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Following restrictions on the use of antimicrobial agents as growth promoters, increasing attention has been directed towards feed additives that may support animal health and production performance. Fumaric acid, vitamin C and vitamin E possess antioxidant properties and may therefore contribute to the mitigation of oxidative stress. However, enrofloxacin, a fluoroquinolone commonly used for the treatment of bacterial infections in poultry, may promote the formation of reactive oxygen species and consequently modify redox homeostasis. The aim of our study was to investigate the effects of a feed additive containing fumaric acid, vitamins C and E on the redox homeostasis of broiler chickens, both alone and in combination with enrofloxacin treatment.

Male Ross-308 broiler chickens were allocated to four experimental groups, with 12 birds in each group. The control group was not subjected to any antioxidant or antibiotic treatment. The antioxidant-supplemented group (Ao) received fumaric acid (20 g/kg feed), vitamin C (0.07 g/kg) and vitamin E (0.038 g/kg) from day 1 to day 21. The antibiotic group (Ab) was treated with enrofloxacin at a dose of 10 mg/kg body weight from day 15 to day 19, while the combined-treatment group (Ao+Ab) underwent both antioxidant supplementation and enrofloxacin treatment according to the respective treatment schedules. On day 21, blood samples were collected from the wing vein. Antioxidant capacity, protein carbonyl, malondialdehyde (MDA), total glutathione, reduced glutathione (GSH) and oxidized glutathione (GSSG) concentrations were determined.

Compared with the control group, all treatments significantly decreased protein carbonyl concentrations. However, the antioxidant-supplemented group showed a significantly higher MDA concentration than the other groups, indicating enhanced lipid peroxidation. Enrofloxacin treatment alone did not significantly alter antioxidant capacity, whereas antioxidant supplementation resulted in a significant decrease compared with the enrofloxacin-treated group. Combined antioxidant and enrofloxacin treatment also significantly decreased antioxidant capacity compared with both the control and Ab groups. Among the investigated parameters of the glutathione system, only total glutathione concentration showed a significant difference: it was significantly lower in the Ab group than in the control and Ao+Ab groups.

Our results indicate that combined supplementation with fumaric acid, vitamins C and E, despite reducing protein oxidative damage, may increase lipid peroxidation and disturb redox homeostasis at the investigated concentrations. The antioxidant supplementation might attenuate the inhibitory effect of enrofloxacin on glutathione synthesis. These findings suggest that the simultaneous administration of different antioxidant compounds should be carefully considered, even when they are applied at recommended concentrations.