N09021: Investigation of the contribution made by food portion size to food and energy intake

Study Duration: July 2005 to September 2007

Contractor: Northern Ireland Centre for Food and Health, University of Ulster

US studies have suggested that increasing food portion size may be facilitating excess energy intake and a higher risk of overweight and obesity. The purpose of this study was to explore the relationship between food portion size and energy intake in a UK setting.

The study included examining existing dietary data to explore the association between portion size of food groups and risk of obesity. The main aspect was a fully residential study to evaluate the extent to which portion sizes influenced food and energy intake over 2 periods of 4 days, within 3 weeks of each other. Subjects were randomly assigned to receive either a 'standard' food portion size followed by a 'large' food portion size of the same food in the second 4 day period, or vice versa.

Methods
Analysis of food intake data from the National Diet and Nutrition Survey (NDNS) of British adults to determine the odds ratio for obesity associated with consuming larger quantities of different foods, using multinomial logistic regression analysis.

The completion of a 4 day residential study to assess the extent to which the amounts of foods consumed are influenced by the availability of different sized food portions. This study used a randomized within subject cross-over design. Forty-three normal weight and overweight subjects were fully residential in the Human Intervention Studies Unit at the University of Ulster, for two four day periods with a 3 week interval between each study period. Subjects were randomly assigned to 2 groups. Group 1 received ‘standard’ portion sizes of foods in the first four day period followed by ‘large’ portion sizes of the same foods in the second 4 day period. The order was reversed in group 2.

Results
The NDNS analyses emphasised that under reporting needs to be taken into account when assessing the associations between portion size of food groups and obesity risk. Specific foods or food groups in isolation may not be major contributing factors to obesity risk, rather it may be the consumption of a wider range of foods and food groups. This analysis also highlighted the need to explore possible associations between the energy density (ED) of foods/ food groups with measures of adiposity, using different methods for the calculation of ED.

The portion size residential study showed that EI over four days was significantly higher on the large portion condition compared to the standard condition in the total group (59.1 (±6.6) MJ vs 52.2 (±14.3) MJ, P = 0.020); men and women increased their EI by 17% (10 (±6.5) MJ P < 0.001) and 10% (4 (±6.5) MJ P = 0.005) respectively when served the large food portions relative to the standard food portions. The increased intakes were sustained over the four days in the large portion condition with little evidence of down regulation of EI and food intake being made by subjects.

This showed showed that subtle increases in the portion sizes of all foods resulted in significant and sustained increases in EI in both men and women that were sustained over 4 days, suggesting that the availability and consumption of larger portions of food may be a major contributing factor in inciting excess EI and adiposity

Analyses of NDNS dietary data indicated that under-reporting may have masked any true associations between food portion sizes with measures of obesity.

The portion size study showed that energy intake over four days was significantly higher when participants were offered large portions compared to the standard portions and that there was little evidence of down regulation of food intake overall.

This study emphasises the complexities of exploring food portion size and obesity risk. The study indicates that the availability and consumption of larger portions of food may be a major contributing factor in inciting excess energy intake.

• MT Kelly, A Fletcher, SF Brennan, JMW Wallace, PJ Robson, RW Welch, MP Hannon-Fletcher and MBE Livingstone (2007)Increased portion size leads to a sustained increase in energy intake over 4 days in normal weight and overweight men and women. Proceedings of the Nutrition Society 66 (Supplement OCA-B) 32A-114A; 77A
• Relationship between portion size of food groups and body mass index in the british national diet and nutrition survey: Kelly M, Rennie K, Wallace JMW, Hannon-Fletcher MP and Livingstone MBE. Northern Ireland Centre for Food & Health, University of Ulster, Coleraine. Public Health Nutrition Volume 9No. 7(A)