High protein diets are associated with weight loss and improvements in body composition. The exact reason for this is unknown, but it is thought that protein provides a number of metabolic advantages over carbohydrate and fat in terms of its ability to favour weight loss. Protein for example, may decrease appetite because it inhibits gastric emptying. In addition, protein is more metabolically costly to metabolise and so provides less energy per gram compared to other macronutrients. Protein is also less likely to be stored, but more likely to be incorporated into structural components such as skeletal muscle. This in turn may then increase the metabolic rate, leading to further energy usage. A number of studies have demonstrated a weight loss effect for protein and the weight loss reported on low carbohydrate diets may be due to the higher protein content. Both animal and plant protein appear to share the beneficial effects and either can be incorporated into the diet to improve body composition and cause a weight loss effect.
High protein diets containing both animals of plant based protein may protect from weight gain and reduce the risk of the metabolic syndrome.
For example, in one study researchers assessed the type and amount of protein as a determinant of metabolic syndrome. The subjects were fed a typical American diet and then allocated to follow either a high plant (18 % protein, two-thirds plant sources), high animal (18.4 % protein, two-thirds animal sources from beef) or moderate animal protein (27% protein, two-thirds animal sources as beef) diet. The subjects followed one of the diets through both weight loss (-500 kcal per day deficit) and weight maintenance phases. All groups achieved a 5 % reduction in weight by the end of the weight loss part of the study and this was maintained during the maintenance part of the study. Both animal and plant protein diets were equally effective at causing weight loss and at reducing the prevalence of metabolic syndrome by 50-60 % following the weight loss phases of the diet. High animal or plant protein diets may therefore be effective at improving health by significantly reducing body weight, when compared to a typical American diet.
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