Degree Name

Doctor of Philosophy


School of Health Sciences


Obesity results from a cumulative positive energy balance over a period of time with resultant increases in adipose tissue. Dietary intervention is crucial in inducing negative energy balance, which is the cornerstone to weight loss. Finding an optimal ratio of macronutrients in a weight loss diet is important as it can potentially enhance weight and fat mass loss. Dietary protein has been shown to be more thermogenic than other nutrients while polyunsaturated fatty acids (PUFA) are capable of elevating human fat oxidation rates. These properties are important in weight management and have received much attention in the scientific literature in recent years. This thesis argues that the dietary protein and PUFA fraction of the diet are especially significant and that manipulation of these fractions will result in metabolic advantages. The overall aim of this thesis was to investigate the position of dietary protein and PUFA in a weight loss diet, using measures of energy and substrate metabolism in studies involving the whole room calorimeter (WRC) facility. From a methodological perspective, the validity of triaxial accelerometers for estimating energy expenditure and in predicting energy requirement in the WRC was investigated. Following this, the usefulness of increased protein proportions in a weight loss diet was examined through two studies. The first study investigated the acute effects of high protein diets from three protein sources (meat, dairy and soy) on human energy and substrate metabolism, while the second study investigated longer term effects of high protein diets on the related clinical outcomes of weight and fat mass loss. The acute study demonstrated that meat protein was more retained in the human body which may result in less fat-free mass loss during weight loss in the longer term. However, this was not observed where all study groups showed similar weight loss and body composition changes in a 12-week weight loss trial. Using a similar experimental approach, the usefulness of increased dietary PUFA in weight management was tested through acute and longer term dietary interventions. Fat oxidation was significantly increased following high PUFA meals and produced significant negative fat balance, but in the longer term this was not translated into greater fat mass loss. Fat mass loss was also not greater in the abdominal region although there is evidence linking higher PUFA intake with lower visceral adipose tissues. In both dietary protein and PUFA manipulation studies, the acute effects on energy expenditure and substrate oxidation did not appear to be sustained over a longer period of time. Therefore, in the final chapter of this thesis, reasons for this failure to extend acute effects to longer term clinical outcomes were explored. A dynamic energy balance framework was proposed to help manage weight more effectively in a clinical setting. Effective weight management should include more specific dietary strategies involving the manipulation of dietary components, instead of simple generic advice to eat less and exercise more.