7. Conclusions
Overall, the evidence base suggests that portion-related interventions in OOH settings can reduce energy purchased or consumed, particularly when they change the default offer, standardise smaller portions, or redesign meals so that lower-energy options are easier to choose and acceptable to consumers. Effects are typically modest, vary by context, and can be undermined by value-for-money offers that encourage upsizing or by compensatory purchasing or consumption later.
For Scotland’s OOH sector, the current evidence is likely to be most useful in informing guidance, piloting, and voluntary implementation rather than immediate mandatory policy, because relatively few studies assess longer-term feasibility, business impacts, and implementation under routine OOH conditions. Within that context, the most implementable intervention approaches are likely to be those that make smaller portions the default and align pricing and promotions so that smaller options retain perceived value for money.
Current evidence is insufficient to specify a single optimal level of portion reduction across OOH settings, because few studies jointly assess energy purchased, compensation, acceptability, and business outcomes over longer follow-up. Approaches to making smaller portions the default could include offering a parallel smaller option, such as a 40% smaller meal for a 40% lower price, and making modest reductions in calories by volume, for example by reducing energy-dense components while offsetting the reduction with other foods to preserve the visual size of the meal.
Pricing and promotional strategies are also likely to matter, for example ensuring that smaller portions are proportionately cheaper than standard serving sizes. Alongside implementation, Scottish evaluations should embed monitoring of energy purchases using sales data, such as total calories sold, together with customer acceptability and waste outcomes. Further Scottish evaluations in OOH settings, with longer-term follow-up, including days for individuals and weeks for businesses, and with business outcomes included, are needed to establish feasible reduction magnitudes and to minimise unintended consequences such as increased burden on vendors or increased calorie consumption through snacking.