Abstract:
Excess energy intake is clearly implicated in the obesity epidemic, but there is uncertainty as to the role of specific macronutrients. Although carbohydrates and fat are major sources of excess dietary energy, there exists a hypothesis that protein is a key determinant of carbohydrate and fat, and hence total energy intake. The Protein Leverage Hypothesis (PLH) suggests that protein plays a key mediating role mediating in the development of obesity, through its interaction with fat and carbohydrates is not well understood. PLH predicts that humans prioritize protein when regulating food intake. In doing so, humans will consequently over-consume fats and carbohydrates when consuming diets low in protein and fats and carbohydrates will be under-consumed on diets that are high in protein. Although evidence exists in support of the PLH in two experimental studies of UK and Australian participants, and from a recent meta-analysis of experimental data, there are not data to support this hypothesis in humans living in their usual living environments. The first aim of this thesis was to test the hypothesis that humans with a population history of severe childhood under-nutrition exhibit characteristics of the PLH. Given the growing body of work showing the importance of developmental nutritional history in predicting energy intake and obesity, it is important to determine macronutrient regulation in a nutritionally deprived population and, in particular, whether the participants exhibited protein leverage as per the PLH. The second aim of this thesis was to test the hypothesis free-living humans in a developing country undergoing the nutrition transition exhibit dietary behavior consistent with the PLH. In this thesis, I report the first evidence in a population-level study that supports the PLH in a free-living population. Together these data contribute to the understanding of how dietary macronutrient content influences energy intake, by extending how PLH can be generalized to a population with a history of severe childhood undernutrition, and showing for the first time that the predictions of PLH are met in population studies outside of an experimental setting. Both the population-level and experimental components of this study provide support for the role of the protein appetite in driving energy intake.