Meeting Abstract
Previously, we found that house sparrows (Passer domesticus)(~28g) can fast for only 24 h, if their fast begins after feeding in the morning and that their fast did not follow the three-phase model found in many species. Recently, however, we found that house sparrows can sustain a 43 h fast if it begins when they go to roost. We tested the prediction that, under such circumstances, fasting house sparrows sequentially oxidize substrates in the classic order: carbohydrates – lipids – protein. We analyzed the birds’ breath for 13CO2 at regular intervals for 43 h after feeding them millet enriched with 13C-(glucose, palmitic acid, or leucine) for 12 days. We also analyzed blood plasma. Contrary to our predictions, the main fuels oxidized during the first 21 h were carbohydrates and lipids, while during the subsequent 22 h, protein and lipids were metabolized. In the final 4 h, protein was the main fuel oxidized. Also, glucose levels followed a circadian rhythm; ketone levels increased until hour-31, then decreased significantly until hour 43; and glycerol levels decreased during the first 13 h and then stabilized at low levels. Thus, house sparrows that begin fasting at sundown have only two phases of metabolic fuel use; first carbohydrates and lipids, then lipids and protein. This enables the birds to fast for longer than when they began to fast after feeding in the morning. Interestingly, these results mesh closely with those of our earlier study.
