The World Health Organization (WHO) describes obesity in children, adolescents, and adults as a serious global challenge for public health in the 21st century. It is a significant determinant of disability and death in the WHO European Region, where almost 60% of adults, one in four adolescents and one in three school-aged children live with overweight or obesity (1). Several medical conditions are associated with obesity, including cancer, cardiovascular diseases, diabetes, musculoskeletal complications, and mental health problems (1). Overweight in children and adolescents not only affects their immediate health but is associated with a higher risk and earlier onset of various non-communicable diseases such as cardiovascular disease and type 2 diabetes. It can also have adverse psychosocial consequences, such as discrimination and bullying (2).

Multicomponent behavioral change interventions, offered as one-to-one sessions or in a group, are the standard treatment of overweight and obesity (3, 4). These interventions include diet, physical activity, and behavior change elements (1). Drug therapy can be considered as a complement to lifestyle therapies if the body mass index (BMI) is above 30 kg/m² or the patient has comorbidities. Bariatric surgery can be an option for adults with a BMI above 40 kg/m² or between 35 and 40 kg/m² with comorbidities (4). For children and adolescents, such multicomponent behavioral change interventions should be family-oriented and multidisciplinary (3).

Often, the providers of such (group) programs face difficulties reaching their target groups and motivating them to participate. These recruiting difficulties have been described for several population groups, e.g., children and their families (5), and young adults (6). Socioeconomically disadvantaged and ethnic minority groups have a higher risk for obesity and the associated morbidity and mortality, but may experience various barriers in accessing specific obesity management health services (1, 7).

Therefore, this article aimed to systematically review the scientific literature on strategies for recruiting children, adolescents, and adults with overweight or obesity and motivating them to take part in multicomponent group programs for overweight and obesity, including nutrition, exercise, and behavioral therapy elements. A particular focus was on reaching socioeconomically disadvantaged people. The information on successful recruitment strategies can be used to develop new or adapt existing programs so that they can reach their target groups more effectively.

This systematic review was conducted as part of a larger project on multicomponent group programs for children, adolescents, and adults with overweight or obesity. The project protocol was published on the Open Science Framework (OSF) website (8). The systematic review was conducted following most of the items of the PRISMA 2020 Checklist for systematic reviews (9).

Multicomponent lifestyle interventions are recommended for children, adolescents, and adults who are overweight or obese. However, program providers often face challenges in recruiting and motivating target groups to participate in such a program. This systematic review identified different strategies that can be used to improve the recruitment of children, adolescents, and adults with overweight or obesity to multicomponent group programs. Based on a systematic literature search, 16 studies were included and analyzed narratively. Recruitment strategies were categorized into active and passive methods. The information on how many of the people approached took part in the program (i.e., recruitment rates) was very heterogeneous, and the studies sometimes came to contradictory results, e.g., as to whether passive (e.g., via the media) or active (e.g., by referral) recruitment methods are more successful. Consequently, a combination of active and passive methods was often used or recommended in the included studies. The identified recruiting strategies did not differ significantly between children, adolescents, and adults. One reason could be that parents and caregivers need to be involved in programs for children and adolescents, and the strategies are therefore also directed at them. For socioeconomically disadvantaged groups, some targeted strategies were identified, e.g., recruitment in specific locations or through trained peers. Several possible barriers to recruitment were mentioned in the studies included, ranging from barriers on a societal level (e.g., stigmatization) to barriers at the level of healthcare professionals (e.g., lack of time and resources, insufficient counselling skills) and the participant level (e.g., organizational barriers, lack of motivation).

The interpretation of the results of this systematic review must be performed against the background of some limitations of the included literature. The assessment of the quality of the included studies using the QuADS tool showed that all studies had limitations in certain areas, e.g., insufficient information on the selection of the study sample or data collection and insufficient addressing of the limitations of the studies. Generally, the methods used for data collection and synthesis were often not described in detail. This is probably because a large proportion of the included studies—except for the two RCTs and the four qualitative studies—present the description of recruitment strategies for other studies (e.g., RCTs), rather than the use and success of strategies being investigated or compared with each other in a suitable study design. It remains unclear whether the results are transferable to program recruitment (i.e., independent of studies or evaluations) or whether different mechanisms or barriers would be relevant. In addition, the recruitment results were reported very heterogeneously in the studies and could therefore not be summarized. Furthermore, ten of the 16 studies were conducted in the US, which could influence the transferability of the results to the context of other countries. The barriers to recruitment were the focus of some qualitative studies, which were systematically surveyed and clustered in these studies (using interviews or focus groups). In the other studies, barriers were only reported narratively (sometimes as part of the discussion), and how they were collected was not described. Overall, the results on the recruitment strategies and the identified barriers should therefore not be seen as reliable, robust evidence, but primarily as a selection of different options that were supplemented with indications of which strategies could work in which areas for which target groups.

Furthermore, some identified recruiting strategies should be critically analyzed from an ethical perspective. For example, two studies on children and adolescents (14, 19) reported good recruitment rates using opt-out methods. In this case, potential participants must contact the program team (e.g., by telephone) within a specified time period if they do not wish to receive information about a study or program; otherwise, they are automatically contacted with further information. From a behavioral economics perspective, this strategy works because people generally tend to choose the option requiring no action (19). Even if the aim here is not to include people directly in a study or program unless they choose the opt-out option, but merely to contact them with information about a program, the question still arises from an ethical perspective whether this approach of providing information without being asked is justifiable.

Besides, as the fear of stigma has been mentioned as an important barrier to participation in overweight and obesity group programs in some of the included studies, the aspect of stigmatization and discrimination needs to be considered within these programs. Often, people who are overweight or obese experience stigmatization and discrimination, also within the healthcare system. Potential consequences of stigmatization in patients with obesity include, e.g., an increase in eating disorders, a decrease in physical activity, further weight gain, increased risk for depression and suicidality, and avoidance of medical consultation (27). Weight-neutral approaches focus on promoting health, improving physical and psychosocial well-being and quality of life. They aim to reduce weight-related stigma (28) and can therefore be considered to address this barrier.

To our knowledge, this is the first systematic review that summarizes recruitment strategies for both children and adults with overweight or grade one obesity to multicomponent obesity group programs. Other reviews on recruiting had other or broader inclusion criteria regarding the intervention and/or target groups (e.g., nutrition and physical activity interventions for all adults, not focusing on overweight/obesity) (29–31), or analyzed barriers and facilitators for children only (32, 33). Overall, these reviews resulted in similar conclusions to our review. For example, the systematic review of Lam et al. (30) regarding recruiting young adults to lifestyle interventions not limited to overweight/obesity programs showed that most studies used two or more strategies for recruiting participants rather than limiting to one strategy. Passive strategies were more common, including print advertising, announcements, mass mailings, and emails, which were the top three methods. Moreover, Guagliano et al. (31) suggested a multifaceted recruitment approach to provide potential participants with repeated exposure to information on the intervention. A systematic review from Clayton et al. (33) on barriers and facilitators of children's participation in nutrition, physical activity, and obesity interventions found time constraints, transportation and lack of childcare to be the most common reported barriers. The authors concluded that facilitating factors involve offering compensation or incentives, referrals by healthcare professionals, support from staff or other parents, and using bilingual staff to recruit participants in some studies (33). Personal and program logistics were also identified as an important recruiting barrier in another systematic review that analyzed barriers and facilitators to initial and continued attendance at community-based lifestyle programs. Further barriers included parental denial of their child's health problem as well as the stigma related to overweight and obesity (32). Generally, recruitment reporting and the effectiveness of different methods should be improved (29, 31).

Despite the unique feature of this systematic review, the results of the review should be viewed in the context of its limitations. Firstly, the review focused on specific interventions, namely, multicomponent overweight and obesity group programs, covering nutrition, physical activity, and behavioral therapy elements. For this reason, only studies reporting on recruitment strategies focusing on these programs were included. Studies on programs in other settings (e.g., individual, inpatient, workplace, school) were omitted, although these studies could also provide valuable information regarding the recruitment of target groups. In addition, behavioral change interventions remain valuable for individuals with higher grades of obesity, particularly as adjunctive treatment and for weight maintenance following more intensive interventions. This represents an important area for future research beyond our current review's scope. Secondly, despite our initial intention to focus on recruitment strategies targeting socioeconomically disadvantaged populations, as stated in the introduction, our systematic review yielded insufficient data specifically addressing this demographic. This represents an important gap in the current literature and highlights the need for future research to explicitly examine effective recruitment strategies for socioeconomically disadvantaged individuals in multicomponent group programs for overweight and obesity. Thirdly, the limited reporting of facilitators compared to barriers in the reviewed literature represents a significant research gap. In addition, quantitative comparison of recruitment success across strategies was hindered by inconsistent reporting in primary studies, with varied definitions of recruitment outcomes and limited information on eligible populations. Moreover, another limitation is the lack of comparative effectiveness data in the included studies, which prevented us from determining which recruitment strategies work best for specific populations or contexts. Future research should evaluate the comparative effectiveness of different recruitment approaches to provide evidence-based guidance for practitioners. Finally, we used the QuADS tool to assess the methodological quality of the included studies because we aimed to consider various study designs (quantitative, qualitative, mixed-methods) and preferred to use the same tool for all studies. However, the QUADS tool is not designed to determine a final score or categorization, making it more challenging to account for methodological limitations. In the summary table, no distinction was made regarding the type of study from which the information was extracted, except for the two RCTs.

This systematic review summarizes methods and strategies for recruiting children, adolescents, and adults with overweight and obesity into multicomponent group programs. It is generally recommended to combine passive and active strategies, e.g., referral by health professionals and advertisements in the media, to reach the respective target groups. Due to some limitations of the included literature, the results of this systematic review are not robust evidence with “guaranteed success” but should be understood as an overview of valuable options that can be applied and considered by program providers when designing or adapting programs, depending on the framework conditions and target groups.