2.2.1. RCT design
Randomization
This RCT was a randomized, single-blind, placebo-controlled, folate-fortified maize intervention. Women of childbearing age were randomly assigned to either the FFM group (treatment group) or the ordinary maize group (control group). Randomization was performed using sealed opaque envelopes [26].
Description of the Intervention
When eligibility was confirmed after the baseline survey, the participants were enrolled in the intervention. At the start of the intervention (1 week after baseline survey), the participants were randomized to consume either the FFM or the ordinary maize for 2 months. Maize was developed and provided by the Biotechnology Research Institute of the Chinese Academy of Agricultural Sciences (CAAS). The cultivar of FFM was Jingkenuo 928; the average weight of one stalk was 115 g, and its average folate content was 96.9 ug per stalk. The cultivar of ordinary maize was Jing 2000; the average weight of one stalk was also 115 g, but the average folate content was 35.6 ug per stalk. Both types of maize were packaged in labeled identical packets identified (for blinding) by the CAAS before sent to the study site (village clinic). Sufficient maize was provided to the participants for each stage of the survey. The participants had to eat 115 g (one stalk) of maize daily. Maize was distributed to the participants 1 week after the baseline survey. The participants were provided seven maize stalks once a week at the same location and time, which lasted for 2 months. The three-stage RCT was conducted from August 13, 2018, to October 19, 2018.
The baseline survey was conducted on August 13, 2018, including screening questionnaires, dietary intake surveys, basic information questionnaires, physical measurements, and fasting venous blood tests. We began the intervention 1 week after the baseline survey, that is, on August 20, 2018. We distributed seven maize stalks per person at the same location and time each week, according to their treatment assignment. Subsequently, we re-examined their serum folate content after 1 month (on September 19, 2018) and 2 months (on October 19, 2018) as the mid stage and final stage results. A similar dietary intake survey, physical measurement, and fasting venous blood tests were also performed in the mid and final stages.
Data Collection
Data were collected during a personal interview at the baseline survey, mid stage survey, and final stage survey. The following information was collected by a researchers through face-to-face interviews with the participants at the baseline survey. The participants completed a sociodemographic and screening questionnaire, including age, residence, education, marital status, number of children, health status, maize preference, height, and weight. The purpose of the screening questionnaire was to select the participants.
Dietary intake was assessed using a past-month food frequency questionnaire (FFQ) [27]. Briefly, the participants were asked to report their consumption of 142 items grouped into 11 categories, including cereals and grain products, beans, vegetables, edible fungi and algae, fruits, dairy products, meat, aquatic products, snacks, beverages, and "other foods". Using the FFQ, we calculated the participants' folate content in the diet during the experimental period. The specific method was to query the folate content of each food item per 100 g through the "China Food Composition" [28], and then combine it with the consumption amount and consumption frequency of each food item. The dietary folate intake of each food item were calculated, and all food items and dietary folate intake were added together to get the total dietary folate intake.
Weight and height were measured using an electronic scale, with participants wearing light indoor clothes and no shoes. The weight was measured to the nearest 0.1 kg; the height was measured to the nearest millimeter.
Fasting venous blood was used to test the folate content in the red blood cells and serum. The serum folate level was considered an indicator of recent folate intake, while red blood cell folate concentrations are useful for indicating long-term folate status. Red blood cell folate concentrations respond slowly to changes in folate intake, since the erythrocytes only accumulate folate during erythropoiesis in its 120-day lifespan. Thus, we used them as the screening criteria; participants were excluded from the study if their red blood cell folate content exceeded 906 nmol/L. Serum folate content was used as an indicator to evaluate the health effects of FFM. Folate content was detected using an improved microbiological assay method [29–30].
Ethical Considerations
This study was approved by the Ethics Committee of the Chinese Center for Disease Control and Prevention (NINH 2018-015). All participants provided written informed consent prior to the study. The reporting of this study adheres to the Consolidated Standards of Reporting Trials (CONSORT) guidelines [31](Schulz et al., 2010).
2.2.2. BDM experiment design
We conducted a basic information survey for all participants in BDM. The subjects were interviewed face-to-face and one-on-one by trained staff following the BDM experiment procedure, which was less prone to group effects [32]. Initially, the trained staff introduced an auction product (namely, one stalk of the FFM), funding (namely, 10 CNY), and the standard BDM experiment procedure. Each trained staff member had a paper bag with quotation notes; the quotation range was 1–10 yuan, with 0.1 yuan as the interval of 100 pieces of paper. First, the participants made bids by writing a random price on a blank paper according to the description of FFM. Then, the trained staff randomly extracted a quote from the folder. If the bid was higher than the quote, participants could buy the FFM at the bid; otherwise, they had no opportunity to buy it.
Moreover, we performed the same bidding procedure for three rounds, and the first two were to familiarize them with the experimental procedures. We also filtered those who could not understand the game by the end of the experiment. Finally, we obtained the actual WTP, which was equal to the bid in the third round of the BDM experiment.
In our BDM experiment, eliciting the WTP for FFM was the final objective. Participants were asked to give the maximum price they would agree to buy for the FFM they had just evaluated since it was in their best interest. If they underestimated it, they might lose an opportunity to buy at a price they would have been pleased to pay. If they overestimated it, they just took the risk to pay more than they would have liked to [33].
After the BDM experiment, the participants completed a questionnaire containing basic information, including age, location, education, job, household size, income, marital status, number of children, maize preference, and the cognition of folate and FFM. The questionnaire can directly reflect the participants' preferences and demands for FFM, which were essential for the follow-up analysis of WTP.