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.

Technical Abstract:
A number of studies show that portion size can have a significant effect on energy consumption; when served larger portions we tend to consume more food. Although reducing portion sizes could be an effective public health approach to tackling weight gain and obesity, we currently lack a psychological theory which a) explains why portion size can exert such a strong influence on energy intake & b) can identify the boundary conditions which determine the magnitude of portion size reductions which will cause sustained and reliable decreases to energy intake. This is problematic because if we do not understand why portion size influence intake, intervention efforts to reduce portion size may fail, due to portion size energy reductions being cancelled out by later compensatory eating.
The aim of this work is to test a proposed theoretical model of the effect that portion size has on energy intake. Our theoretical model builds on visual perception research and is based on the premise that portion sizes are categorically perceived; in other words, when provided with a food portion, individuals make a simple perceptual categorisation of the portion being either 'normal' or abnormal in size. Consistent with previous work, we predict that if a portion is perceived as being 'normal' an individual would be likely to eat the majority of the food provided. A key point of our proposed theory is that although this kind of perceptual categorisation is cognitively efficient, a relatively wide range of portion sizes appear 'normal'. This in turn results in marked effects on energy intake, providing that any increase or decrease to portion size falls inside of a food's 'normal range'. We will test this theory across three laboratory studies. Guided by these insights we will then conduct a large scale 7-day energy balance experiment, in which we examine the size of portion size reductions required to promote reliable and long-term decreases to energy intake.

Potential Impact:
Excess energy consumption and weight gain are major causes for concern, because of the large effect that obesity continues to have on the economy and public health. The present work provides critical information about an approach which could be used to directly address overconsumption and obesity on a population level. To date, we have a very poor understanding of why portion size influences energy intake or how much portion size can be reduced by in order to promote sustained reductions to energy intake. The present research will provide novel information which addresses these needs.
The influence that portion size has on energy intake and its potential public health application has been studied as far afield as America, Europe, Australia and Asia. Thus, as this research programme proposes a novel theoretical model of the influence that portion size has on energy intake, it has potential to benefit the work of scientists across the world. In particular, this research grant will provide critical information about how to reformulate portion size in a way that promotes sustained reductions to energy intake. Given the applied relevance of this work we also envisage it will impact on a wide spectrum of obesity and public health researchers, who will be able to use this new knowledge to guide future intervention efforts against obesity.
Now that we know the influence that overconsumption and weight gain have on health and well-being, there is a pressing need to reduce energy intake at a population level. Focusing on portion size is a potentially far reaching approach. Our work will act as exemplar of how we can use methods from different disciplines (e.g. visual science and feeding behaviour) to understand a complex behaviour and then use this information to start to examine how we can attempt to improve public health. Thus, our findings have potential to be far reaching in the long term, in terms of improving population health and the drain that obesity has on the economy.
As we believe the longer-term implications of this work will have potential to benefit nutrition and public health, a strong component of this project is ensuring that we actively engage with the general public and the types of bodies able to harness portion size principles to improve public health. We have planned a dissemination event to target key stakeholders at the end of the programme and we will supplement this with events and activities targeted at the general public (e.g. British Science Festival). Supplementary dissemination activities will also ensure we have regular communication with key stakeholders. Thus, our work will be delivered in way that maximises its potential impacts on traditional academic circles, public health initiatives and the food industry.