Background – The role of protein in the obesity crisis has, until recently, been largely ignored. This is for two reasons. First, protein provides the minor part of the human energy budget. Second, protein intake has remained far more constant over time and across populations than either fat or carbohydrate, both as a percentage of energy in the diet and in terms of absolute amounts eaten. Hence, while the obesity epidemic has spread, protein intake has remained relatively unchanged – giving the impression that protein cannot be responsible.
Method – We use state-space models (the Geometric Framework) developed from extensive animal studies to postulate a key role for protein appetite in the obesity epidemic, and provide supporting evidence from experimental, nutritional survey, and animal studies.
Results – Protein is the most satiating macronutrient group for humans and is the most tightly regulated post- absorptively. Results from comparative studies of other vertebrates, human experiments, and population-level data strongly suggest that the response of humans when faced with imbalanced diets is to prioritise protein intake. Hence, when the percent protein in the diet is low, non-protein energy is overeaten, whereas when dietary % protein is high, energy intake is limited: in both cases, absolute intake of protein is maintained near constant.
Conclusion – We show how, paradoxically, it may be because protein comprises a relatively small component of the human diet and is tightly regulated that it has sufficient leverage over human ingestive behavior to explain obesity. Focusing on this leverage over intake clarifies the role of dietary protein in the development of obesity, provides a possible means of ameliorating the problem, and explains the effectiveness of high-protein diets as weight loss regimes.