The effect of protein or carbohydrate breakfasts on subsequent plasma amino acid levels, satiety and nutrient selection in normal males.

Normal subjects were fed protein or carbohydrate breakfasts. Both meals were in the form of a chocolate pudding and had similar sensory qualities. At lunchtime subjects were allowed to select from a buffet. The protein breakfast had a greater satiating power than the carbohydrate breakfast, but there was no difference in overall selection of protein or carbohydrate at lunchtime. However, the carbohydrate breakfast did decrease selection of apple, the only pure carbohydrate food available at lunchtime. In a second experiment changes in plasma amino acid levels were studied after subjects received carbohydrate breakfasts containing 0, 4, 8 or 12% protein, or a danish pastry. Only the 0% protein breakfast increased tryptophan availability to the brain. These experiments were performed to test the hypothesis that alterations in brain 5-hydroxytryptamine, brought about by dietary alterations in brain tryptophan, regulate selection of protein and carbohydrate. The results suggest that this mechanism was not operating in our experiments.