The effects of high-carbohydrate vs high-fat breakfasts on feelings of fullness and alertness, and subsequent food intake.

Fourteen subjects consumed four realistic isoenergetic (2035 kJ) breakfasts, varying in macronutrient content (two fat-rich, two carbohydrate-rich (low- and high-fibre)), in random order on separate mornings. After breakfast, subjects left the laboratory and completed appetite and alertness ratings at specific times and recorded all subsequent fluid and food intake for the rest of the day. The high-fibre, carbohydrate-rich breakfast was the least palatable but most filling meal and was associated with less food intake during the morning and at lunch. Hunger returned at a slower rate after this meal than after the low-fibre, carbohydrate-rich meal. Both fat-rich breakfasts were more palatable but less satiating than the carbohydrate-rich meals and were followed by greater food intake during the morning, which may be a compensatory response to ingest a sufficient amount of food and/or carbohydrate to match the level of fullness produced by the subjects' habitual breakfasts. By the end of the day, the average total energy intake was significantly greater after the fat-rich EB meal than after the high-fibre, carbohydrate-rich meal (P < 0.05). Total day fat intakes were also significantly greater when the high-fat breakfasts were eaten. For every individual test, alertness ratings increased immediately after breakfast was consumed. On average, the high-fibre carbohydrate-rich meal was associated with the highest post-breakfast alertness ratings and with the greatest cumulative amount of alertness during the period between breakfast and lunch (AUC). Alertness AUC values up until lunch correlated positively with fullness AUC values (r = 0.36, P < 0.01, n = 56). The results confirm the relatively weak satiating power of fat-rich meals observed in controlled laboratory-based studies and indicate that a high-fibre, carbohydrate-rich breakfast may assist weight control efforts by maintaining fullness. Further research is required to determine whether satiety directly enhances alertness and whether low-GI carbohydrate-rich meals enhance alertness to a greater degree than high-GI meals.