Environmental impacts of food systems have stimulated research to examine how to create healthy diets that will be more sustainable while meeting nutrient requirements. Increasing compliance with existing food-based dietary guidelines in most jurisdictions could be a first step to improve health and reduce environmental impact. MyPlanetDiet was an all-Ireland 12-week randomised controlled trial designed to inform sustainable healthy dietary guidelines. Healthy adults (n 355) aged 18–64 years with moderate-to-high greenhouse gas emitting (GHGE) diets were recruited from three study sites on the island of Ireland. The aim of this research is to assess the relationship between dietary intakes, diet-related environmental impacts and metabolic health using baseline data collected during the MyPlanetDiet study. Dietary assessments collected using Foodbook24 were used to calculate diet-related GHGE, adherence to healthy eating guidelines (HEG) and healthy eating index (HEI) score. Anthropometrics and metabolic health markers (e.g. lipids, glucose and insulin) were included. Overall HEG adherence was low, with 43 % meeting zero or one HEG food group recommendations. Adherence to 4 + HEG food group targets was associated with 31 % lower diet-related GHGE compared with those with lowest adherence. Higher HEG adherence was associated with lower BMI and waist circumference and higher HEI scores. While our findings suggest HEG adherence is associated with positive health and environmental impacts, substantial behaviour change will be needed to meet existing HEG. Further research is needed to assess response and acceptability to HEG. However, adherence to HEG may be an important first step to reducing the environmental impact of food consumption.