Personal and social norms for food portion sizes in lean and obese adults

H B Lewis, S E Forwood, A L Ahern, K Verlaers, E Robinson, S Higgs, S A Jebb, H B Lewis, S E Forwood, A L Ahern, K Verlaers, E Robinson, S Higgs, S A Jebb

Abstract

Background: Portion size is an important component of dietary advice for weight control, but little is known about what portion sizes people consider 'normal'. This study determined the effect of body mass index (BMI), gender, dietary restraint and liking of the food on personal and social portion size norms for a range of foods and the degree of certainty over the norms.

Methods: Thirty lean (BMI 20-25 kg m(-)(2)) and 30 obese (BMI 30-35 kg m(-)(2)) men and women (aged 18-60 years) viewed 17 different portion sizes of 12 foods on a computer screen on two occasions a week apart. Participants responded 'more' or 'less' to each photograph reflecting personal portion size preference or perceived portion sizes of others. Personal and social norms for portion sizes of each food were determined using the method of constant stimuli giving a sigmoidal curve of the probability of answering 'less' over a range of portion sizes. The slope of the sigmoid at the norm gave a measure of certainty about the norm. Regression models were used to examine the effect of BMI, gender, dietary restraint and liking of the food on personal norms, social norms, the relationship between norms, and the slopes.

Results: Personal norms were significantly larger in the obese (P=0.026), men (P<0.001), those with lower dietary restraint (P<0.001), and those with higher liking for the food (P<0.001). Social norms were larger for women (P=0.012). The slopes at the norms were 30% shallower in the obese and in men (P<0.001).

Conclusion: Larger personal norms for portion size among the obese, men, those with lower dietary restraint and those with higher liking for a food imply greater consumption, which may undermine weight control. Shallower slopes for norms in the obese and in men may imply less clearly defined habitual portion sizes.

Figures

Figure 1
Figure 1
Use of the method of constant stimuli task to derive a norm for portion size and associated sensitivity to the stimulus (the example shown is for the modelling of the personal norm for pasta for one participant).
Figure 2
Figure 2
Geometric means and 95% confidence intervals for portion size index for personal norms and social norms according to A) BMI group, B) gender, C) level of dietary restraint, and D) level of liking. *p

Figure 3

Illustration of the nature of…

Figure 3

Illustration of the nature of the difference in the slopes of the response…

Figure 3
Illustration of the nature of the difference in the slopes of the response curve between lean and obese participants.

Figure 4

Geometric means and 95% confidence…

Figure 4

Geometric means and 95% confidence intervals for the slope of the modelled curve…

Figure 4
Geometric means and 95% confidence intervals for the slope of the modelled curve at the norm according to A) type of norm, B) BMI group, C) gender, D) level of dietary restraint, and E) level of liking. *p
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References
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Figure 3
Figure 3
Illustration of the nature of the difference in the slopes of the response curve between lean and obese participants.
Figure 4
Figure 4
Geometric means and 95% confidence intervals for the slope of the modelled curve at the norm according to A) type of norm, B) BMI group, C) gender, D) level of dietary restraint, and E) level of liking. *p

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