Objective: To compare the effect of isoenergetically-dense, high-protein (HP), high-fat (HF) or high-carbohydrate (HC) breakfasts (at 08.30) on subjective hunger, fullness and appetite (measured hourly on a 100 mm visual analogue scale), macronutrient balance and ad libitum energy intake (EI), at a test meal (13.30) and throughout the rest of the day (until 23.00).
Design: Six men each spent 24 h in a whole-body indirect calorimeter on three separate occasions during which they received breakfasts designed to match 75% of BMR and that comprised, on average 3.1 MJ of protein (HP), carbohydrate (HC) or fat (HF), respectively, the remainder being split between the other two macronutrients. Every item of the ad libitum diet comprised 13% protein, 40% fat and 47% carbohydrate by energy, with an energy density of 550 kJ/100 g.
Results: Subjectively-rated pleasantness did not differ between the breakfasts, or any of the subsequent ad libitum meals. Subjective hunger was significantly greater during the hours between breakfast and lunch after the HF (26) treatment relative to the HP (18) or HC (18 mm) meals (P < 0.001), although the HP treatment suppressed hunger to a greater extent than the other two treatments over 24 h. However, mean ad libitum lunch intakes were similar at 5.38, 5.30 and 5.18 MJ (NS) on the HP, HC and HF treatments, respectively. After-lunch intakes were also very similar at 6.14, 6.18 and 5.83 MJ (NS). Mean 24-h energy expenditure amounted to 11.12, 11.14 and 10.93 MJ, respectively, producing energy balances of 5.71, 5.83 and 5.04 MJ (NS), respectively. The HP, HF and HC breakfasts led to enhanced P, F and C oxidation, respectively (P < 0.003).
Conclusions: Large HP, HC or HF breakfasts led to detectable changes in hunger that were not of sufficient magnitude to influence lunch-time intake 5 h later, or EI for the rest of the day. A single positive balance of each macronutrient can be buffered by oxidation and storage capacity, without leading to changes in meal-to-meal EI, when subjects feed ad libitum on unfamiliar diets of fixed composition.