Dietary protein plays a multifactorial role in weight loss and weight maintenance as it influences crucial targets for body weight regulation such as body composition; satiety; thermogenesis and energy efficiency. Protein induced changes in body composition favor an increase of fat free mass, thereby enhancing basic metabolic rate as fat free mass is its main determinant, and a decrease in fat mass that may be mainly due to amino acids fed in excess which under normal leisure time physical activity are used for muscle protein synthesis. Protein induced satiety may be mainly due to oxidation of amino acids fed in excess, especially with diets with ‘incomplete’ proteins, and to gluconeogenesis restoring glucose homeostasis. Protein induced energy expenditure may be mainly due to protein and urea synthesis; ‘complete’ proteins having all essential amino acids, show larger increases in energy expenditure than lower quality proteins. This review provides an overview of how sustaining absolute protein intake affects metabolic targets for weight loss and weight maintenance during negative energy balance, i.e. sustaining satiety, energy expenditure, sparing fat free mass, resulting in energy inefficiency, and showing that long term relationship between net protein synthesis and sparing fat free mass remains to be elucidated.
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