Growing evidence supports early eating to control appetite and energy balance but there are few controlled studies to assess the amount and/or type of breakfast meal. This randomized, within-participant, diet intervention examined the effects of higher-fibre (HF) and higher-protein (HP) breakfasts in adults with overweight/obesity. Nineteen healthy adults consumed two randomized 28 day weight loss (WL) diets; as higher-fibre (HFWL) or higher protein (HPWL), with all food provided. Both WL diets were designed as 45%, 35% and 20% of calories to be consumed at the morning, afternoon and evening, respectively. The primary outcome was energy balance, analysed by body weight changes. The secondary outcomes were gut health (assessed by changes in faecal microbiota composition and microbial metabolite concentrations) and subjective appetite assessed with visual analogue scales (VAS). There was a diet effect on WL, with mean loss of -4.87 kg and -3.87 kg for the HFWL and HPWL diets, respectively (P=0.002). The HPWL diet was superior to the HFWL diet for suppressing subjective appetite (P=0.003). The faecal microbiota analysis showed beneficial groups of bacteria, including bifidobacteria, and the butyrate-producers Faecalibacterium and Roseburia, were significantly increased in proportional abundance on the HFWL diet. Breakfast composition has an important role in influencing subjective appetite with the higher-protein diet promoting greater feelings of satiety. The proportional abundance of putatively beneficial groups of gut microbiota was markedly higher on the fibre-enriched diet, which may be preferable for gut health.