Portion Size & Energy Intake: A key nutritional concept we must now understand to address obesity

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Rises in obesity have been caused by increases in energy intake and a key environmental factor which may responsible for this is portion size. Over the last thirty years portion sizes for a wide variety of foods have increased. This is a cause for concern because experimental data shows that people 'clear their plate' and eat most of the food they are served. To date, we have very convincing acute laboratory data showing that portion size is likely to be a key driver of energy intake; the amount of energy we are served at a meal or provided in a food portion influences how much energy we consume. Because of this, targeting reductions to portion size could be a key approach to tackling obesity and recently commissioned public health reports support this view. However, we currently lack a theoretical model which is able to 1) explain why portion size exerts such a strong effect on energy intake and, 2) identify the magnitude of reductions to portion size we can make to promote reliable long-term reductions to energy intake. This is a major stumbling block because before we can be confident that exploiting portion size will help to tackle obesity, we need to understand why portion size influences energy intake and how to reformulate portion sizes. Recent research has shown that it is critical that reductions made to portion size are informed by a theoretical model, because if not, it is likely that any energy reductions made could be cancelled out by later compensatory eating. The aim of the present research is to develop and test a theoretical model to explain the effect that portion size has on energy intake, before applying this knowledge to test how we can reduce portion sizes to promote reliable and sustained reductions to energy intake. Our theoretical model builds on an established framework and is based on the premise that when presented with a food portion, individuals make a simple perceptual categorisation of the portion being either 'normal' or abnormal in size. If a portion is categorised as being 'normal' a consumer would be likely to eat the majority of the food provided. Although this kind of perceptual categorisation is cognitively efficient, a relatively wide range of portion sizes appear 'normal'. We predict that this causes any increases or decreases to portion sizes which sit inside this 'normal range' to have marked effects on energy intake. Thus, our proposed model attempts to explain why portion size can exert such a strong effect on intake for a variety of foods, with consumers still believing they have eaten a 'normal' sized meal. This research grant will empirically test our proposed theory and examine the effect of reducing portion size on energy balance. In doing so, this work brings together experts in the fields of appetite, health psychology, visual perception and public health, in order to understand portion size and energy intake. Critically, our theoretical model suggests that there are likely to be boundary conditions which determine when a decrease to portion size will cease to have a reliable effect on energy intake. By testing these boundary conditions we will be in a unique position to examine the size of portion size reductions required to promote a sustained decrease in energy intake. In doing so we will generate important knowledge about how we can go about reformulating portion sizes for public health benefit. By the end of the three year programme we will have tested a theoretical account of why portion size influences energy intake and applied this knowledge to test whether it is possible to use portion size principles to promote sustained reductions to energy intake. These novel insights will have direct relevance to the food industry and public health initiatives which aim to make reductions to portion size in order to help tackle the widespread obesity problem.