Obesity, Portion Sizes and High Energy Density at Meals

Over the past few decades, increased body weight among Americans has coincided with significant changes in the availability and types of food and the eating environment. One change has been the increased portion sizes available at fast-food restaurants and at home. The variety of foods and beverages available to consumers has also increased dramatically. The easy access to highly palatable, energy-dense foods available in large portions at relatively low cost may be contributing to the rapid growth in the incidence of obesity among all ages, particularly children, according to a group of researchers at Penn State.

Energy and portion size important to overeating

According to Dr. Barbara J. Rolls and co-workers at Penn State’s Laboratory for the Study of Human Ingestive Behavior, State College, PA, the energy density and the portion size of a food act independently to affect energy intake (Am J Clin Nutr 2004;79:962). Thus, large portions of food with a high-energy density may facilitate the overconsumption of energy.

The researchers used a within-subjects study design to determine the combined effects of energy density and portion size on intake in women. Once a week, for 6 weeks, 39 women volunteers were served breakfast, lunch, and dinner, and were allowed to eat freely with no restrictions on amount. However, the main entre at lunch was served in two different versions that varied in energy density (5.23 or 7.32 kilojoules per gm), each of which was served in 3 different portion sizes: 500, 700, or 900 gm. The amount of food and calories offered in the smallest portion size of the entre was enough for the subjects to feel full and satisfied at the end of the meal.

Visual analog scale recorded the degree of hunger

Before and after each meal, the subjects completed a series of 100-mm visual analog scales that rated their degree of hunger, thirst, perception of how much they could eat, and feelings of nausea and fullness. For example, hunger was rated on a line preceded by the question, ‘How hungry are you right now?” with “not at all” on the far left of the line and “extremely hungry” on the right end of the line.

The portion size and energy density had a significant effect on the amount of food eaten at lunch. The interaction between the two factors was not significant, which indicated that the portion size and energy density of foods acted independently to affect the amount of food the participant ate.

When served the largest portion of the entre, subjects ate 20% more than when they were served the smallest portion. They ate 10% less food when served the entre with higher caloric content compared with an entre of lower energy density.

The results from this experiment may have important implications for weight management. Increasing the portion size and the energy density of a main entre resulted in significant increases in energy intake during a single meal. There was no evidence that the women compensated for the increased energy intake at subsequent meals.