Background
Fruits and vegetables (FVs) are essential components of a healthy diet and can reduce risk for many chronic illnesses; [
1,
2] yet, most children do not consume the recommended number of servings of these foods [
3‐
5]. This is concerning, as inadequate FV intake is linked to increased risk for cardiovascular disease, [
1,
2] and certain cancers [
6]. Children from minoritized backgrounds living in communities characterized by high poverty have the lowest FVs intake [
7]. These children often live in food deserts with limited FV affordability and access [
8,
9]. They are also the most likely to participate in the National School Lunch Program (NSLP), [
10] making the school food environment a critical target of public health efforts to enhance dietary quality [
11,
12].
A major goal of the NSLP is to increase FV intake, with a long-term aim of reducing obesity, [
12] yet it is unclear how increasing children’s FV intake relates to energy intake in the NSLP [
13]. Greater FV consumption could decrease energy intake if these foods replace higher-calorie items; however it could increase energy intake if FVs are merely added to regular meals [
13‐
16]. Understanding how FV consumption influences energy intake in school lunch is particularly important to examine in schools serving predominantly minoritized children living in communities characterized by high poverty, as this population faces disproportionately high obesity risk [
17] and is most likely to be impacted by school food policies, given its reliance on the NSLP [
10].
Salad bars are promoted as a strategy to increase students’ FV intake, variety, and choice within the NSLP [
18‐
22]. For example, the Chef Ann Foundation raised >$15.8 million and donated 6,083 salad bars to schools, with 260 schools on a waiting list to receive one [
23]. The Centers for Disease Control (CDC) notes that it supports the mission of
Salad Bars to Schools (a public-private partnership which includes the Chef Ann Foundation, the Whole Kids Foundation, and government programs) as a strategy, “to ensure every child has the choice of healthy fruits and vegetables each day at school.” [
23]. Yet, rigorous evaluation of school salad bars to determine their impact on dietary intake patterns is needed to ensure they meet their intended purpose.
Increasing accessibility to a variety of FVs and fostering choice are two mechanisms proposed to explain how salad bars might positively impact dietary consumption [
19‐
22,
24‐
27]. However, the limited research investigating this relation has yielded somewhat mixed results. For example, results of one study indicated that greater variety of FVs offered (independent of salad bar status) was associated with higher FV intake among elementary school students [
22]. Similarly, the introduction of salad bars into Title I schools was associated with increases in the variety of FVs offered and selected [
28]. Yet, self-served
portions were smaller and FV intake
decreased [
28].
Fostering choice might be an especially helpful in promoting FV intake in children. Indeed, in one study conducted with 4th and 5th grade students (
N = 1193), 85% reported that they liked having the option to choose FVs from their school salad bar; however only 44% said they used the salad bar at least once per week [
28]. A limitation of that investigation was that salad bars were offered
in addition to fixed portions of FVs on the lunch line, increasing variety, yet precluding isolation of presentation methods; this design also introduced confounds related to FV familiarity (e.g., of canned [
29] and/or heated FVs) competing with salad bars.
In addition to fostering choice, scholars have posited that salad bars might impact dietary consumption by increasing FV intake, and decreasing overall caloric consumption, via energy displacement [
30]. Yet it is unclear how increasing FV intake affects children’s overall energy consumption, in part due to methodological limitations of extant research. [
16,
31,
32]. Even less is known about how increasing FV intake within the NSLP (which limits calories available per meal) influences energy intake at lunch. One of the few existing studies in this area found that increased lunch FV intake was associated with decreased self-reported energy intake in students at schools with salad bars [
33]. In contrast, a subsequent study [
16] found that students who consumed the most FVs at lunch had the highest total energy intake. However, non-FV energy decreased across FV intake groups (thus at least some energy consumed from other sources was displaced by FVs). This study was conducted prior to the current NSLP guidelines, and only 54% of the schools it evaluated had salad bars. In a cross-sectional investigation conducted under the current NSLP, students in schools with salad bars consumed more energy from vegetables compared with students from schools without salad bars. Yet, evidence was inconsistent regarding FV displacement of other lunch calories [
34].
Other limitations of previous research include a lack of longitudinal and objective data [
35]. For example, in multiple cross-sectional studies, middle and high school students self-reported greater FV intake in schools with salad bars [
36‐
38]. However, because dietary intake was assessed via self-report only it is more subject to bias than objectively measured consumption. In contrast, 1st -5th grade students’ FV intake (assessed via objective plate waste) was no higher in schools with salad bars compared with schools serving pre-portioned FV only [
22]. However, this latter work was conducted > 15 years ago and might not be generalizable to today’s children, particularly given significant changes to the NSLP. There are also several program evaluations available online, although methodological concerns limit their internal validity (e.g., lack of comparison groups, post-only assessments, or low validity of FV assessment methods) [
35].
Only two quasi-experimental studies [
28,
33] have prospectively examined the impact of salad bars on dietary intake among elementary school students, and they yielded conflicting results. Moreover, only one of these investigations, [
28] was implemented under the current NSLP standards and assessed dietary intake objectively. In the first of these studies, Slusser et al. [
33] compared FV intake before and after salad bar installation in three schools. FV intake increased by 1.12 servings per day, as measured by 24-hour recalls; however, objective assessments of FV consumption were not conducted. There was also a 2-year gap between baseline and post assessments (and a 30% student transience rate), introducing potential history effects and reducing the likelihood that the same children were assessed at both time points. Bean and colleagues recently assessed FV intake before and one month after salad bars were installed in two Title I elementary schools serving predominately Black children, all of whom received free meals [
39]. Using objective, digital imagery plate waste methods, they found that students selected significantly more
types of FVs after the introduction of salad bars. However, at post, self-served FV portions were significantly smaller than those served by food service personnel, and mean FV intake
decreased by 0.65 cups (c), compared to when FVs were pre-portioned exclusively [
28]. These results suggest that increasing access to FVs might not be sufficient on its own to shape consumption patterns in this population. Importantly, neither of these prior studies included comparison groups.
Bean and colleagues subsequently compared FV intake in schools with salad bars with matched schools serving proportioned FVs only, within this same district. Although there was some evidence that vegetable consumption was higher in salad bar schools, different patterns of FV selection and consumption were observed across school pairs, suggesting that school environment factors other than salad bar access influenced FV intake [
40]. Importantly, salad bars in this district were offered
in addition to pre-portioned FVs on the serving line, and salad bar usage varied widely (8–64%) between schools. The continued availability of pre-portioned FVs precluded the ability to isolate the effects of salad bars on dietary consumption. There were also between-school differences in salad bar location, a factor demonstrated to impact usage [
41]. These results highlight the need for rigorously-designed, prospective evaluations of school salad bars, that include larger numbers of schools and comparison groups, use robust scientific methods, assess dietary intake objectively, and place salad bars in a consistent location.
Finally, salad bars are also often proposed as a mechanism to enhance NSLP participation [
35]. NSLP participation is a priority across districts, as it enhances the economic stability of school nutrition departments [
42]. Greater NSLP participation could also have positive public health implications, as school meals offer superior nutrition, compared with meals brought from home [
42]. Despite these benefits of NSLP participation, it has consistently declined over time [
43]. Moreover, there are no empirical data to support the claim that salad bars increase participation in the NSLP. The current study will examine how salad bars impact cafeteria sales and NSLP participation within a diverse district, with wide variations in NSLP participation, enhancing generalizability of findings related to these critical revenue sources.
In sum, there is an urgent need to improve the quality of children’s dietary intake. Optimizing school meals within the NSLP can have a significant public health impact. Tremendous resources have been invested in school salad bars as a means to increase FV intake, yet it is not known if they achieve this goal. By randomly selecting schools receiving salad bars, matching them with those serving pre-portioned FV only, and conducting a comprehensive, longitudinal evaluation that includes objective, validated assessments of dietary intake, this investigation will address these critical gaps. Specifically, this study will identify: (1) how salad bars impact dietary consumption in NSLP lunches, and (2) consequences of, and barriers and facilitators associated with, salad bar implementation. It will also examine the potential moderating role of the school-level sociodemographic factors of percent high obesity risk racial/ethnic minority students and Title I status on dietary consumption, and salad bar implementation barriers and facilitators. In the current study, all FVs on the salad bars will be fresh and
replace all fixed portion FVs on the serving line. All schools will operate under a policy requiring students to take at least one FV serving [
44]. This will allow examination of the independent and combined impact of variety and choice, two mechanisms with potential to increase FV intake, within the NSLP. We hypothesize that schools with salad bars will manifest greater increases in both FV selection and consumption, and decreases in FV waste, compared with schools without salad bars. In addition, we hypothesize that schools with salad bars will have greater improvements in dietary quality and reductions in total energy intake at lunch, compared with schools without salad bars.
Data from this trial will yield some of the clearest evidence related to salad bars to date, providing a strong evidence base to evaluate school nutrition policies designed to enhance dietary intake and reduce health disparities within the NSLP. Results can guide resource allocation and inform targeted interventions and policies designed to reduce obesity.
Discussion
This investigation responds to the urgent need to conduct rigorous research on policy and environmental approaches to obesity prevention. It has high public health significance, given the extent of the risks unhealthy dietary intake poses to children, especially those from systematically oppressed and minoritized backgrounds, who are most likely to participate in the NSLP. It also capitalizes on a unique natural experiment by systematically evaluating schools with and without salad bars in a large, diverse district.
The practice of installing salad bars, although intuitively appealing, has advanced well-ahead of the evidence. This investigation will make a significant contribution to policy research by providing the empirical data necessary to evaluate whether salad bars help achieve the HHKFA’s major goal of increasing FV intake within the NSLP [
14]. It will also enhance understanding of the role of FVs and salad bars on energy intake at lunch in the NSLP. Moreover, results will identify consequences of and barriers and facilitators to salad bar implementation, including investigating how salad bars impact sales and NSLP participation, informing sustainability efforts. This application improves upon prior research in this area via its rigorous experimental design (RCT); large sample of students nested within matched schools; use of psychometrically sound, objective assessments of dietary intake by blinded assessors; consistent salad bar implementation; and a detailed analysis plan, all of which will minimize the influence of bias on results. Findings will be disseminated via policy briefs and to scientific, school, and community outlets. Only one prior RCT has prospectively examined the effects of salad bars on objectively assessed FV intake, and none has evaluated their effects on energy intake in school lunch. Examination of the potential moderating role of school-level sociodemographic factors further strengthens the significance of this application, enhances its external validity, [
70] and will ultimately lead to targeted efforts to enhance dietary intake among children at greatest risk for obesity.
Since the initiation of this study, we have completed collection of Year 1 and the majority of Year 2 data, yielding > 14,000 images of school lunches. Data collection was paused in March 2020 when schools closed due to COVID-19, canceling post-ratings of one pair of schools scheduled to be rated that month. Students returned to school in Fall 2020, yet salad bars have not yet re-opened. The COVID-19 pandemic highlighted the critical role of school meals in addressing food security, and simultaneously placed enormous pressure on school nutrition programs to ensure students were fed [
71]. When school buildings were closed in the Spring and Summer of 2020, schools set up meal distribution sites and delivery systems (all while understaffed and under-resourced). The return to school in 2020-21 brought new challenges related to increased demands for school meals, risk mitigation, staffing shortages, and supply chain disruptions. In response, the USDA issued national meal pattern waivers, allowing schools to veer from NSLP nutritional standards—including those related to milk, whole grains and sodium set forth by the HHKFA—and mandated universal free meals [
71‐
73]. Many districts, including the one in this investigation, transitioned to more pre-packaged meals during this time and temporarily eliminated a la carte offerings. Although these temporary waivers helped schools meet the meal demands during that time, there is concern that diet quality has been compromised [
71‐
73]. This is a particular concern given extension of nutrition waivers and the enhanced role of schools in addressing food security [
71‐
73]. There is thus a great need to investigate how these changes have impacted children’s nutrition.
Given that surfaces are not the primary mode of COVID-19 transmission, and because a healthy diet is important to support immune functioning and overall health, some have argued that salad bars should return to schools [
74]. However, it is unclear how potential barriers to the re-implementation of salad bars, such as staffing shortages and parents’ fears of the virus, might affect this process. Personal communication with this district’s Food and Nutrition Services stated plans to resume salad bar operations, yet acknowledged facing the same challenges as those encountered nationwide—adjusting to changes in school food policy mandates, supply chain disruptions, and staffing shortages. The resumption of data collection in the current application depends on timing of the return to salad bars in schools. Importantly, however, current estimates suggest that in our two years of data collection, we far exceeded projected numbers, even when removing data from the pair of schools for whom post-ratings were not conducted. To date, we have applied laboratory rating procedures on an estimated 11,000 school lunches across time points, from an estimated 5,500 students. Thus, even if data collection does not resume, we are optimistic that we will be adequately powered to respond to our study aims and hypotheses.
In sum, this study stands to be one of the most definitive investigations of school salad bars to date and will inform school nutrition policies and programming designed to enhance dietary intake and reduce obesity. The vital role of schools in promoting children’s nutrition and preventing food insecurity has become clearer to the nation since the onset of the COVID-19 pandemic, making this work particularly timely [
71]. Future directions for this line of research include using these data, combined with results from our formative work, to design targeted interventions (adjunctive or alternative to salad bars) to optimize children’s FV intake within current school policy mandates.
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