Food-anticipatory rhythms under 24-hour schedules of limited access to single macronutrients.

Food-restricted rats anticipate a fixed daily mealtime by entrainment of a circadian timekeeping mechanism separate from that which generates daily light-entrainable activity rhythms. The entrainment pathways and rhythm-generating substrates for food-anticipatory rhythms are unknown. In this study, we attempted to define minimal food-related stimuli necessary or sufficient for food anticipation by employing schedules of restricted macronutrient availability, with or without free access to a complementary diet. Rats did not anticipate a daily meal of protein, carbohydrate, or fat, as measured by tilt-cage, running-wheel, or food-bin activity, when they had free access to other nutrients. However, rats did anticipate single-macronutrient meals when they were limited to only two, larger, complementary meals each day (protein-fat, protein-carbohydrate) providing a reduced total number of calories. Previous work has shown that caloric restriction per se is not a prerequisite for food anticipation. In combination with that study, the present results indicate that the size of a nutrient meal, in absolute terms or relative to total daily nutrient intake, is of pre-eminent importance in determining its value as a synchronizer of anticipatory rhythms. The results further suggest that physiological responses unique to the ingestion and absorption of any particular macronutrient are not necessary components of the entrainment pathway.