Diets Low in Fat, Carbohydrate, or Energy Density
A variety of diets, including low-fat foods, low-carbohydrate foods, or a balanced reduction of all macronutrients, have been used to treat obesity. Table 8.3 is a compilation of several of these diets. A meta-analysis of low-fat vs. conventional studies identified five studies lasting up to 18 months.
Fig. 8.3 Algorithm for diagnosis and treatment of obesity. Adapted from 
In comparing the weight loss at 6, 12, and 18 months, there were no statistically significant differences from control, leading the authors to conclude that low-fat diets produce weight loss, but not more so than other diets. In a meta-analysis comparing “named” diets, Johnston et al.  showed no consequential differences in weight loss at the end of 1 year.
Fat is an important component of energy density. If the diet is high in fat or low in water content, then it will have a high energy density (i.e., more calories per gram). In a recent trial, Ello-Martin et al.  reported a weight loss of 7.9 kg after 1 year by feeding a diet with a low energy density. The diet was low in fat diet and rich in fruits and vegetables with high water content. This underscores the role of energy density of the diet as a factor in weight loss. It is important to appreciate that little weight loss will occur unless the diet induces an energy deficit, but there may be a number of different ways to do that. This idea of low energy density is developed in the Volumetries diet (Table 8.3).
Several controlled trials showed more weight loss with a low-carbohydrate diet than the control diet in the first 6 months but no difference at 12 months (Table 8.3). In two head-to-head comparisons of four popular diets, the average weight loss at 6 and 12 months was the same [25, 26]. The best predictor of weight loss for each of the diets was the degree of adherence to the diet [25, 26].