Norwegian patients with severe mental illnesses (SMI), such as schizophrenia spectrum or bipolar disorder, lose on average 10 years of life compared to mentally healthy individuals. Much of this gap is due to heart disease. Unhealthy lifestyle habits, including poor diet and physical inactivity, contribute to higher levels of metabolic risk factors for heart disease in this population. The goal of this clinical trial is to find out if a lifestyle program including dietary counselling and regular physical exercise can help people with SMI to improve their physical and mental health. The main questions it aims to answer are: * Does adherence to a healthy lifestyle program lead to reduced estimated risk of heart disease? * Does it change lifestyle habits, body weight and composition, and metabolic risk markers over six months? * Can participants with severe mental illness complete a healthy lifestyle program, and do they find it acceptable? Researchers will compare two groups: one that receives the lifestyle program in addition to regular mental health care, and one that receives regular care only. During the six month program, participants in the lifestyle group will: * Meet with a clinical dietitian once a month for dietary counselling * Take part in group-based physical activity sessions once a month, and receive support to follow a personal training plan Around 70 adults will take part in the study. The results may help improve the way lifestyle support is offered to people living with severe mental illness and inform health care providers about strategies to improve physical health in this vulnerable group.