Background: Popular beliefs that ‘breakfast is the most important meal of the day’ are grounded in cross-sectional observations linking breakfast to health, the causal nature of which remains to be explored under real-life conditions.
Objective: To conduct a randomized controlled trial examining causal links between breakfast habits and all components of energy balance in free-living humans.
Design: The Bath Breakfast Project is a randomized controlled trial with repeated-measures at baseline and follow-up amongst a cohort in South-West England aged 21-60 y with Dual-Energy X-Ray Absorptiometry (DXA)-derived fat mass indices ≤11 kg·m-2 (women; n=21) and ≤7.5 kg·m-2 (men; n=12). Components of energy balance (resting metabolic rate, physical activity thermogenesis, energy intake) and 24-h glycemic responses were measured under free-living conditions with random allocation to daily breakfast (≥700 kcal before 1100 h) or extended fasting (0 kcal until 1200 h) for 6 weeks, with baseline and follow-up measures of health markers (e.g. hematology/biopsies).
Results: Contrary to popular belief, there was no metabolic adaptation to breakfast (e.g. resting metabolic rate stable within 11 kcal·d-1), with limited subsequent suppression of appetite (energy intake remained 539 kcal·d-1 greater than fasting; 95%CI=157, 920). Rather, physical activity thermogenesis was markedly higher with breakfast than fasting (442 kcal·d-1; 95%CI=34, 851). Body mass and adiposity did not differ between treatments at baseline or follow-up and neither did adipose tissue glucose uptake or systemic indices of cardiovascular health. Twenty-four hour glycemia was more variable during the afternoon/evening with fasting than with breakfast by the final week of intervention (2%; 95%CI=0.1, 8).
Conclusions: Daily breakfast is causally linked to higher physical activity thermogenesis in lean adults, with greater overall dietary energy intake but no change in resting metabolism. Cardiovascular health indices were unaffected by either treatment but breakfast maintained more stable afternoon/evening glycemia than fasting.