The protein leverage hypothesis suggests that an organisms appetite is regulated by its requirement for protein (here). As the percentage of protein in the diet goes down, compensatory mechanisms are put in place to increase appetite and thus increase protein intake. This then lifts the amount of protein required above the set threshold and satisfies the organisms need for protein. Diets with low percentages of protein therefore encourage overeating of other micronutrients in the quest for adequate protein. The protein leverage hypothesis explains why high protein diets cause increases in satiety and thus induce weight loss. In addition, the requirement for protein during periods of low calorie dieting has been understood for decades by strength and power athletes that manipulate protein intakes to maintain nitrogen retention and improve lean mass during times of negative energy balance. That the understanding of this concept has taken so long to reach the research arena is not surprising given the ivory towers most academic find themselves inhabiting.
The benefits of higher protein intakes are now reasonably well established in the nutritional literature. For example, in a recent paper published in the Journal of Nutrition, researchers investigated the effects of dietary protein content on body weight loss variables during a period of negative energy balance over a 6 month period1. Overweight and obese men and women were given either a high protein (1.2 grams per kg body weight) or low protein (0.8 grams per kg body weight) diet and then placed on a weight loss regimen centred around calorie restriction. For two weeks the subjects ate 100 % of their normal calorie requirement. A 6 week phase of weight loss was then initiated by feeding the subjects 33 % of their normal calorie requirement for 6 weeks. Finally for 17 weeks the subjects ate 67 % of their normal calorie requirement in a maintenance phase. The researchers then assessed the subjects body composition and metabolic responses to the calorie restriction on the two protein intakes.
The results of the study showed that the body weights and body mass indexes of the subjects decreased in both high and low protein treatments to the same extent (~7 kg and ~2.5 kg/m2). However, those on the high protein diet lost less lean mass than those on the low protein diet (0.1 versus 0.8 kg, respectively) and more fat mass (6.9 kg versus 6.4 kg, respectively). Those on the higher protein diet also experiences less reductions in resting metabolic rate compared to those on the lower protein diet (47.8 versus 129 kcal per day). From this data the authors concluded that the higher protein diet was more beneficial at preserving lean mass and preventing falls in resting metabolic rate when compared to the lower protein diet. In this respect the study supports other research in this area in that body composition improvements are superior when higher protein diets are ingested. Measurements of nitrogen retention showed that both diets had a muscle sparing effect but that the higher protein diet was superior in this respect.
High protein diets are beneficial at causing weight loss because they satisfy the need for protein and so, based on the protein leverage hypothesis, satiety is increased which prevents overeating of other macronutrients. In this regard, high protein diets can decrease intakes of fat and carbohydrate, the combination of which may contribute to obesity. In fact, lowering carbohydrate intakes is likely the primary driver of fat loss in high protein diets, as such diets are by their very nature low carbohydrate diets. This is an interesting point, because in essence, studies feeding high protein diets are actually feeding low carbohydrate diets. That refined carbohydrates and sugars have been implicated as a primary driver of obesity and weight gain, it is no surprise that such diets cause weight loss. High protein diets can therefore cause weight loss without the need to restrict calories because they limit carbohydrate intake (here). High protein diets also improve nitrogen retention, and this has hypertrophy stimulating effects, which can beneficially raise resting metabolic rate.
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