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

Project Code: N09021

30/09/2007

University of Ulster
Livingstone, B

Introduction
The burgeoning rates of obesity in genetically stable populations suggest that an increasingly obesogenic environment is the major driving force behind this epidemic. Increased food portion size may be facilitating excess energy intake (EI) and a higher risk of overweight and obesity. The assessment of associations between food portion size and adiposity in free-living adults is extremely limited. Portion size has been demonstrated to positively influence EI in adults under semi-controlled laboratory conditions, but these findings have been specific to the US and the extent to which these findings would be relevant to the UK population is unknown at present.

Methods
1. Analysis of food intake data from the National Diet and Nutrition Survey (NDNS) of British adults (n=1519) to determine the odds ratio for obesity associated with consuming larger quantities of different foods, using multinomial logistic regression analysis.
2. The completion of an intensive 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 of presentation was reversed in group 2.

Results
Few associations were observed between portion size of food groups consumed and measures of body fatness (Body Mass Index; BMI and Waist Action Level; WAL) in the NDNS analyses. In this study it is highly likely that under-reporting (UR) may have masked any true associations between food portion size with measures of obesity.

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.

Conclusions
The NDNS analyses emphasised that UR 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.

The portion size residential study 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.

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