Glucose, but not fat, phase shifts the feeding-entrained circadian clock

To study the ability of single macronutrients to entrain or phase shift the feeding entrainable circadian oscillator, rats with lesions of the suprach iasmatic nucleus were first maintained on a single daily meal of lab chow u ntil robust anticipatory approaches to the feeder or anticipatory wheel run ning was established. The meal time was then delayed by 8 h and chow was re placed with a 25-mL solution of 0.2% saccharin or 25 ml,of saccharin plus 1 5 g of glucose. For other phase shifts, rats received either 6 mt of vegeta ble oil or mineral oil for 2 consecutive days. Consumption of about 6 g (24 kcal) or more of glucose resulted in robust delaying transients on the day s after ingestion, whereas saccharin induced only small delays consistent w ith the initiation of a free-running rhythm with a period greater than 24 h . Surprisingly, consumption of 5.5 g of vegetable oil (47 kcal) did not res ult in delays greater than those in rats receiving mineral oil. The introdu ction of oil also produced a severe reduction in approaches to the feeder w hich could be alleviated by placing inaccessible chow in the feeders betwee n oil meals. Phase shifts with oil were repeated with rats housed in wheels using anticipatory wheel running as a phase marker to assess whether the r ack of phase shifts with fat was apparatus dependent. As was the case with approach behavior, anticipatory wheel running was not significantly delayed by vegetable oil consumption. These results indicate that a simple monosac charide, glucose, has zeitgeber properties for the feeding entrainable osci llator. Vegetable oil, despite a higher caloric content, may be ineffective because of slower gastric emptying and nutrient absorption or because fat is not a good zeitgeber for the feeding entrained circadian oscillator. (C) 1998 Elsevier Science Inc.