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Nutritional epidemiology
For many decades, beginning with the altercation between Ancel Keys and John Yudkin(1), the scientific community has debated whether the principal driver of obesity is the excessive consumption of fats or carbohydrates(2-5). Historically, the role of protein has received comparatively little attention, both because it is proportionally a minor part of the energy content in the diet and because its absolute and relative intakes have remained more constant over time and across populations than those of either fats or carbohydrates(5-7). More recently, however, attention has turned to dietary protein, particularly in the context of body weight management, with specific emphasis on the role of this macronutrient in appetite regulation(8,9).
The fact that all three macronutrients have now been implicated in obesity suggests that there is a need to broaden the focus from specific nutrients to questions of how each contributes both individually and in interaction with others to drive energy overconsumption. In 2005, Simpson and Raubenheimer postulated the 'protein leverage hypothesis' (PLH) to address this issue. Like numerous other animal species(10-14), human macronutrient regulation minimizes variation in absolute protein intake. Consequently, any factor that causes a decrease in the dietary proportion of protein energy will also result in increased absolute intakes of fats and/or carbohydrates, and total energy; conversely, increased dietary protein will result in decreased energy intake ('protein leverage')(5,15-18). The PLH proposes that this characteristic of human macronutrient regulation has interacted with an extrinsic cause of dietary protein dilution, for example increased availability of cheap low-protein foods, to drive energy overconsumption and the rise in obesity(5,14).
Protein leverage in human subjects is supported by a recently published compendium of experimental studies(15), which shows how decreases in percentage of protein across the range typically seen in human populations with adequate food supply (10-25 % of total energy from protein) resulted in increased intake of energy in the form of either carbohydrates or fat(5). Increases above 20 % protein yielded a lesser...