The aim of the study is to provide proof of the effectiveness, acceptability, healthfulness and nutritional adequacy of dietary guidelines to reduce greenhouse gas emissions. The study will compare the effect of dietary advice based on 1) healthy climate-friendly dietary guidelines (intervention group) or 2) standard healthy dietary guidelines (control) on greenhouse gas emissions associated with dietary intake over 12 weeks.

A randomised, parallel controlled trial will recruit n=360 healthy adults (age 18-64) across three research centres. After successful screening and receiving informed consent, participants will be randomised into the intervention or the control group for the duration of the 12-week trial described in detail in the text and figure below. The intervention group will receive dietary advice based on climate friendly eating recommendations. The control group will receive dietary recommendations based on Healthy Ireland's Healthy Eating Guidelines and Food Pyramid (FSAI, 2019).

Prior to their first onsite visit, participants will be asked to complete a Health and Lifestyle questionnaire and dietary assessment. The dietary assessment will include

1. 3 online 24-hour recalls (2 non-consecutive weekdays and 1 weekend day) (via web-based dietary recall tool, Foodbook24)
2. a food frequency questionnaire (Foodbook24)

Baseline, Visit 1 (Week 0): We will collect blood and urine samples from participants and record anthropometric measurements. Participants will be asked to complete a Stage of Change questionnaire to assess their openness to a diet change. All participants will complete a Food Waste Questionnaire at baseline for later comparison at endpoint. At the end of Visit 1 participants will receive a personalised feedback report and advice from nutritionists based on their habitual diet.

Week 6: Participants will be asked to complete a second dietary assessment during week 6. 3 24-hour recalls and a food frequency questionnaire will be hosted on Foodbook24, a web-based dietary recall tool.

Week 11: Before attending endpoint onsite visit, participants will complete a final dietary assessment. The same assessment as Pre-baseline and Week 6 will be used.

Endpoint, Visit 2 (Week 12): At the end of 12 weeks, participants will be invited back for a follow-up assessment. Blood and urine samples and anthropometrics will be collected/recorded. Participants will complete the Stage of Change questionnaire and Food Waste questionnaire. A Tolerance questionnaire will be administered to see how willing participants would be to continue with their prescribed diet.

Inclusion Criteria:

• Provide written informed consent
• Be a healthy adult between the ages of 18-64 years old
• Consume a diet associated with moderate-high greenhouse gas emissions
• Be in good general health

Exclusion Criteria:

• Are pregnant, lactating or planning to become pregnant
• Are following a medically prescribed diet
• Have a diagnosis of an acute or chronic medical condition that could interfere with the outcomes of the study. Such diagnoses include (but are not limited to) cardiovascular disease, diabetes mellitus, inflammatory bowel disease, cancers (within the last 5 years), etc.
• Are immunocompromised or have a suspected immunodeficiency
• Have excessive alcohol intake (>28 units per week)
• Have a known food allergy
• Regularly consume a single high-dose vitamin or mineral supplement
• Are participating in another research study
• Are unable to read, write or understand English.

At the start of the study each participant will have a study code assigned to them. All of the data from the trials will be stored using these unique study codes. A file will be set up which will be stored in a location separate to the study data. This file will contain a list which will link each participants contact details to the corresponding ID code. All stored information will be encrypted and protected by a password that only the researcher and PI will know.

Single-blinded parallel study