WIC-Based Intervention to Promote Healthy Eating Among Low-Income Mothers
Phase: NA
Conditions: Cancer Prevention, Diet Modification, Vegetable Intake
Sponsor: William Paterson University of New Jersey

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Study Title: WIC-Based Intervention to Promote Healthy Eating Among Low-Income Mothers 
NCT Number: 040383385 
Document Type: Study Protocol and Statistical Analysis Plan 
Document Date: October 19, 2021 
 
 

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Objectives 
This study examined the preliminary efficacy of a novel, theory-driven intervention designed for 
adults served by the Special Supplemental Nutrition Program for Women, Infants, and Children 
(WIC). Primary outcomes were vegetable intake (objectively assessed using dermal carotenoids 
as a biomarker of intake and via self-report) and the redemption of WIC Farmers’ Market 
Nutrition Program (FMNP) vouchers for fruit and vegetable purchases at farmers’ markets 
(objectively assessed using data provided by WIC). Potential hypothesized mediators of 
intervention effects were vegetable-related knowledge, attitudes, and behaviors; physical activity, 
and weight status. Intervention effects on the redemption of WIC cash value vouchers (CVV) at 
farmers' markets, participant satisfaction with the program, and the cost-effectiveness of the 
intervention also were examined. The CVV are issued monthly by WIC for fruit and vegetable 
purchases and are redeemable at farmers’ markets in New Jersey, the location of the study. 
 
Setting, Design, and Sample 
The setting for the research was a large, New Jersey-based WIC agency located in a densely 
populated primarily Hispanic urban area. The intervention was evaluated in three of the agency’s 
17 sites selected based on similarity in size and the demographics of participants served (one 
randomized to the intervention study group and two to the control group). As the intervention was 
delivered at the site level, randomization was by site to prevent contamination of the control group. 
Inclusion criteria were 1) English- or Spanish-speaking adult, 2) FMNP voucher recipient or 
caregiver of a child voucher recipient, 3) no known restrictions on food intake, 4) not less than 3 
months from expected delivery date (if pregnant), and 5) eligible to receive WIC benefits for at 
least 6 months after study entry. Recruitment occurred from June 3, 2019, to August 1, 2019. 
Across sites, trained bilingual English/Spanish-speaking research staff contacted participants 
considered eligible to receive FMNP vouchers based on information provided by WIC by 
telephone prior to forthcoming appointments, provided a description of the study, and confirmed 
eligibility. Interested adults were orally administered an outcome battery of self-report measures. 
During appointments, biometric measures (height, weight, and carotenoid levels) were taken. 
Participants also were recruited from the WIC clinic where they completed all measures in person. 
All participants provided informed consent verbally, prior to completing telephone assessments, 
and in writing prior to completing in-person assessments. Thereafter, a WIC administrator 
confirmed whether adults completing all assessments had received FMNP vouchers. Those who 
did not were informed that they were ineligible and were thanked for their involvement.  
In total, 297 adults were enrolled. Participants enrolled at the intervention site (n = 160) 
received routine services provided by WIC and the intervention. Those enrolled at control sites (n 
= 137) received routine services only. Across sites, all participants were contacted to complete 
follow-up measures at mid- and post-intervention (3 and 6 months post-baseline, respectively). As 
at baseline, research staff contacted participants by telephone prior to forthcoming appointments 
and orally administered the outcome battery; during appointments, biometric measures were taken. 
To enhance retention, across sites, research staff mailed appointment reminders to participants and 
confirmed appointments via telephone one day in advance, maintained multiple contacts for 
participants, and updated participants’ contact information at each assessment. Honoraria, gift 
cards redeemable at local supermarkets and discount chain stores, also were provided to 
participants for completing successive assessments. Participants received a $10 honorarium at 
baseline and at mid-intervention; to enhance response rates, the amount was increased to $20 at 
post-intervention (participants received up to $40 in honoraria in total). For self-report data 
assessed via the outcome battery, 273 participants completed at least one post-baseline assessment 

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(143 participants in the intervention group and 130 in the control group); for biometric measures, 
the corresponding figure was 261 (137 in the intervention group and 124 in the control group). 
 
Intervention 
Designed to address individual and broader systems-level influences on vegetable intake, the 
intervention was conceptually grounded in the Social Ecological Model and Social Cognitive 
Theory.1 According to the Social Ecological Model, behavior is shaped by influences operating at 
multiple levels, i.e., intrapersonal and interpersonal factors, community and organizational 
factors, and public policies.2 Social Cognitive Theory emphasizes targeting the environment 
(factors external to a person), behavioral capacity (knowledge and skills to perform a behavior), 
and self-efficacy (confidence in the ability to perform a behavior) using self-control strategies 
such as monitoring and feedback to regulate behavior, observational learning (the acquisition of 
behaviors by observing outcomes of others’ behavior, ideally, credible and relatable role models); 
and reinforcement (incentives, rewards, and feedback) to increase the likelihood of a behavior.3,4 
The program consisted of 1) a WIC-based farmers’ market to improve community access to 
vegetables, and among those purchasing items at the market, home vegetable availability; 2) 
behaviorally focused instruction to enhance social support for vegetable consumption and build 
vegetable-related knowledge, skills, and self-efficacy; 3) field trips to an area farmers’ market to 
further improve vegetable access, provide opportunities for experiential and hands-on learning, 
and enable participants to apply knowledge and skills learned at the WIC-based market to a real-
world setting; 4) telephone coaching and support before and after trips to facilitate plans to 
incorporate vegetables into daily meals, 5) recipe demonstrations and tastings to build vegetable 
knowledge, preparation skills, and taste preferences; and 6) handouts to reinforce vegetable 
knowledge and preparation skills. The program logic model is shown in the figure. 
The WIC-based market was implemented at the intervention site in the summer of 2019 
during the FMNP voucher issuance period. Participants presenting for appointments (during 
which they would receive FMNP vouchers) were directed to wait in a classroom with the market. 
Nutrition educators provided group-based instruction to participants while waiting for 
appointments, conducted recipe demonstrations and tastings (in total, 3 recipes were 
demonstrated), and instructed participants to return to the classroom after appointments to receive 
additional instruction and a recipe pack containing handouts and the ingredients for one of the 
recipes to try at home. Participants also learned that they had the option to purchase fruits and 
vegetables at the WIC-based market with their FMNP vouchers. Nutrition educators provided 
personalized, 1:1 instruction to participants returning to the classroom after appointments. 
Thereafter, participants completed up to three field trips to the area farmers’ market (one 
each in September, October, and November). The same individuals who delivered nutrition 
education at the WIC-based market scheduled the trips and provided group-based instruction to 
participants during trips. At the market, the educators conducted recipe demonstrations and 
tastings (3 recipes per trip, differing each month), distributed recipe packs containing the 
ingredients for one of the recipes, and provided 1:1 instruction to participants. The educators also 
provided telephone coaching and support to participants before and after trips. The 5-month 
intervention was implemented between July 1, 2019, and November 30, 2019. 
 
Measures 
At baseline, participants reported their age, race, ethnicity, nativity, language preference and 
years in the US (if foreign born), pregnancy and breastfeeding status, educational attainment, 
car ownership and access, supplement use, smoking status, and past 7-day exposure to 

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secondhand smoke. Food security status was assessed with an item from the Household Food 
Security Survey Module (“Have you or other adults in your household worried whether your 
food would run out before you got money to buy more?”).5 Participants also completed a 
validated measure of social desirability trait, the tendency to respond to self-report measures in 
a manner consistent with expected norms.6 As children aged 2 to 5 years were eligible to 
receive FMNP vouchers, a measure of the number of children per household aged 2 to 5 years 
was constructed based on FMNP voucher data provided by WIC. 
 
Vegetable intake was objectively measured with pressure-mediated reflection spectroscopy, a 
safe, reliable, and valid method for noninvasively assessing dermal carotenoids as a biomarker 
of intake.7 Carotenoid levels were assessed with a portable device, the Veggie MeterTM 
(Longevity Link Inc., Salt Lake City, UT), by scanning the tip of the finger. Scans were 
conducted in triplicate and the average of the three scans was recorded. Scores can range from 
zero to 800, with higher scores indicating higher dermal carotenoid levels. Vegetable intake also 
was assessed via self-report with one of two items in a brief fruit and vegetable screener 
developed by the National Cancer Institute (“How many cups of vegetables [including 100% 
vegetable juice] do you eat or drink each day?”).8 To facilitate the estimation of food portions, 
participants were told that a cup was about the size of their fist.9 
 
FMNP voucher redemption was objectively assessed once at the end of the voucher redemption 
period (June 1 to November 30, 2019) using data provided by WIC. WIC reported whether 
participants redeemed any vouchers (yes/no) over this period.  
 
Knowledge of vegetable intake recommendations was assessed with an item from the Food 
Attitudes and Behaviors Survey.10 Based on their responses, participants were classified as 
knowledgeable of the recommended cups/day of vegetables (yes/no) as per the My Plate 
amounts for women aged 19 to 50 years.11  
 
Home vegetable availability was measured with the Home Health Survey, shown to have 
moderate reliability and validity.12 Participants responded to items querying whether they had 
any fresh, canned, and frozen vegetables at home (yes/no). Those responding affirmatively were 
asked to report which vegetables they had at home. Two measures of availability were 
constructed based on the responses: 1) the quantity or total number of vegetables at home, and 2) 
the variety or number of different items at home. 
 
Vegetable taste preferences were assessed with a single item, “Overall, how much do you like 
the taste of vegetables?” Responses were on a 7-point scale ranging from not at all (1) to a lot 
(7). Higher scores indicated greater taste preferences.  
 
Social support for vegetable consumption was assessed with the item, “How much would you 
say people in your life support you to eat vegetables?” Responses were on a 7-point scale 
ranging from not at all (1) to a lot (7). Higher scores indicated greater social support. 
 
Vegetable preparation practices were assessed with three items from the Project EAT-II survey.13 
Participants reported how often they performed the following behaviors in the past month: 1) 
bought fresh vegetables, 2) prepared a green salad, and 3) prepared a dinner with vegetables. 
Responses were on a 5-point scale ranging from never (1) to more than once a week (5). 
 

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Vegetable preparation skills were assessed with the following item: “How would you rate your 
overall skill in preparing vegetables?” Responses were on a 7-point scale ranging from poor (1) 
to excellent (7). Higher scores indicated greater preparation skills. 
 
Self-efficacy for vegetable consumption was assessed with an adapted version of a 5-item 
measure of self-efficacy for fruit and vegetable consumption.14 The measure was adapted by 
revising items referencing fruits and vegetables to reference vegetables only (sample item: “I feel 
I can prepare recipes with vegetables.”).14 Responses were on a 4-point scale ranging from 
strongly disagree (1) to strongly agree (4); higher scores indicated greater self-efficacy.  
 
Physical activity was operationalized as meeting current physical activity guidelines and was 
assessed with a validated 2-item measure.15 Participants reported the frequency and duration of 
moderate and vigorous intensity physical activity, respectively, in a typical week. Item responses 
were assigned point values and summed to derive a total score. Score ranges were used to 
classify participants as meeting physical activity guidelines (yes/no).15 
 
Weight status was measured as body mass index (BMI), calculated as weight in kilograms 
divided by height in square meters.16 Height and weight were measured with participants 
wearing light clothing without shoes using standardized methods and equipment (Seca 876 scale 
and Seca 213 stadiometer [Seca Corp., Chino, CA]).17  
 
Cash value voucher redemption was assessed using data provided by WIC and was tracked over 
the same period as FMNP voucher redemption (June 1, 2019, to November 30, 2019). WIC 
reported whether participants redeemed any CVV at farmers’ markets (yes/no) over this period.  
 
Satisfaction with the intervention was assessed at post-intervention among participants who 
received the intervention. A single item measure (with response options on a 7-point scale ranging 
from very dissatisfied [1] to very satisfied [7]) was used to assess overall satisfaction with the 
program. Participants also completed a 5-item satisfaction measure developed by the investigators 
(sample item: “The program was interesting;” Cronbach’s α = 0.90). Responses were on a 7-point 
scale ranging from strongly disagree (1) to strongly agree (7). Ratings were averaged across items 
to derive a total score. Higher scores indicated greater satisfaction. Participants also responded to 
open-ended items on what was liked most and least about the program, what, if anything, could be 
done to improve it; and the most important thing learned in the program. 
 
Hypotheses 
Primary (Outcome Evaluation) 
H1: At mid- and post-intervention, greater improvements in vegetable intake and FMNP voucher 
redemption will be found among participants who receive the intervention relative to those who do not. 
 
 
Secondary (Mediation Analyses) 
H2: Program effects on vegetable intake will be mediated by improvements in vegetable-related 
knowledge, attitudes, and behaviors; physical activity, and weight status. 
 
Exploratory Aims 
1. To evaluate intervention effects on the redemption of CVV at farmers’ markets. 
2. To examine satisfaction with the intervention among participants who receive it. 
3. To determine the cost-effectiveness of the intervention. 
 

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Intervention Effects on Primary Outcomes and Potential Mediators 
Linear mixed-effects models were used to relate two repeated measures of primary outcomes and 
potential mediators to time, trial arm, and a time-by-arm interaction. Covariates included baseline 
measures of each outcome and prognostic factors or potential influences on intake. A common set 
of prognostic factors was identified for inclusion across analyses, i.e., age, race, breastfeeding 
status, and exposure to secondhand smoke. Supplement use, a potential confounder of carotenoid 
data, and social desirability trait, shown to influence self-reports of behavior, also were included in 
analyses of objectively measured and self-reported vegetable intake, respectively. Intervention 
effects on binary measures of knowledge of vegetable intake recommendations and meeting 
physical activity guidelines were examined with generalized linear mixed models with binary error 
distributions. In addition to tests of the significance of differences between least square means by 
trial arm at each time point, 95% confidence intervals were estimated for the differences. To 
quantify the magnitude of between-group differences, adjusted Cohen’s d was calculated as the 
difference between least square means divided by the square root of the residual variance.  
Logistic regression analysis was used to relate FMNP voucher redemption to study arm. 
The analysis was adjusted for the aforementioned covariates, i.e., age, race, breastfeeding 
status, and exposure to secondhand smoke. Odds ratios (ORs) were estimated with 95% 
confidence intervals. For FMNP voucher redemption, effect size was measured using ORs, 
where 1.5 is a small effect, 2.5 is a medium effect, and 4 is a large effect. 
 
Exploratory Analyses 
Preliminary analyses revealed that 12 participants redeemed CVV at farmers’ markets, all in the 
intervention arm. Thus, a formal analysis of intervention effects on CVV redemption was not 
conducted. Mean ratings on measures of satisfaction with the intervention (on a 7-point scale) 
were computed. Scores at or above 5.0 were considered evidence of a high degree of satisfaction. 
Responses to open-ended items were examined with thematic analysis. Total and per participant 
intervention costs and cost-effectiveness ratios (expressed as cost per intervention effect) were 
estimated in 2019 US dollars over a 6-month period from the perspective of the agency 
implementing the intervention. Although not originally planned, we also examined the 
relationships between measures of vegetable intake and potential hypothesized mediators at 
baseline, and screened baseline values of the mediators for a possible ceiling effect. 
 
 
 

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