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Restricted daily food access acts as an entraining stimulus (zeitgeber) for a circadian clock, the feeding-entrainable oscillator (FEO). There are many properties of a daily meal that could potentially convey timing information to the FEO. Olfactory cues associated with feeding are one such property. In order to rule out olfaction as a necessary entraining stimulus, olfactory bulbectomized and sham-operated male Sprague–Dawley rats had access to food for 2 h each day. Food bin approach behavior was monitored as an index of food-anticipatory activity (FAA). Both groups entrained to the daily meal with an increase in feeder approach time several hours before meal onset. There were no significant differences in the timing or the amount of FAA between groups. Furthermore, FAA was maintained during 3 days of food deprivation in both groups. In accordance with previous studies, the results show that olfactory cues are not necessary for the entrainment of FAA.

To study the ability of single macronutrients to entrain or phase shift the feeding entrainable circadian oscillator, rats with lesions of the suprachiasmatic nucleus were first maintained on a single daily meal of lab chow until robust anticipatory approaches to the feeder or anticipatory wheel running was established. The meal time was then delayed by 8 h and chow was replaced with a 25-mL solution of 0.2% saccharin or 25 mL of saccharin plus 15 g of glucose. For other phase shifts, rats received either 6 mL of vegetable 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 days after ingestion, whereas saccharin induced only small delays consistent with 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 result in delays greater than those in rats receiving mineral oil. The introduction of oil also produced a severe reduction in approaches to the feeder which could be alleviated by placing inaccessible chow in the feeders between 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 lack 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 monosaccharide, glucose, has zeitgeber properties for the feeding entrainable oscillator. 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.