Background: The majority of adults in the UK consume breakfast daily , yet this meal is typically lower in protein and higher in carbohydrates than other eating occasions. While carbohydrate-rich breakfasts can result in higher physical activity thermogenesis and glycaemia than morning fasting, less is known about whether enriching the morning meal with protein may enhance energy expenditure, moderate glycaemia, and improve appetite control.

Aims/Objectives: The aim of this study was to establish the short/mid-term (i.e., days; weeks) free-living behavioural and metabolic responses when fortifying breakfasts with protein, relative both to isocaloric carbohydrate-rich breakfasts and to complete omission of morning meals.  

Methods: Thirty-four healthy adults (11 M/23 F; 35 ± 6 years; 23.6 ± 2.9 kg·m^-2) were randomised to one of three 28-day interventions: (1) a daily carbohydrate-rich breakfast (providing 7.3 mg∙kJ RMR^-1 of carbohydrate per breakfast meal) before 1200 h (n = 11), (2) a daily whey protein-enriched breakfast (~6 mg∙kJ-1 of carbohydrate per breakfast meal with an additional 15 g of whey protein to match the carbohydrate-rich breakfast for energy content) before 1200 h (n = 10), or (3) daily extended morning fasting until 1200 h (n = 13). Participants attended the laboratory on three occasions: preliminary screening, baseline postprandial and anthropometric assessment, and follow-up assessment. Participants also completed 7 days free-living monitoring of physical activity and energy intake prior to, and during weeks 1 and 4 of the intervention.

Results Four weeks of daily carbohydrate-rich (1264 ± 341 kcal·d⁻¹), protein-enriched (1061 ± 470 kcal·d⁻¹) or extended morning fasting (1261 ± 470 kcal·d⁻¹) did not result in significant differences in physical activity thermogenesis (Group p = 0.51), self-reported energy intake (Condition x Time p = 0.18), body mass (Group × Time p = 0.68), or postprandial metabolic responses (Condition x Time x Timepoint p > 0.05). No changes in resting metabolic rate, dietary-induced thermogenesis, substrate oxidation, or appetite were observed.

Summary: The current study provides no evidence that daily consumption of either a protein-enriched, or carbohydrate-rich breakfast did not result in differences in energy balance, or metabolic control compared to extended morning fasting.