Background

Advertising of foods and non-alcoholic beverages, (hereafter food advertising), particularly for items high in fat, salt and/or sugar (HFSS), has been identified as a factor contributing to obesity and associated non-communicable diseases globally [1]. People from more deprived backgrounds or ethnic minority groups are disproportionately targeted and exposed to greater food marketing across a range of platforms [2], and this may contribute to social gradients in obesity and associated health inequalities [3]. Marketing is defined by the American Marketing Association (AMA) as “the activity, set of institutions, and processes for creating, communicating, delivering, and exchanging offerings that have value for customers, clients, partners, and society at large” [4], and advertising is a key aspect of marketing, which seeks to “inform and/or persuade members of a particular target market or audience regarding their products, services, organizations or ideas” [5]. The World Health Organization (WHO) assert that the impact that food marketing has on consumer behaviour is dependent on both ‘exposure’ and ‘power’ [6]. Exposure is the frequency and reach of the marketing messages and power is the creative content and strategies used, both of which determine the effectiveness of marketing [6]. Hierarchy of effects models of food marketing consider that the pathways for these effects are likely to be complex [7], with evidence demonstrating that food marketing impacts food purchasing [8], purchase requests [9], consumption [10, 11] and obesity prevalence [12].

Evidence suggests that children are likely to be more vulnerable to marketing messages than adults [13,14,15]. Furthermore, it has been proposed that the scepticism towards advertising that is developed in adolescence does not equate to protection against its effects [16], leaving both young children and older adolescents vulnerable to the effects of food marketing [17]. For this reason, policies enacted generally aim to decrease the exposure or power of food marketing to children, and so this is where much of the research is focused. Despite this, it is apparent that adults are similarly affected by food marketing [18], and therefore also likely to benefit from restrictions [19].

In 2010, WHO called on countries to limit the marketing of unhealthy foods, specifically to children [6]. Various policies have since attempted to enforce restrictions on HFSS advertisements [20], however, restrictions outdoors remain scarce [21] and implementation and observation of such restrictions has been found to be inconsistent and problematic [22].

Previous reviews have collated the evidence on the exposure, power and impact of food advertising on television [23,24,25], advergames [26, 27], sports sponsorship [28, 29] and food packaging [30, 31] and in some cases across a range of mediums [2, 32]. An existing scoping review [33] documents the policies in place globally to target outdoor food marketing, and the facilitators and barriers involved in implementing these policies. The lack of effective policies for outdoor food marketing may reflect the comparatively little evidence or synthesis of evidence on outdoor marketing or its potential role in contributing to overweight and obesity, relative to that for other media. Additionally, there are challenges in measuring outdoor marketing exposure compared to television and online [34]. As countries such as the UK and Chile [35] move to strengthen restrictions on unhealthy food marketing via television, digital media and packaging, it is plausible that advertisements will be displaced to other media such as outdoor mediums so that brands can maintain or increase their exposure [36, 37].

Despite being a longstanding and widely used format [38] there is no agreed definition for outdoor food marketing. This may have implications for the comparability of data across study designs, which has been reported as a limitation in previous reviews [11, 39]. Identifying the common criteria used to define outdoor food marketing, alongside considering best practice methodologies for outdoor marketing monitoring and impact research, are important steps to support the generation of robust, comparable evidence to underpin public health policy development.

Given that 98% of people are exposed to outdoor marketing daily [40], it is an efficient form of marketing for brands [41], and is likely successful in influencing purchase decisions through targeting potential shoppers in places the brands are sold [42]. Food marketing through media such as television and advergames have been shown to impact eating and related behaviours such as purchasing [43,44,45,46], and the evidence on this marketing and body weight has satisfied the Bradford Hill Criteria [47], which is used to recognise a causal relationship between two variables. However, the impact that outdoor marketing has on eating related outcomes is less clear.

Therefore, this scoping review aims to (i) identify common criteria used to define outdoor food marketing, (ii) summarise research methodologies used, (iii) identify available evidence on the exposure, power (i.e. persuasive creative strategies within marketing) and impact of outdoor food marketing on behaviour and health with consideration of any observed differences by equity characteristics such as socioeconomic position and (iv) identify knowledge gaps and directions for future research.

Methods

Approach

Given the broad objectives, a scoping review [48] was conducted and reported in accordance with the Joanna Briggs Institute (JBI) methodology for scoping reviews [49] and the Preferred Reporting Items for Systematic reviews and Meta-Analyses extension for scoping reviews (PRISMA-ScR) [50]. The review was pre-registered on the Open Science Framework (https://osf.io/wezug).

Search strategy

A detailed search strategy was created by the research team (see supplementary material 1), which included an experienced information specialist (M.Ma), to capture both published and unpublished studies and grey literature. Search terms related to food, outdoor and marketing were developed based on titles and abstracts of key studies (identified from preliminary scoping searches) and index terms used to describe articles. For grey literature sources simple terms “outdoor food marketing” and “outdoor food advertising” were used. Searches were conducted between 21st January and 10th February 2021.

Databases searched for academic literature included Medline (Ovid), Scopus, Science Direct, Proquest, PsycINFO, CINAHL, the Cochrane Database of Systematic Reviews and the Cochrane Central Register of Controlled Trials. An additional supplementary PubMed search was conducted to ensure journals and manuscripts in PubMed Central and the NCBI bookshelf were captured. Grey literature searches were conducted of databases Open Access Theses and Dissertations, OpenGrey, UK Health Forum, WHO and Public Health England. Other searches for grey literature included government websites (GOV.uk), regulatory and industry body websites (World Advertising Research Centre Database, Advertising Standards Authority) and NGO sites (Obesity Health Alliance, Sustain).

Eligibility criteria

Primary quantitative studies assessing marketing of food and non-alcoholic beverage brands or products encountered outdoors in terms of exposure, power or impact were considered for inclusion. We defined both marketing and advertising as per the AMA definitions [4, 5].

Examples of outdoor marketing included billboards, posters, street furniture and public transport. Exposure was defined as the volume of advertising identified, with consideration of the brands and products promoted. Power of outdoor marketing was defined as the strategies used to promote products (e.g. promotions, characters) [51]. Eligible behavioural impacts of outdoor marketing were food preference, choice, purchase, intended purchase, purchase requests and consumption. Health-related impacts were body weight and prevalence of obesity or non-communicable diseases. Non-behavioural outcomes were ineligible, e.g., brand recall, awareness, or attitudes.

Studies in which outdoor marketing could not be clearly isolated from other marketing forms [46], or food could not be isolated from other marketed products (e.g. alcohol and tobacco) were excluded. Studies of health promotion (e.g., public health campaigns) were ineligible. Qualitative studies and reviews were not eligible for inclusion; however, reference lists of relevant reviews were searched.

Selection of sources of evidence

The full screening process is shown as a PRISMA flow diagram (Fig. 1). Titles and abstracts were screened by one researcher (AF). Full text review was conducted independently by two researchers from a pool of four (AF, M. Mu, RE & HM). Disagreement was resolved by discussion and where necessary (n = 4 articles) a third reviewer (EB) was consulted. Covidence systematic review software was used to organise the screening of studies. Inter-rater reliability for the full-text screening was high, with estimated agreement of 95.7% and a Kappa score of 0.91.

Fig. 1
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PRISMA flow diagram

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Data charting

The extraction template was developed and piloted prior to data extraction. For more detail on the information extracted from each article see supplementary material 2. Discrepancies in extraction were resolved by discussion. As the aim was to characterise and map existing literature and not systematically review its quality, as is common in scoping reviews [52] quality assessment (e.g. risk of bias) was not undertaken.

Synthesis of results

Studies that defined outdoor food marketing were grouped to identify common criteria used in definitions. Methodologies used to measure exposure, power and impact are summarised. Studies were grouped into exposure, power and impact for synthesis, with relevant sub-categories to document findings related to equity characteristics. We deemed foods classed as “non-core”, “discretionary”, “unhealthy”, “less healthy”, “junk”, “HFSS”, “processed”, “ultra-processed”, “occasional”, “do not sell”, “poorest choice for health”, “less healthful”, “ineligible to be advertised”, and “not permitted” as unhealthy.

Results

Study selection

After removal of duplicates from an initial 4177 records, 3093 records were screened. Ninety-eight articles were then full-text reviewed. Fifty-four studies were excluded here (supplementary material 3). After grey literature and citation searches, the final number of included studies was 53.

Characteristics of included studies

All studies (n = 53) measured exposure to outdoor food marketing, n = 12 also measured power of outdoor food marketing, and n = 3 measured impact. N = 15 studies provided at least one criterion through which outdoor food marketing was defined, beyond stating the media explored.

Studies were conducted across twenty-one countries, the majority took place in the USA (n = 16) [53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68], and other high-income countries (n = 23) as categorised by the world bank [69] (Tables 1, 2 and 3).

Table 1 Studies measuring exposure and impact of outdoor food marketing
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Table 2 Studies measuring exposure and power of outdoor food marketing
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Table 3 Studies measuring exposure to outdoor food marketing
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Of studies including participants (n = 7), three measured exposure of children between the ages of 10 and 14 [89, 93, 102], two surveyed caregivers of children aged 3–5 [70] (79.2% mothers) and 0–2 years [85] (100% mothers) and two studies collected data from adults in select census tracts [53, 71].

What common criteria are used to define outdoor food marketing?

As shown in Tables 1, 2 and 3, the majority of studies (n = 33) encompassed a combination of outdoor media [53,54,55, 58, 60,61,62, 65,66,67,68, 72, 73, 75, 76, 78,79,80, 80,81,82,83,84, 87,88,89, 91, 92, 94, 97, 98, 102,103,104]. Many (n = 11) focused on advertising solely on public transport property [64, 70, 71, 86, 90, 93, 95, 99,100,101, 106], five studies focused exclusively on billboards [63, 74, 77, 85, 96] and four measured advertising outside stores or food outlets [56, 57, 59, 105].

Outdoor food marketing was inconsistently defined across studies. All studies stated the media they were measuring and some defined marketing or advertising generally, but often not how it related to the outdoor environment. Studies that provided specific criteria for outdoor food marketing (n = 15) or an equivalent term (i.e. outdoor food advertising) beyond simply stating the media recorded are listed in Table 4. Figure 2 represents the criteria referred to most frequently when defining outdoor food marketing.

Table 4 Outdoor food marketing definitions
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Fig. 2
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Common criteria used to define outdoor food marketing

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What methods are used to document outdoor food marketing exposure, power, and impact?

Most included studies (n = 49) were cross-sectional, although four were longitudinal [68, 90, 95, 100]. The methods used to classify foods were inconsistent, for example, often local nutrient profiling models were used to classify advertised products as healthy or not healthy (e.g. [80]), however in some cases the number of advertisements for specific food groups were tallied (e.g. [56]). Forty-two studies assessed the frequency of food advertising through researcher visits to locations. In four [82, 83, 86, 93] studies, researchers visited streets virtually, through Google Street view. Real, rather than potential exposure was measured in three studies [89, 93, 102]. In two cases, children wore cameras which documented advertisements encountered in their typical day [89, 102], and in a final study, children wore a global positioning system (GPS) device so researchers could track when they encountered previously identified advertisements [93]. Self-reported retrospective exposure (frequency of encountering outdoor food advertising) was measured in three studies [70, 71, 85].

When measuring advertising around schools/places children gather, researchers typically created buffer zones, ranging from 100 m [81] to 2 km [105], with 500 m being the most frequent buffer size (n = 8) [77, 78, 86,87,88, 95, 97, 104]. Four studies used multiple buffers [82, 87, 93, 97], allowing for comparison between the area directly surrounding a school (e.g. < 250 m) with an area further away (e.g. 250-500 m) [87], one study compared advertising in Mass Transit Railway stations in school and non-school zones [90], and another used GPS point patterns to determine the extent of advertising around schools [92].

Content analysis was used to characterise the food types promoted and strategies used in the advertising. Two studies investigated price promotions [55, 56], two identified promotional characters and premium offers [75, 78], one specifically assessed child-directed marketing [57]. Others examined a mix of strategies including sports or health references, cultural relevance and emotional, value or taste appeals [54, 73, 74, 76, 79].

All three studies measuring the impact of outdoor food advertising used self-reported data. In a study conducted in Indonesia [70], caregivers reported the frequency of food advertising exposures in the past week, and their children’s frequency of intake of various confectionaries at home in the last week. In a study conducted in the US [53], individuals reported consumption of 12 oz. sodas in the last 24 hours, and odds of exposure was assessed by the extent of advertising in surrounding areas. In the UK study [71], participants reported exposure to HFSS advertising in the past week, and body mass index was calculated from participants’ reported height and weight.

What is known about exposure to outdoor food marketing?

Content of food marketing

Fifty-three studies investigated outdoor marketing exposure (Tables 1, 2 and 3), n = 22 reported specifically on exposure around schools or places children gather, n = 9 documented exposure on public transport and n = 4 outside stores/establishments. The remaining n = 21 measured exposure across multiple settings.

Food products were promoted in between 7.8% [64] and 57% [91] of advertisements, the mean across studies was 22.1%. Of food advertisements, a majority (~ 63%: range 39.3% [64] - 89.2% [89]) were categorised as unhealthy. Healthier foods were advertised far less, with studies generally reporting between 1.8% [97] and 18.8% [91] of food advertisements being for healthier products. Fast food (n = 17) and sugar-sweetened beverages (SSBs; n = 22) were frequently n amed as some of the most advertised product types.

Coca Cola was frequently stated as the most prominent brand advertised [73, 75, 88, 91, 100]. Around 5% of all outdoor food advertisements in New Zealand [78] and Australia [100] were promoting a brand (rather than a specific product), however there were no brand only advertisements identified in a UK study [80].

Marketing to children

Over half of the studies included (n = 29) sought to examine children’s exposure to food advertising. One UK study [93] concluded that while it was unlikely that unhealthy products were advertised on bus shelters surrounding schools (100-800 m), children, particularly in urban areas, were likely to encounter advertising on their journeys to and from school. All other studies found food advertising to be prevalent around schools, often promoting unhealthy products, although in three studies, a minority of schools (20.4% [78], 15.4% [79], 33.3% [103]) did not have any food advertising nearby. Four studies found that there was more food advertising closer to schools or facilities used by children and adolescents, compared to areas further away from these facilities [87, 97], specifically for unhealthy or processed foods [87, 90] and snack foods [82], however one study found SSB advertisements increased as distance from schools increased [92].

Differences by socioeconomic position/ethnicity

Eight studies considered differences in exposure by ethnicity. Three of these found that ethnic minority groups were exposed to more food advertising [55, 58, 62], for example, schools in the US with a majority Hispanic population were found to have more total advertisements and establishment advertisements in surrounding areas [55], whilst in New York City, for every 10% increase in proportion of Black residents there was a 6% increase in food images and 18% increase in non-alcoholic beverage images [58]. In addition to this, associations were found between sugary drink advertisement density and Percentage of Asian or Pacific Islander residents and percentage of Black, non-Latino residents [62]. Two studies found that multicultural neighbourhoods had a higher proportion of food advertisements [94] and higher density of unhealthy beverage advertisements [61]. Unhealthy food [63] and beverage [61] advertising were found to be more prevalent in ethnic minority communities. Low-income communities with majority Black or Latino residents had greater odds of having any food advertising [53], generally more food and beverage advertising and greater unhealthy food space [61] compared to white counterparts. A US study [56] found that differences in exposure to food and beverage, and soda advertisements by ethnicity were no longer significant after controlling for household income.

Twenty-six studies considered differences in exposure by SES, five of these did not find a relationship [60, 71, 83, 93, 99]. Two studies showed that food and beverage advertisements were more prevalent in low SES communities [56, 94]. Schools characterised by low SES had a higher proportion unhealthy food advertising nearby in two studies [78, 104], although in one instance there was no significant difference in the number of unhealthy food advertisements [78]. One study conducted in Sweden [84] found no significant difference in the proportion of food advertisements by SES, however there was a significantly greater proportion of advertisements promoting ultra-processed foods in the more deprived region.

Foods more frequently advertised in low SES areas were: SSBs, hamburgers and kebabs, diet soft drinks, vegetable snacks, dairy with no added sugar [103], staple foods [91], flavoured milk and fruit juice [101]. Low income communities in the US had lower odds of fruit and vegetable advertisements at limited service stores [56] and a higher density of unhealthy beverages [61, 62] compared with higher-income communities.

Two studies found no significant difference in the number of core advertisements by SES [98, 100], however a study of outdoor food advertising in Uganda found that there were more healthy food advertisements in high income areas [75]. Advertisements for fast food, takeaways, hot beverages and soft drinks were found to be more frequent in high-income areas [91, 96, 101].

A study comparing four schools of varying deprivation found the school with lowest deprivation had no advertisements but there was no clear trend in extent of advertising by deprivation [105]. Two studies conducted in Mexico [81] and New Zealand [86] found outdoor food advertising to be more frequent around public schools than private schools, however a study in Uganda [75] found no significant difference in the number of core foods advertised around private and government funded schools, in all three studies private school was considered a proxy of high SES. In this New Zealand study, low decile areas had the greatest number of advertisements for non-core food, core food and non-core food and beverage, however when high decile schools were combined with areas around private schools, the greatest number of all food and beverage advertisements and non-core advertisements were found in high SES areas [86].

What is known about the power of outdoor food marketing?

Twelve studies documented the power of outdoor food marketing (Table 2). This was measured by quantifying the use of a range of persuasive creative strategies and child-directed marketing. The persuasive creative strategies observed across studies are shown in Fig. 3.

Fig. 3
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Powerful creative strategies observed in studies

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Observed power

There was evidence of variation in the use of persuasive creative strategies in outdoor advertising, with premium offers (e.g. buy one get one free [78]) utilised in between 7.84% [74] and 28.1% [78] of food advertisements, and the proportion of advertisements featuring a person or promotional character ranging from 2.8% [79] to 46.8% [73]. Other strategies frequently identified were appeals related to price [55, 56, 72, 74, 76], emotion [72, 74, 76, 77] and taste [72, 74, 76, 77].

The proportion of advertisements considered to be targeted just at children or young people ranged from less than 1% [58] to 10.4% [73]. Studies assessing appeals to children considered the use of cartoon characters, popular figures, child models or characters, colours or images, toys and the placement of the advertisement [58, 64, 73, 83].

Often, the foods promoted using persuasive creative strategies were soft drinks [73, 76], non-core foods [77] and fast foods [57, 76], however one study [75] observed outdoor food advertising in Uganda and found that 15% of healthy food advertisements used promotional characters.

Differences by socioeconomic status/ethnicity

A US study [55] found that schools with a majority Hispanic population (vs. low Hispanic population) had significantly more advertisements featuring price promotions within half a mile of the school. Price promotions were also more frequent outside supermarkets in non-Hispanic Black communities in the US [56], although this was no longer significant after controlling for household income. Supermarkets in low-income communities were significantly more likely to have price promotions [56] and being located in middle-income (compared to high) and black communities was marginally associated with increased odds of child-directed marketing [57]. Sometimes, local culture was referenced in food advertising through persuasive creative strategies [54, 76, 77], for example a US study quantifying advertisements in a Chinese-American Neighbourhood [54] found food advertisements were frequently relevant to Chinese culture (58.9% of food and 59.04% of non-alcoholic beverage advertisements), often featuring Asian models.

What is known about the impact of outdoor food marketing?

Three studies (Table 1) [53, 70, 71] explored associations between exposure to outdoor food advertising and behavioural or health outcomes, two of these found a significant positive relationship. Lesser et al. (2013) [53] found that for every 10% increase in outdoor food advertisements present, residents consumed on average 6% more soda, and had 5% higher odds of living with obesity. In Indonesia [70] self-reported exposure to food advertising on public transport was associated with consumption of two specific HFSS products. No associations were found between exposure and consumption of the other eight products considered. A UK study [71] found no significant association between self-reported exposure to HFSS advertising across transport networks and weight status. No studies measured differences in impact in relation to equity characteristics.

Discussion

Summary of main results

This review is the first to collate the criteria used to define outdoor food marketing, document the methods used to measure this form of marketing, and identify what is known about its exposure, power and impact.

Fifty-three studies were identified which met all eligibility criteria. In brief, of studies with a definition, the criteria referenced most were; on or outside stores/establishments; and stationary signs/objects. The methods used to research outdoor marketing include self-report data, virtual auditing, in-person auditing, and content analysis. There was little consistency in the approach used to classify foods as healthy or unhealthy, although nutrient profiling models were used in some studies.

Food accounted for an average of 22.1% of all advertisements, the majority of foods advertised were classed as unhealthy (63%). Ethnic minority groups were generally shown to have higher exposure to outdoor food advertising, but findings on differential exposure by SES were inconsistent.

Studies showed frequent use of premium offers, promotional characters, health claims, taste appeals and emotional appeals in outdoor food advertisements. There was limited evidence of relationships between exposure to food marketing and behavioural or health outcomes.

Common criteria used to define outdoor food marketing

Eight out of fifteen studies (Fig. 2) stated that outdoor food marketing must be on or outside of stores or establishments, seven studies included stationary signs or objects in their definition and five studies stated that advertisements must be visible from the street or sidewalk. However, the defining criteria was inconsistent across the fifteen studies, and some of the most referenced criteria are problematic. Although stationary signage is an important aspect of outdoor marketing, this excludes forms of marketing on transport e.g. the exterior of buses. Equally, not all outdoor marketing may be “visible from the street or sidewalk”, this could exclude advertising on public transport property, i.e. station platforms. Additionally, the share of digital out-of-home advertising rose from 14% in 2011 to 59% in 2020 [107]. Three studies did aim to document digital advertising [60, 63, 90] through observing a digital board for a set amount of time. This medium is likely to become more prevalent over time globally, and there are challenges due to its changing and interactive nature [108]. The literature appears dominated by studies of advertising. This may reflect that most marketing encountered outdoors is advertising, conversely, it may be that the literature is yet to consider some newer forms of marketing, such as increased digital platforms. It will be important for future research to consider the evolving nature of outdoor marketing and how this should be measured.

Only fifteen studies defined outdoor food marketing as a term. This has likely been a factor influencing the heterogeneity observed across studies (e.g. differences in scope), as inconsistencies in defining a factor can negatively impact the development of an evidence base [109]. Researchers should endeavour to work towards an agreed definition, perhaps through use of the Delphi method of consensus development [110], in order to improve consistency in the resulting research. However, this method can be open to bias if the researchers are of the same background as the experts involved [109] therefore it is important that any definition developed aligns with criteria used by industry to reduce likelihood of bias.

Methods used in outdoor food marketing research

Outdoor food marketing exposure and impact were measured using self-reported data, which may lack validity, as advertising can influence brand attitudes whether consciously or unconsciously processed [111]. While it can be useful to know the extent that individuals process advertising, this may not be a true representation of exposure. Equally, participants may alter their response to appear socially desirable which has previously resulted in misreporting of height and weight data [112].

Using Google Street View as an auditing tool is beneficial in saving time and resources whilst gathering large samples [113], however almost one third of advertisements in one study were unable to be identified [83], therefore systematically searching the streets in sample areas, and taking photographs for later reference is a more reliable method. Buffer areas are a useful tool for measuring advertising, particularly around specific sites such as schools, although stating advertising was present “around schools” has different meanings when comparing 100 m to 500 m, or to 2 km. GPS and wearable camera technology can identify how individuals encounter food marketing in the routes they use to travel through their environment. These methods should be replicated globally as a more objective measure of individual exposure to outdoor food marketing, although care must be taken in regard to privacy and ethical considerations.

There was little consistency in the methods used to identify persuasive creative strategies, which is typical in the field of food marketing [23]. The heterogeneity observed could be reduced through adherence to protocols for the monitoring of food marketing such as those developed by WHO [51] and INFORMAS [114]. This would improve comparability of future outdoor food marketing data across countries and time points which would better support policy action in this area. Nutrient profiling models are a useful tool for food categorisation, as opposed to grouping foods as “everyday” and “discretionary” or “core” and “non-core”, however, profiling models differ due to cultural differences in diet [106, 115]. There is a need to balance the data required for country-level policy relevance with international comparability. Watson et al. (2021) [106] propose an amalgamation of the WHO EURO NPM and WHO Western Pacific models.

Exposure to outdoor food marketing

Marketing platforms outdoors remain accessible for the food industry and are relatively unrestricted. This is reflected by the extent of advertised food products (22.1%) and the proportion of those that were unhealthy (63%), which is problematic as discrepancies between the food types frequently promoted and dietary recommendations have been linked to changes in dietary norms and food preferences [111]. Whilst fruits and vegetables should make up 40% of daily intake [116], these products were rarely promoted. These findings are comparable to global data of other marketing formats, for example, a benchmarking study found that on average, 23% of advertisements on TV were for foods or beverages, and other studies have found 60–70% of food advertisements to be unhealthy across social media [117,118,119] and in print [120].

This knowledge adds to the existing evidence reporting the extent of children’s exposure through multiple forms of marketing [2, 13, 121]. Whilst efforts are being made to restrict their advertising exposure through other sources such as TV, for consistency, more must be done to protect children in the outdoor environment.

There is no consensus on clear trends in exposure by SES. In part, contradictory findings within this review, such as targeting of wealthier consumers, may reflect the occurrence of a nutrition transition occurring in low income countries, characterised by increased reliance on processed foods [122] which are more available to those with more disposable income. Further research should attempt to develop clear consensus on the differential exposure to outdoor food marketing by SES in both high- and lower-income countries.

Power of outdoor food marketing

The lack of research into the power of outdoor food marketing is most likely a result of the lack of established definitions and classifications for the powerful characteristics of marketing and in particular, child appeal of marketing [123]. The most frequent persuasive creative strategies identified across the twelve studies documenting power were premium offers, promotional characters, health claims, taste appeals and emotional appeals, similar to those identified in television food marketing [23]. These strategies are particularly salient to children: spokes-characters can be effective in influencing children’s food choice, preference, awareness and attention [124], whilst premium offers (e.g. collectible toys) can influence children’s likeability and anticipated taste of the promoted food [125] and can prompt choice of healthier meals [125, 126]. One study found that children were more likely to choose unhealthy food products if they featured nutrient content claims such as “reduced fat, source of calcium” [127]. In this study participants were exposed to unknown brands, it is anticipated that larger responses would be present in brands recognised by participants. Future research should attempt to determine the success of different strategies in influencing behaviour, particularly as the rise in digital media used outdoors may increase the potential for power through increasing the variation and sophistication of outdoor marketing techniques. Policy in this field is largely focused on advertising directed at children, although it is important for research and policy to reflect that due to persuasive creative strategies used, advertising not wholly directed at children can still appeal to them [83].

Impact of outdoor food marketing

There is evidence that outdoor advertising exposure is related to consumption of SSBs and odds of obesity, Previous reviews on the impact of food marketing on television and digital media have found compelling evidence of a relationship between exposure and food intake [44], attitudes and preferences [24]. However, the small number of studies measuring impacts of outdoor food marketing in this review were correlational and therefore cannot demonstrate causality. This lack of evidence is likely preventing policy progress in this area. It is likely that the lack of studies measuring impact of outdoor food marketing is due to the difficulty in controlling for confounding variables in external settings [128] or replicating this form of marketing in a lab compared to other formats such as television. It is clear that unhealthy food marketing is prevalent outdoors, but our understanding of the resultant impacts is underdeveloped and must be further examined through experimental research.

Experimental research will enable clearer understanding as to whether outdoor food marketing influences behaviour as television and digital marketing do [10, 24, 129]. Understanding the impact of outdoor food marketing on body weight would require longitudinal research, although it is difficult to separate the impact of marketing from secular trends. Additionally, there is increasing recognition that attributing a behavioural outcome to a single marketing communication can be problematic and does not appropriately reflect the cumulative effects of multiple, repeated exposures [7]. Purchase data in response to marketing campaigns could be a useful indicator of marketing impact [130], however gathering sales data from industry is problematic. This could be made possible through changes such as those proposed by the UK national food strategy, and supported by the NGO sector [131], calling for mandatory annual reporting of product sales for large food companies [132]. Although this is only proposed in the UK, if the strategy is successful in encouraging companies to make changes to formulations or the proportion of healthy products available, this strategy may be adopted elsewhere.

Strengths

The review was pre-registered, allowing for transparency in approach and reporting of results and the methodology and reporting of the review were robust and consistent with guidelines from both the PRISMA extension for scoping reviews and the JBI methodology. The systematic search strategy ensured a wide range of databases were searched and the identification of a large number of potentially relevant studies. The use of multiple independent reviewers in the full-text screening and data extraction ensured all relevant data was captured accurately.

Limitations

As this is a scoping review, non-peer reviewed sources such as letters to editors [96], conference abstracts [99] and grey literature [104] were included if they met inclusion criteria. Government websites beyond the UK were not included, which is a limitation of our searches, however multiple grey literature sources that were not UK based would have captured relevant international materials. The majority of studies included are focused on advertising and while some marketing aspects are considered, this review does not encompass all marketing communications. However, our searches were designed with thesaurus terms to capture words related to marketing that might not have been realised from a public health perspective. Therefore, it is likely that the relevant literature from marketing disciplines was identified, and is just limited.

As quality assessment was not deemed appropriate, there is potential for error and bias within the included studies, similarly, inaccuracies may arise from the self-reported data used in four studies. Although there were no limitations by language, no translation was required and all eligible studies were published in English. Further, discrepancies in the conduct and reporting of studies make it difficult to collate data and draw firm conclusions.

Conclusions

This review has documented the research on outdoor food marketing exposure, power, and impact. There is substantial heterogeneity in the criteria used to define and methods used to measure outdoor food marketing. Future research will benefit from using a consistent definition and measurement tools to allow for improved comparability between studies. Whilst all the studies documented exposure, few recorded the powerful strategies used in outdoor food marketing and it is still largely unknown how this marketing influences behaviour and ultimately health. In order to inform policy, further research will benefit from examining the causal processes through which outdoor marketing may influence behaviour and health outcomes.