Diet can play a key role in atherosclerosis and coronary heart disease (CHD), but little interventional data exists, and the mediators of possible anti-atherosclerotic effects of diet are unclear. The investigators will investigate if a healthy Nordic diet (HND) reduces plaque volume, coronary artery calcification (CAC), and plaque inflammation (FAI) in CHD, and examine if changes in gut microbiota may be linked to plaque progression over time. The investigators will also explore if the diet response can be predicted by the metabolic phenotype. In total 150 CHD patients is randomized to a HND rich in unsaturated fat and fibre from plants, or to a usual care diet for 18 months. Plaque volume and composition is assessed by CT, and fecal microbiota composition is determined by deep metagenome shotgun sequencing. CHD and metabolic risk factors, liver fat, muscle fat and biomarkers of diet adherence (plasma fatty acids, whole-grain metabolites) are measured. Machine-learning is used to identify diet responders on plaque progression, based on the individual microbiome and metabolome. If a HND reduces plaque progression, this would be novel information of clinical importance. Also, if the diet alters microbiota that are linked to plaque progression, this would be of high scientific interest. Finally, potential prediction of the diet-response would open up for more personalized treatment of atherosclerosis.