Sustained Wellbeing Benefits of Red Meat Consumption in a Modern Flexitarian Diet: A Study Protocol for a 10 Week Randomised Clinical Trial

Study Design and Setting The study is a parallel RCT comprising a 2-week pre-intervention assessment, a 10-week intervention period and a 22-week post follow up. Participants will all eat a balanced plant-based basal diet and be randomised 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 to our specifications in New Zealand, while the meat substitute will be commercially available plant-based products.

Recognising 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 no 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. Each couple/household unit (see Recruitment) will receive a weekly delivery of a vegetarian meal kit containing complete ingredients for cooking 3-4 evening meals. Depending on their allocation, the couple will also receive an allotment of frozen red meat or meat-analogue to use for preparing 3 additional evening meals. To facilitate cooking and keep the behaviours of the arms aligned, we will provide recipe cards, shopping guidance and video instructions. All other meals during the week are self-catered and must adhere to study guidelines.

The quantity of meat will be sufficient to provide 350-500 g cooked weight per person per week. This conforms to the latest international recommendations for maximum intake 7 and the reported typical frequencies of red meat consumption. The meat alternative will be similar in quantity, form and function. It is vegetarian-friendly and serves to balance the total protein and fat intake of the arms.

Changes in objective and self-assessed subjective health variables over time and between arms will be investigated. These include physical function, body composition, metabolic health biomarkers and psychological wellbeing.