Introduction

Worldwide, approximately 8.2% of the population, or an estimated 630 million people were living in extreme poverty (< US $1.90/day) in 2019 [1, 2]. An additional ~ 97 million people fell into poverty in 2020 due to the COVID-19 pandemic [3]. Social protection programs, which provide an important safety net for those living in poverty, comprise a wide variety of measures such as cash transfer (CT) programs, school feeding, public works programs, pensions, and unemployment insurance [4]. Social protection is a fundamental part of the United Nations Sustainable Development Goal 1 to end poverty in all its forms everywhere. The goal includes the implementation of nationally appropriate social protection systems and measures for all to achieve substantial coverage of the poor and the vulnerable by 2030 [1].

CT programs are direct, regular and predictable non-contributory payment of money to eligible individuals. CT programs can be either unconditional or conditional. In unconditional CT programs, cash is provided to beneficiaries without any specific obligations to fulfill. In conditional CT programs, cash is provided to beneficiaries under conditions such as health care and education. The World Bank reports that 142 countries have CT programs, of which 70% have unconditional CT programs and 43% have conditional CT programs [5]. The percent of gross national product (GNP) spent on social protection programs in low-, middle-, and high-income countries is 1.5%, 1.6%, and 1.9%, respectively [4]. The number of planned and actual cash CT beneficiaries is 1.8 and 1.5 billion people, respectively [6]. In 2020, over US $1.7 trillion were spent on social protection programs, of which CT programs accounted for 42% of programs, or about US $700 billion [6]. The amount of spending and number of recipients are increasing in the face of the COVID-19 pandemic [6].

CT programs have been shown to reduce the risk of child undernutrition in a recent systematic review and meta-analysis [7]. CT programs targeted to households with young children improved linear growth and reduced stunting in lower- and middle-income countries [7]. The double burden of malnutrition, defined as the simultaneous manifestation of both undernutrition and overweight and obesity, has been increasing worldwide [8]. Whether CT programs increase the risk of the DBM has not been well characterized, mainly because few studies have examined the impact of CT programs on the DBM. However, some studies have described the relationship between CT programs and overweight and/or obesity. Whether CT programs increase the risk of overweight and obesity in children and adults is unclear. Our specific aim was to conduct a systematic review of CT programs and the risk of overweight and obesity in children and adults.

Materials and methods

For this systematic review, we searched articles in PubMed, Embase, Cochrane, EconLit, Global Health, CINAHL Plus, IBSS, Health & Medical Collection, Scopus, Web of Science, and WHO Global Index Medicus in August 2021 using search terms as shown in Supplementary Table 1. In addition, we hand-searched reference lists of articles identified through the systematic search. CT programs were defined as those programs that provided direct, regular and predictable non-contributory payments of money to eligible individuals. The inclusion criteria for studies were: a control/comparison group was present in the study design, the total sample size was > 300, published after January 1, 1997, written in English, Spanish, or Portuguese, and from the peer-reviewed or gray literature. The exclusion criteria for studies were: pensions, cash-for-work programs, payment-in-kind programs, CT issued in temporary emergency situations or disaster relief, CT programs limited to adults with pre-existing conditions, i.e., human immunodeficiency virus infection, diabetes, etc., or with limited disbursements (< 3).

For children < 5 y, the main outcome measures were overweight (weight-for-height > 2 standard deviation [SD] above the World Health Organization [WHO] growth standards median) and obesity (weight-for-height > 3 SD above the WHO growth standards median) [9]. For children 5–19 y, the main outcome measures were overweight (body mass index [BMI]-for-age > 1 SD above the WHO growth standards median) and overweight (BMI-for-age > 2 SD above the WHO growth standards median) [9]. For adults, the main outcome measures were body weight as a continuous variable, BMI as a continuous variable, overweight (BMI ≥ 25 kg/m2) and obese (BMI ≥ 30 kg/m2) [10].

Articles from each database search were transferred into EndNote (EndNote 20, Clarivate, Philadelphia, PA, USA), and duplicates were eliminated. Unique references were uploaded into Covidence systemic review software (Veritas Health Innovation, Melbourne, Australia) for title/abstract screening, full-text screening, and finally data extraction of the included studies. Screening and data extraction were conducted by two independent reviewers (RDS, NR). The PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) flow diagram was used to summarize the methods [11].

Quality assessment was conducted using the Newcastle–Ottawa scale (NOS) for cohort studies and a modified NOS for cross-sectional studies [12]. The NOS for cohort studies involves evaluation of three factors by giving stars or no stars for: (1) selection (maximum 4 stars), based upon the representativeness of the exposed cohort, the selection of the non-exposed cohort, the ascertainment of exposure, and a demonstration that the outcome of interest was not present at the beginning of the study; (2) comparability of the exposed and nonexposed cohorts, based upon the study design or analysis controlling for confounders (maximum 2 stars); (3) outcome (maximum 3 stars), based upon the method of assessment, whether the follow-up was long enough for the outcome to occur, and the adequacy of follow-up of the cohorts. The modified NOS for cross-sectional studies involves the evaluation of three factors: (1) selection (maximum 4 stars), based upon the representativeness of the sample, the sample size, characterization of non-respondents, and ascertainment of exposure; (2) comparability (maximum 2 stars) based upon comparability of the outcome groups with controlling of confounders; (3) outcome (maximum 3 stars), based upon outcome assessment and statistical testing. The NOS gives a maximum total of 9 stars for each study. NOS assessment of each study was conducted by two independent reviewers (RDS, NR). A consensus discussion was used to reach a final agreed-upon rating for each study.

Results

The initial search of 11 databases yielded 4550 references. There were 2355 unique references after removing duplicates. Seventeen references fit the inclusion/exclusion criteria, and three additional references that fit the inclusion/exclusion were identified outside of the systematic search. A summary of the review and reasons for excluding studies are shown in the PRISMA flow chart in Fig. 1. The location, study design, methods, outcomes, and results of the twenty eligible studies are summarized in Table 1, grouped by children or adults as the target population. There were eleven studies that examined the relationship between CT programs and child overweight and/or obesity [13,14,15,16,17,18,19,20,21,22,23], eight studies that examined the relationship between CT programs and adult overweight and/or obesity [24,25,26,27,28,29,30,31], and one study that characterized the relationship between CT programs and overweight and/or obesity in both children and adults [32]. The location of most of the studies was in Latin America and the Caribbean [13,14,15,16,17,18,19, 22, 24,25,26,27,28,29,30, 32]. Of the different major CT programs, there were four studies of Bolsa Família in Brazil [13,14,15,16], four studies of Oportunidades in Mexico [22, 27, 29, 30], two studies of Familias en Acción in Colombia [17, 32], and two studies of Juntos in Peru [18, 24]. The relationship of CT programs with BMI, overweight and/or obesity, was described in studies from South Africa [21], Japan [20], Canada [31], and the United States (Alaska) [23]. Fourteen of the studies focused on conditional CT programs [13,14,15,16,17,18,19, 22, 24, 27,28,29,30, 32], and six studies described unconditional CT programs [20, 21, 23, 25, 26, 31].

Fig. 1
figure 1

PRISMA flow diagram of studies evaluating the association between CT programs and overweight and/or obesity. Abbreviation: PRISMA, Preferred Reporting Items for Systematic Reviews and Meta-Analysis

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Table 1 Summary of twenty studies reporting the relationship of CT programs with body weight, body mass index, overweight, and obesity
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The NOS assessment of the studies are shown in Table 2. Eleven studies used a cross-sectional design [13,14,15,16, 19,20,21, 24, 29, 31, 32]. Nine studies used a longitudinal cohort design [17, 18, 22, 23, 25,26,27,28, 30].

Table 2 Newcastle–Ottawa Scale assessment of studies
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Of the cross-sectional studies, five studies received 2–3 stars, four studies received 4–5 stars, and two studies received 6–7 stars. Of the cohort studies, one study received 2 stars, two studies received 4–5 stars, four studies received 6–7 stars, and two studies received 8 stars. The mean score of the twenty studies was 4.75 stars. Nearly all of the cross-sectional studies were representative of the specific population that was the focus of the study. Ascertainment of exposure was mainly self-report through interviews with participants. Few of the cross-sectional studies provided sample size and power calculations. Most of the studies did not describe the characteristics of non-respondents. Nearly all the cross-sectional studies used anthropometry to assess outcomes, but no studies reported that the study team members who conducted anthropometry were blinded to the exposure assessment. Most of the cross-sectional studies had comparable control groups and adjusted for covariates in the outcomes analyses. Most of the cohort studies were representative of the specific population that was the focus of the study and selected non-exposed controls from the same population. Ascertainment of exposure was by self-report through interviews in most studies.

Given the great deal of heterogeneity in target study populations, study design, statistical methods, CT programs, and outcomes (Table 1), a meta-analysis was not conducted. The relationship of CT programs with overweight and/or obesity as categorical outcomes in seventeen studies is summarized in Fig. 2. One study showed a significantly higher OR for obesity in women > 18 y whose household participated in the CT program in Colombia [28]. Thirteen studies showed that CT programs were significantly associated with lower overweight and/or obesity in children, adolescents, or adults compared with controls [14, 16, 18,19,20,21,22,23,24, 26, 29, 31, 32]. Eight studies showed no significant association between CT programs and overweight and/or obesity in the specific target groups that were studied [13, 15,16,17, 22, 24, 26, 31]. There were three studies that reported the outcome as a continuous variable. In a study from Mexico, the CT program was not associated with a significant change in BMI in adults > 70 y [25]. Another study reported that the CT program was associated with a significant decrease in BMI in a nationally representative sample of adults in Mexico [27]. Among women, aged 18–49 y participating in a community-randomized, controlled intervention trial in rural Mexico, those receiving CT had a significant increase in body weight, especially women who were already obese at baseline [30].

Fig. 2
figure 2

The association between CT programs and overweight and/or obesity as categorical outcomes in fourteen studies. Results that were statistically significant are shown as horizontal bars. Results showing no significant difference between CT programs and overweight or obesity are shown as a dot on the vertical line. Abbreviations: SE southeast, NE northeast. *Studies that report OR

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Discussion

The present review shows that the impact of CT programs on overweight and obesity in both children and adults is not conclusive due to the limited number of studies and mixed results regarding the direction of the association. Thirteen of the twenty studies showed that CT programs were significantly associated with a lower risk of overweight and obesity, while eight studies reported no significant associations. Only one study showed that a CT program was significantly associated with an increased risk of obesity, and the risk was found in women. Overall, the results are suggestive that CT programs either have no impact or decrease the risk of overweight and/or obesity in children, adolescents, and adults, but due to the small number and heterogeneity of studies, no firm conclusions can be drawn from the available evidence.

The CT programs included in this review varied considerably by location, recipients, cash amount, and other characteristics as shown in Supporting Table 2 [33,34,35,36,37]. Most of the conditional CT programs included the present review are located in Latin America and the Caribbean. There was only one study of the association of CT programs with overweight and obesity from Africa [21], although there are currently over forty CT programs in Africa [6]. No CT programs from Asia were identified through the systematic search in the present study.

Overall, the quality of the studies as assessed by the NOS not high, with an average score of 4.75 out of a 9 star rating. The quality evaluation of the studies using the NOS assessment revealed several weaknesses in study design that could be addressed in future research conducted with CT programs, including provision of sample size and power calculations, using measures to ensure blinding of study team members who are measuring the outcomes using anthropometry, describing the characteristics of subjects who are lost to follow-up or refuse participation, and validating the measure of exposure.

A recent meta-analysis showed that CT programs have a small but significant impact on reducing stunting by 2.1% [7]. Another recent review of experimental or quasi-experimental studies showed the CT programs increased birthweight, although the number of studies was small [38]. The double burden of malnutrition, which is the simultaneous manifestation of both undernutrition and overweight and obesity, can occur on the individual, household, and population level [8]. CT programs appear to reduce undernutrition [7, 38], and overall, the twenty studies in the present analysis do not provide strong support for the idea that CT programs increase the risk of overweight and obesity.

The present study was limited to CT programs because these programs represent a major proportion of social protection expenditures worldwide [6]. Other types of social protection, such as school feeding programs, food supplements, and cash-for-work were not included, as the programmatic implications are quite different from CT programs. The present study did not examine the pathway between the increase in household income through CT programs and body mass index or body weight. Factors in the pathway include nutrition education, growth monitoring, household spending, and food expenditures, such as spending on sugar-sweetened beverages [39].

CT programs continue to evolve worldwide, with some accelerated changes due to the COVID-19 pandemic [6]. The CT program in Mexico, most recently known as Prospera and formerly Oportunidades, was considered a model for CT programs worldwide [40]. Prospera was abruptly terminated by the government in 2019 and replaced with an unconditional CT program [41]. The removal of health and education conditions for the CT program in Mexico resulted in a large drop in attendance at health centers and layoffs of frontline healthcare workers [41]. Concerns have been raised that removal of conditions from the CT program in Mexico will adversely affect civic participation among the poor [42]. The role of health conditions of conditional CT programs and the risk of overweight and obesity is not clear and could be addressed in future studies.

Worldwide, CT programs are becoming a preferred form of food assistance, as many countries are shifting from food vouchers and food transfers to CT [43]. CT have a favorable impact upon food consumption and dietary diversity and are more cost-efficient than food-based interventions [44]. Digital payments have improved speed and transparency of CT programs and have achieved deeper financial inclusion [6]. In the age of COVID-19, digital payments have also reduced person-to-person exposure and minimized health risks [6].

Obesity and overweight are important risk factors for chronic diseases such as hypertension [45], diabetes [46], and cardiovascular disease [47]. Thus, CT programs, through their potential impact on overweight and obesity, have the potential to affect long-term health of millions of participants worldwide. CT programs are considered the most important social safety net for social protection programs [4]. Despite the large expenditure on CT programs worldwide (~ US $700 billion in 2020) [6], there are only a limited number of studies that used a randomized controlled design to examine the impact of CT on nutritional outcomes [48].

In conclusion, there were a limited number of studies that described the relationship between CT programs and the risk of overweight and obesity. Overall, CT programs appear to have either no impact or a decreased risk of overweight and/or obesity in children, adolescents, and adults. Due to the small number of studies, heterogeneity of studies, and limited quality of studies, no firm conclusions can be drawn from the available evidence. Further work is needed to characterize the relationship of CT programs with overweight and obesity using more rigorous methodology and inclusion of distal outcomes such as hypertension, diabetes mellitus, and cardiovascular disease.