This clinical trial investigates whether a high-protein diet combined with resistance training is more effective than a standard low-calorie diet for maintaining fat mass loss in individuals with obesity. A total of 60 adults with obesity (BMI ≥30 kg/m²) will be randomly assigned at baseline to one of two groups: 1. A control group receiving a balanced low-calorie diet. 2. An intervention group receiving a high-protein, low-calorie diet and supervised resistance training. All participants will follow their assigned low-energy diets for the first 10 weeks, which constitutes the weight loss phase. The subsequent 42 weeks comprise the weight maintenance phase, where participants will follow an energy-balanced diet adapted to their individual needs and continue in their assigned group. The intervention group will also complete supervised resistance training three times per week during the first 10 weeks, with continued follow-up throughout the maintenance phase. Assessments will take place at baseline, week 11, 6 months, and 12 months, and include measurements of fat and lean mass, resting metabolic rate, appetite hormones, glucose metabolism, inflammation, gut microbiota and its metabolites, hedonic appetite, eating behavior, and quality of life and physical function. Participants will also provide food intake records, physical activity dat. The primary aim is to determine whether the combined high-protein diet and resistance training intervention results in greater long-term fat mass loss and muscle mass preservation than the control diet. Secondary aims include effects on appetite regulation, eating behavior, metabolic health, and patient-reported outcomes. The study is conducted at NTNU and St. Olavs Hospital in Trondheim, Norway, and registered under the acronym PRO-FIT.