reduced metabolic rate and energy requirements vastly increases the risk of weight regain because of the diminishing amount of energy required to sustain normal metabolic function. Such individuals regularly turn to exercise when results cease, which increases the thermic effect of exercise, only for the hypothalamus to countermeasure the lost energy through further reductions in resting metabolic rate or the thermic effect of food (here).
The detrimental changes to resting metabolic rate are the main cause of failure on most forced calorie restrictive diets. It is unfortunate that to most physical activity is synonymous with aerobic type exercise because activity can play a role in preventing declines in resting metabolic rate. Resistance training causes unique physiological changes that can stimulate protein synthesis and reduce insulin resistance. It has been shown that obese individuals who undertake resistance training can increase their lean muscle mass and reverse previous damage to their resting metabolic rates. This results in a requirement for more calories which can further stimulate the metabolism by increasing the thermic effect of food, particularly if protein intake is increased. However, no exercise programme can be successful without dietary changes to improve the quality of the diet. Resistance training requires higher protein diets to maintain a positive nitrogen balance. In addition, removal of the refined carbohydrates and sugar would still be necessary to reverse the metabolic abnormalities that diminishes the ability to generate energy and produce force for intense exercise.
