The dataset contains data for the study investigating the effect of 3-weeks high (SWEET) versus low (PLAIN) sugar breakfast on energy balance, metabolic health, and appetite. 29 healthy adults (22 women) completed this randomised crossover study. Participants had pre- and post-intervention appetite, health and body mass outcomes measured, and recorded diet, appetite (visual analogue scales) and physical activity for 8 days during each intervention. Interventions were 3-weeks iso-energetic SWEET (30% by weight added sugar; average 32g sugar) versus PLAIN (no added sugar; average 8g sugar) porridge-based breakfast. The main finding from the data is that energy balance, health markers, and appetite did not respond differently to 3-weeks of high or low sugar breakfasts.