Mechanisms of the portion size effect. What is known and where do we go from here?

Childhood obesity is a persistent problem worldwide, and of particular concern in the United States. Clarifying the role of the food environment in promoting overeating is an important step toward reducing the prevalence of obesity. One potential contributor to the obesity epidemic is the increased portion sizes of foods commonly served. Portion sizes of foods served both at home and away from home have dramatically increased over the past 40 years. Consistently, short-term studies have demonstrated that increasing portion size leads to increased food intake in adults and children, a phenomenon known as the portion size effect. However, the mechanisms underlying this effect are poorly understood. Understanding these mechanisms could assist in clarifying the relationship between portion size and weight status and help inform the development of effective obesity interventions. First, we review the role of visual cues, such as plate size, unit, and utensil size as a potential moderator of the portion size effect. In addition, we discuss meal microstructure components including bite size, rate, and frequency, as these may be altered in response to different portion sizes. We also review theories that implicate post-ingestive, flavor-nutrient learning as a key moderator of the portion size effect. Furthermore, we present preliminary data from an ongoing study that is applying neuroimaging to better understand these mechanisms and identify modifiable child characteristics that could be targeted in obesity interventions. Our tentative findings suggest that individual differences in cognitive (e.g. loss of control eating) and neural responses to food cues may be critical in understanding the mechanisms of the portion size effect. To advance this research area, studies that integrate measures of individual subject-level differences with assessment of food-related characteristics are needed.