Glucose utilization rate and pancreatic hormone response to oral glucose loads are influenced by the migratory condition and fasting in the garden warbler (Sylvia borin).

Substrate utilization and regulatory mechanisms of metabolism were studied in migratory garden warblers by measuring plasma levels of glucose, free fatty acids (FFAs), beta-hydroxybutyrate, insulin and glucagon in response to oral glucose loads. Three different physiological states were examined: (a) the autumnal migratory period on a high and (b) on a fasted low body mass level, and (c) the postmigratory period with low body mass. Glucose tolerance was better in the postmigratory lean than fat condition. However, total food deprivation of 5-7 days with fat birds reaching their lean body mass further reduced the glucose utilization rate. Initial levels of FFAs were highest in the starved, intermediate in the fat and lowest in the lean condition. Changes in plasma FFAs during glucose tolerance tests were opposite to those of the glucose levels. Ten minutes after the glucose load plasma glucagon levels decreased and insulin increased. These effects were larger in the fat than in the postmigratory lean condition. There were no differences between sexes. It appears that during premigratory and migratory periods glucose utilization may be inhibited by a more favorable oxidation of fatty acids as would be predicted by the glucose fatty acid cycle. However, the inhibition of glucose utilization seems to be counterregulated by stronger responses of insulin and glucagon. These results may be important also in the consideration of food selection during premigratory periods and refueling abilities of birds crossing ecological barriers.