Rapid Evidence Review: Implementation of Portion Size and Portion-Related Energy Reductions in Out of Home Settings

Across intervention types, the evidence most strongly supports interventions that change what is offered by default and make smaller portions easy to choose. When presented, smaller portions tended to be popular, especially when proportionately cheaper. Interpretation is complicated by differences between studies of snacks, individual meal components and complete meals, and by variation in whether changes were measured in calories, volume, or both. The principal threats to effectiveness are promotions or other value-related messages that encourage upsizing, opportunities for compensation through add-on items, and limited measurement of longer-term outcomes and commercial feasibility. For Scottish implementation, the most informative future evaluations should measure total energy purchased per transaction, including side dishes and additional items, and assess whether any immediate reductions are offset by compensatory intake later in the day. They should also prioritise business-relevant outcomes, including sales patterns, operational burden, customer acceptability, food waste, and the feasibility of proportionate price reductions. The current evidence is more immediately informative for guidance, piloting, and voluntary implementation in OOH settings than for mandatory policy, because studies remain heterogeneous and relatively few assess longer-term business feasibility in real-world conditions.

Two evidence gaps stand out. First, future studies should measure compensatory responses more completely, including additional purchases at the same eating occasion, later intake, and plate waste. Second, more evidence is needed on business feasibility, including operational burden, impacts on sales and customer satisfaction, and whether proportionate price reductions are commercially sustainable.

Across the included OOH evidence base, direct physical portion reduction and portion standardisation most consistently led to lower energy selected, purchased and/or consumed in the immediate eating occasion. Evidence for this comes from a mix of real-world interventions in cafeterias, restaurants and takeaway settings, alongside shorter laboratory studies. The real-world studies are particularly informative for implementation and feasibility, while the laboratory studies are useful for understanding the likely direction of effect and the potential for compensation.

Evidence relevant to feasibility and commercial viability is limited. Few studies reported revenue impact, repeat purchasing, or customer satisfaction, and where acceptability or value-for-money was measured it often depended on whether price changed in line with portion size. Across studies, dissatisfaction risk appeared more likely where portion size was reduced without a price reduction, whereas smaller portions could be acceptable when framed as a standard option, when quality was perceived to have been maintained, or when portion reduction was accompanied by changes that preserved perceived value. Smaller portions did tend to be popular and even a reduction in the size of a subset of dishes had average benefits for energy intake.

Reductions tended to range from 10% to 40%, with most studies showing a more modest reduction of 10-20% based on volume reduction. Overall, the evidence is currently insufficient to specify an ‘optimal’ reduction level across OOH settings because few studies jointly measure energy purchased, compensation, acceptability, and business outcomes over longer follow-up.

Where compensation was assessed, it occurred through additional items chosen at the same occasion (for example, taking an additional portion, or adding sides, snacks, desserts). This means that evaluations focusing only on a single target item or range of items may overstate effects on reduced calorie intake relative to those measuring total energy purchased per transaction or the total intake over a day. For interpretation and policy relevance, the most informative outcomes are therefore total calories purchased per transaction and any evidence on later compensation, not only portion size of the target dish.

However, in the studies that monitored additional purchases or choices, the overall energy consumed during the immediate eating occasion was still, on average, lower when participants were given smaller portions of the targeted foods. This was the case even when additional side dishes or desserts were chosen.

Practical implementation examples reflected in this evidence include:

• Reducing default portion sizes. Reductions can be modest (e.g., 10-15% calories by volume)
• Reducing unit sizes and using smaller and standardised utensils and container sizes
• Revising how meal components are bundled in meal deals to reduce the overall size or energy content of the meal.
• Engaging outlet owners, managers, or staff in the rationale for portion changes and supporting implementation through standardised serving approaches and promotion of smaller options.

Studies of portion composition and meal design suggest that the energy content of OOH meals can also be reduced without simply shrinking the visible size of the portion. These interventions typically reduced higher-energy meal components, increased lower-energy components, or combined modest portion reduction with replacement of higher-energy items. This may be relevant where preserving perceived value, meal completeness, or customer acceptability is important. However, the evidence base for these approaches is smaller and more heterogeneous than for direct physical portion reduction, and includes a mixture of real-world and controlled studies, making it harder to draw firm conclusions about feasibility and effect size across OOH settings. These approaches should therefore be considered a distinct but relevant strand of evidence rather than as equivalent to direct physical portion reduction.

This suite of interventions includes approaches that did not necessarily remove larger portions, but aimed to shift selection of foods by altering defaults, presentation, or perceived norms. Across the studies, this was a common strategy and often overlapped with direct physical portion reduction.

Evidence from the broader synthesis literature supports the proposition that changing portion norms and related choice architecture can change selection and intake, and that sustained, system-level changes to the “size, availability and appeal” of larger portions are likely to be required to produce meaningful population-level effects. Much of the evidence in this category comes from short-duration laboratory or simulated ordering studies. These studies are useful for showing that portion norms and menu architecture can influence choice, but they provide less certainty about whether effects would be maintained over time and about implementation in routine OOH practice.

The most policy-relevant insight from this evidence family is that portion selection is not purely preference-driven. It is shaped by cues that signal what is normal or good value. Where smaller portions are made prominent, framed as a standard choice, or presented as the default, selection can shift towards smaller options. However, the evidence base for pure “format-only” changes (for example plateware size alone) is less consistent and often based on short-term or controlled settings. This reduces confidence that format manipulation alone will deliver reliable effects in high-throughput OOH environments. 

Practical implementation examples from norms and behavioural interventions include:

• Increasing the prominence of smaller portions
• Using a variety of language to promote smaller portions to appeal to different audiences
• Re-framing smaller portions as the norm using visual cues to set expectations.
• Pairing smaller portions with proportionately lower prices where appropriate, so that smaller options do not appear to offer worse value for money.

Across the included evidence, information-only effects were variable and typically appeared more plausible when paired with changes to what is offered. Much of the evidence in this category comes from menu-based or simulated ordering studies. Evidence from rapid evidence synthesis in the OOH context suggests calorie labelling can reduce calories ordered or consumed at population level, but average effects are modest and heterogeneity is substantial. Where information is provided in isolation, it relies on consumers noticing, understanding, and acting on it in the moment, which may be less likely in routine and time-pressured OOH decisions. This limits confidence that information-only approaches will have consistent impact in real-world OOH environments without accompanying structural changes.

For Scotland, a key point is that information provision often takes place in OOH settings where “on the go” choices are common and are shaped by price, promotions, and time constraints. In these contexts, pairing information with structural changes is likely to be more effective than information alone. In practice, this could mean using information strategies to support and legitimise portion reductions, for example by reinforcing that a smaller portion is a normal standard option, while ensuring that pricing and menu architecture do not undermine the message.

Practical implementation examples reflected in this evidence include:

• Pairing information with direct interventions to enhance impact.