The study is a parallel RCT comprising a 2-week preintervention assessment, a 10-week intervention period, and a 22-week posttrial follow‑up. Participants will all be maintained on a balanced vegetarian basal diet and be randomized to also consume either red meat (“Flexitarian” arm) or a meat substitute (“Vegetarian” arm). The meat will be pasture-raised beef and lamb butchered and packaged in accordance with our specifications in New Zealand, while the meat substitute will be commercially available plant-based products (Multimedia Appendix 1). The SPIRIT guidelines were followed when designing the research [23].

Recognizing that there are many interpretations of the terms, “vegetarian” and “flexitarian,” in this study we define vegetarian as ovo-lacto vegetarian and flexitarian as “a vegetarian diet with moderate amounts of red meat.” Both arms can consume eggs and dairy products, but not chicken, pork or fish, and no red meat other than that supplied by the researchers.

The study is designed around pairs of young adult consumers who share meals and are therefore likely to engage over the values and challenges of developing a healthy lifestyle. Couples/household units (see Recruitment) will be randomized to the same intervention arm. Depending on their allocation, the couple will receive regular allotments of either frozen red meat or meat-analogue, which will be sufficient to provide 3 meals (approximately 350-500 g of cooked meat or vegetarian alternative per person per week) [8,24,25]. We will evaluate changes in objective and self-assessed subjective health variables between the 2 intervention arms. These include physical function, body composition, metabolic health biomarkers, and psychological well-being.

We will recruit pairs of individuals (spouses, partners, or companions) who cohabitate and typically share evening meals. Participants will be GenZ (18-23 years old) and millennial (23-34 years old) individuals. This population demographic has the greatest variation in meat intake [8]. Recruitment will be advertised with posters placed around the University of Auckland and using social media websites and tools.

Potential participants will meet with researchers responsible for recruitment in person, where eligibility will be confirmed, and details of the study will be discussed with opportunity for questions to be clarified. At this in-person screening visit, participants will provide written informed consent (Multimedia Appendix 2).

All participants are required to be considered omnivores if, in the last 2 months, they consumed at least 2-3 meals per week which included meat of any description (red or white fleshed meat, including fish). They must be willing to consume both red meat and meat analogues for the purposes of the trial. Those with chronic health conditions, obesity (BMI≥30 kg/m²), hyperlipidemia, disordered eating patterns, history of anosmia and ageusia (issues with smell and taste), use of medications (except for occasional nonsteroidal anti-inflammatory drugs and antihistamines), or recreational drugs, or those who smoke tobacco will be excluded from participating. Potential participants who use dietary supplements were asked to abstain for the month prior to the beginning of the study. Participants must own a mobile phone with a camera and be proficient with using Facebook and Facebook messenger. Women must confirm they are neither pregnant nor intending to become pregnant during the trial.

Participants will complete a web-based eligibility screening, which includes the Three-Factor Eating Questionnaire (TFEQ-R18), a Self-Efficacy questionnaire, a Food Frequency questionnaire, and a health survey. Participants with a TFEQ score greater than 75% will be excluded on the basis that their perception of food is potentially influenced by underlying psychological issues or that they demonstrate disordered eating patterns. Given the routine monitoring of food intake and subjective experience to food required in this research, this cut-off was deemed clinically relevant for the purpose of this study by the research team, including psychologists and registered dietitians [26]. Participants must own a mobile phone with a camera. Women must confirm they are not pregnant, nor intending to become pregnant during the trial.

Participants will be monitored for their adherence to the study guidelines through the dietary recording smartphone app Easy Diet Diary and were prompted for adherence via emails, social media, and SMS text messages. They will have the right to withdraw from the study at any time without any explanation. The principal investigator (PI) will have the right to discontinue participants’ involvement if they become ineligible or when any significant protocol deviations occur in the study. The data of participants who withdraw will be retained and might be used in exploratory analyses, unless the participant requests data to be deleted.

The primary biomarker for calculating the sample size is the concentrations of LCn3 in erythrocyte membranes, and a sample size of 63 will provide sufficient power to detect changes in fatty acids, which might be due to the intervention. Published data indicate a change of 3.01 µg/mL (SD 1.1 µg/mL) in erythrocyte fatty acid composition (20:5) following a 2-week dietary intervention and crossover meat trial among young adults [27-29]. The Cohen effect size of 0.2 (small) of the variability in erythrocyte fatty acids was used to consider the smallest worthwhile effect. To allow for dropouts, a total of 80 participants will be recruited.

The 80 participants (self-organized as 40 couples/household units) will be randomized evenly to the vegetarian or flexitarian arm, with a random allocation sequence (1:1 ratio) generated using Random.org. Researchers responsible for participant recruitment will be blinded to allocation until participants begin their 2-week lead-in period. Owing to the nature/form of the food provided, participants will not be blinded to their intervention; nonetheless, they will not be aware of this until the intervention begins.

Couples will be further organized into “teams,” which are groupings of 5 couples who consume the same diet. There will be 4 such teams per intervention arm. A team serves as another layer of structure and mutual support during the intervention period. Team members will engage virtually through social media, which was explained to participants during recruitment.

The designed intervention has been assessed by a nutrition expert panel. The trial was approved by the New Zealand Ministry of Health’s Health and Disability Ethics Committees (protocol 20/STH/157). The investigators will ensure that this study is conducted in accordance with the principles of the Declaration of Helsinki, with relevant institutional regulations. Results arising from this study will be submitted for publication in scientific journals and presented at meetings. Authorship will be determined in accordance with the guidelines of the International Committee of Medical Journal Editors.

Each participant will be coupled with another participant within a team in each arm (Figure 1). The trial commences with a 2-week preintervention period when participants consume their usual diet. Following allocation, couples will begin to receive allotments of either meat or the meat alternative (3 meals per week) and weekly vegetarian meal kits plus (additional 3 meals per week). The dietary intervention continues for 10 weeks.

At intervals during the trial, participants will take part in assessments of nutritional status and mental well-being in accordance with the schedule shown in Table 1. A follow-up evaluation (on the internet or in person) at week 22 will conclude the study.

Participants will visit the University of Auckland Clinical Research Centre and a blood testing facility in Auckland on three occasions: at baseline (t₀), midway through intervention at week 5 (t₅), and at the end of intervention at week 10 (t₁₀). A blood sample will be collected from an antecubital vein by a trained phlebotomist following an overnight fast. Body weight, blood pressure, grip strength, and body composition will be measured as described below. Participants will wear a wristband accelerometer for the duration of the study to estimate their level of physical activity and sleep patterns. All participants will complete the suite of mental health questionnaires on up to 7 occasions either on the internet or in person, which takes approximately 15 minutes. From t₀ to t₁₀, participants will record their dietary intake using Easy Diet Diary. Two members of the research team will be responsible for participant recruitment, clinic visit assessments, and nutrition education delivery via the web-based platform. Participants will also be in contact with technical staff at the blood testing facility.

Household units will receive regular deliveries of either red meat or vegetarian analogues sufficient for 3 evening meals per week. The quantity of meat will provide 350-500 g cooked weight per person per week, which conforms to the latest international recommendations for maximum intake [8] and the reported typical frequencies of red meat consumption [24,25]. The meat alternatives are commercially available soy protein–based products which been selected on the basis of similar quantities (350 g cooked weight), form (“beef” mince and patties which can be interchanged in recipes between red meat and vegetarian groups), macronutrient composition with a focus on total protein and fat, as well as taking product availability in the local food supply into account (Table 2).

Participant’s background diet can affect the physiological and psychological outcomes of interest. Efforts have been made to align participants’ diets with healthy eating guidelines and to increase consistency between intervention arms, while still providing a realistic examination of the effects of consuming moderate amounts of red meat compared rather than its vegetarian analogue. To achieve this, household units will receive a weekly vegetarian meal kit delivery (Woop Ltd) containing complete ingredients and recipe cards for cooking 3 evening meals, which will be matched across all participants. Beyond the protein and meal kit provided, participants do have a degree of choice within their diet but are provided with a cookbook and web-based nutrition support package to facilitate healthy meal choices and preparation (see Nutrition Education). Collectively, the objective is to ensure that participants are meeting local recommendations for servings of core food groups consumed each day (fruits/vegetables, wholegrains, protein/alternatives, and dairy/alternatives), and minimizing the intake of discretionary foods (eg, takeaways and fizzy drinks) [30].

A secondary outcome of this study is the subjective ease, compliance, and satisfaction of following a prescribed healthy eating pattern with or without red meat. This aligns with the trial objective of understanding the psychological response to following a diet with moderate amounts of red meat or vegetarian analogues.

We have designed a web-based nutrition education package specifically for young adults, following a literature review and focus group needs assessments. The nutrition package was developed and reviewed by expert registered dietitians in the team, who are appropriately placed to ensure the accuracy and quality of information provided. The Nine Principles framework, incorporating behavioral theory and user-centered design, was used to guide its development [31]. The aim of the package is to (1) support young adults’ adoption and maintenance of healthy dietary behaviors, such as those outlined in the Eating and Activity Guidelines for New Zealand Adults [30], and (2) support adherence to respective dietary arms. Each team will have a private Facebook page and messenger chat which will be utilized to deliver evidence-based nutritional information and meal ideas. Behavior change techniques used in this study include goal-setting, behavior demonstration, and setting graded tasks. The nutrition package content and delivery will be standardized across arms to encourage consistency in the adoption of healthy dietary behaviors. Participants will only be prompted to engage with nutrition education by way of notifications through Facebook and Facebook messenger as posts are made in accordance with a predetermined schedule. eHealth use will be monitored in accordance with participant’s having viewed messages in the Facebook messenger chat, and engagement will be monitored in accordance with their “likes” and comments on Facebook messages and posts.

Participants will be emailed a link to register for Easy Diet Diary (Xyris Software Pty Ltd) at –t₂. This app enables researchers to see all data entered in real time, allowing compliance to be routinely monitored during the intervention. Instructional support on using the app will be provided at –t₂ and t₀. From t₀, participants are required to record all dietary intake manually through text insertion into the app 2 days a week (Sunday and Monday). For the remaining 5 days of the week (Tuesday to Saturday), participants are required to enter intake through a choice of photographs or manual text insertion to monitor participant compliance with consuming all red meat or meat analogues provided each week. Participants will receive regular text reminders to complete their diaries. Compliance to dietary recording and the dietary intervention will be monitored twice a week (every 3-4 days). Adequate dietary recording is defined as entering at least 1 full day (ie, 3 meal occasions) per 3-4–day period. If participants are noncompliant with recording or the dietary intervention within a 3-4–day period, they will be sent a reminder through SMS text message. If they are noncompliant for 3 consecutive periods, the pair will be contacted and will discontinued from the study if adherence is not improved.

Using a free-living, mixed meal approach has its advantages in real life applicability of study findings but may hinder the clarity in outcomes. A key aim was to compare the health effects of red meat and alternatives in combination with a healthy diet, using a clearly planned nutrition education and adherence strategy. However, we acknowledge that compliance is voluntary, and the degree to which participants comply will impact the outcomes. To address this, daily food intake will be assessed using images based or mobile phone app technology and stop-go strategies in place to best support the participants in adhering to the dietary intervention and provide confidence in participant compliance [42,43].

Concurrent analysis of physiological and psychological well-being provides an extraordinary opportunity to understand the broad benefits of a healthy diet. If the inclusion of pasture-raised red meat improves compliance, enjoyment, and measures of mental and physical health, then this can be communicated to markets and demographics that are important to industries in New Zealand. A sequence of scholarly publications will underpin, and be a prerequisite for, any marketing messages.

The PI is responsible for project coordination and will oversee the operational aspects of the trial. A scientific advisory group will regularly monitor study implementation, as well as data generation, documentation, and reporting. The PI will communicate protocol amendments to the ethics committee and clinical trial registration. Access to data will be granted to appropriate members of the research team and to authorized representatives from the host institution to monitor or audit the study and ensure compliance with regulations. Data will be made available to external academics on reasonable request.