Migratory refueling affects non-enzymatic antioxidant capacity, but does not increase lipid peroxidation

• Migrating birds engage in over-eating, which in mammals causes oxidative stress.
• We experimentally tested if over-eating causes oxidative stress in migratory birds.
• Individuals over-eating had higher antioxidant capacity than control individuals.
• There was no difference between these individuals in oxidative damage to lipids.
• Migrants may use the antioxidant properties of uric acid to forego oxidative damage.

All aerobic organisms are to some degree affected by oxidative stress, which is an imbalance between pro-oxidants and antioxidants in favor of the former. Pro-oxidants can damage DNA, proteins and lipids, and as such oxidative stress can carry considerably fitness costs. In mammals excessive calorie intake is a known cause of oxidative stress. We investigated whether in migrant birds, which typically engage in over-eating in between flights (refueling), high food intake causes oxidative stress. In an experiment we compared levels of plasmatic total non-enzymatic antioxidant capacity (AOX) and oxidative damage (lipid peroxidation) between migrants repeatedly fasted and refed (simulating the flight-refuel cycle of wild migrants), and migrants on ad libitum food. We found that refueling increased AOX, an effect mainly attributable to an increase in uric acid level, an antioxidant that is produced during protein metabolism. Accordingly, variation in AOX was mainly explained by the refueling birds' food intake. However, food intake in migrants on ad libitum food did not explain any variation in AOX. Refueling did not affect lipid peroxidation, nor were its levels explained by food intake. We propose that over-eating migrants retain uric acid, which might be a very low cost mechanism to forego oxidative damage.