Low and very low calorie diets continue to be used as the main treatment against obesity. This is despite evidence published by the nutritional sciences demonstrating that forced energy restriction diets cause muscle loss and lower resting metabolic rates (RMR) in obese individuals. For example, in a study published in the American Journal of Clinical Nutrition in 19901, researchers fed 13 obese women a low calorie diet (1000-1200 kcal/d) for 4 weeks. Following this time the RMR of the subjects decreased, suggesting that muscle loss had occurred. One group of women were then fed a very low calorie diet (500 kcal/d) for the next 8 weeks and their RMR decreased further, indicating that lean tissue levels had also decreased. In fact the reduction in RMR in this group was almost 20 % from baseline, suggesting that their metabolism had been seriously affected by the very low calorie diet.
The very low calories group then had a incremental increase in calories (200 kcal/d) until week 24, when they would again be on the same energy intake as the low calorie group. During this time their RMR recovered somewhat, but never returned to the same levels as subjects of the less severe energy restriction. By week 24 both groups had a reduction in RMR compared to baseline, with the low calorie group showing a 2.0 % reduction and the very low calorie group showing a 11.3 % reduction. Both groups lost fat during this period, but at week 24 both groups had also lost lean tissue. The low calorie group lost a total of 2.3 kg of lean tissue (-4.1%), whereas the very low calorie group lost a total of 2.1 kg of lean tissue (-3.6 %). At the end of the study at 24 weeks, the low calorie group required 41 less kcal/d to maintain weight, while the very low calorie group required 206 kcal/d less.
These results support other studies that show forced energy restriction diets to be a useless tool for treating obesity. Low and very low calorie diets have been shown to cause serious metabolic modifications. Treating a metabolic disorder such as obesity with a diet that causes further detrimental changes to metabolism is of questionable benefit. The high failure rate of such diets is explained by the changes to the RMR that increases the likelihood of subsequent weight regain. For example, while the very low calorie group were consuming the 500 kcal/d diet, they were losing weight which included fat free mass. However, when they started to increase their calories the RMR began to increase, but this was accompanied by weight regain. Because both the low and very low calorie groups had significant muscle loss during the study, both required fewer calories to maintain their metabolic rate, indicating serious loss of lean tissue.