Reductions to main meal portion sizes reduce daily energy intake regardless of perceived normality of portion size: a 5 day cross-over laboratory experiment

Abstract

Background: Smaller portions may help to reduce energy intake. However, there may be a limit to the magnitude of the portion size reduction that can be made before consumers respond by increasing intake of other food immediately or at later meals. We tested the theoretical prediction that reductions to portion size would result in a significant reduction to daily energy intake when the resulting portion was visually perceived as 'normal' in size, but that a reduction resulting in a 'smaller than normal' portion size would cause immediate or later additional eating.

Methods: Over three 5-day periods, daily energy intake was measured in a controlled laboratory study using a randomized crossover design (N = 30). The served portion size of the main meal component of lunch and dinner was manipulated in three conditions: 'large-normal' (747 kcal), 'small-normal' (543 kcal), and 'smaller than normal' (339 kcal). Perceived 'normality' of portion sizes was determined by two pilot studies. Ad libitum daily energy intake from all meals and snacks was measured.

Results: Daily energy intake in the 'large-normal' condition was 2543 kcals. Daily energy intake was significantly lower in the 'small-normal' portion size condition (mean difference - 95 kcal/d, 95% CI [- 184, - 6], p = .04); and was also significantly lower in the 'smaller than normal' than the 'small-normal' condition (mean difference - 210 kcal/d, 95% CI [- 309, - 111], p < .001). Contrary to predictions, there was no evidence that the degree of additional food consumption observed was greater when portions were reduced past the point of appearing normal in size.

Conclusions: Reductions to the portion size of main-meal foods resulted in significant decreases in daily energy intake. Additional food consumption did not offset this effect, even when portions were reduced to the point that they were no longer perceived as being normal in size.

Trial registration: Prospectively registered protocol and analysis plan: https://osf.io/natws/; retrospectively registered: https://ichgcp.net/clinical-trials-registry/NCT03811210.

Keywords: Downsizing; Food environment; Food portion; Norms; Obesity; Portion size.

Conflict of interest statement

ER and JCGH have also received research funding from the American Beverage Association and Unilever. CAH has received research funding from the American Beverage Association and speaker’s fees from the International Sweeteners Association. JCGH currently receives research funding from the American Beverage Association, Astra Zeneca and Bristol Meyers Squib and has been an advisory board member for Novo Nordisk and Orexigen.

Figures

Fig. 1

CONSORT Flow diagram depicting flow of participants through study recruitment, enrolment, completion, and analysis

Fig. 2

Overview of daily assessment of energy intake

Fig. 3

Effect of portion size on daily energy intake. a 95% CI [− 418, − 192], d = 1.01. b 95% CI [− 309, − 111], d = 0.79. c 95% CI [− 184, − 6], d = 0.40. Error bars represent standard errors and values on comparison bars = p for pairwise comparisons

Fig. 4

Effect of portion size on immediate additional intake of other meal food at lunch (left) and dessert food at dinner (right). a 95% CI [179, 281], d = 1.69. b 95% CI [106, 184], d = 1.40. c 95% CI [51, 117], d = 0.95. d 95% CI [15, 88], d = 0.53. e 95% CI [− 20, 38], d = 0.12. f 95% CI [12, 73], d = 0.53. Error bars represent standard errors and values on comparison bars = p for pairwise comparisons

Fig. 5

Effect of portion size on total meal intake (sum of intake from initial portion and additional intake of other meal food) at lunch (left), and (sum of intake from initial portion and additional intake of dessert food) at dinner (right). a 95% CI [− 163, − 68], d = 0.92. b 95% CI [− 82, − 4], d = 0.41. c 95% CI [− 106, − 40], d = 0.84. d 95% CI [− 299, − 179], d = 1.49. e 95% CI [− 178, − 100], d = 1.32. f 95% CI [− 141, − 57], d = 0.88. Error bars represent standard errors and values on comparison bars = p for pairwise comparisons