Of the importance of metabolic phases in the understanding of oxidative stress in prolonged fasting and refeeding.

Life phases such as migration or reproduction may partly (or totally) prohibit food accessibility, making regulation of the energy balance one of the main challenges faced by wild organisms. Although long-term fasting is common in a number of species, it has been reported to be detrimental for the organism, notably because it induces oxidative stress. However, fasting metabolism is characterized by successive metabolic adaptations that are likely to be different in terms of stress, and no previous studies, to our knowledge, have tested the dynamics of changes in oxidative balance in relation to the metabolic phases of fasting. Our study provides a first insight into this relationship by inducing prolonged fasting and subsequent refeeding in captive mallards (Anas platyrhynchos). Both plasmatic antioxidant and oxidative damage levels were observed to be lower during fasting. Oxidative damage levels decreased by 95% during fasting, and they surprisingly remained at low levels even after 3 d of refeeding. In contrast, restoration of the antioxidant barrier was observed from the third day of refeeding onward, and it thus preceded the increase in oxidative damage levels. Therefore, under our experimental conditions, long-term fasting was not associated with marked oxidative stress in mallards. Because data interpretations may be influenced by concomitant metabolic states, we underline the importance of taking metabolic phases into account when studying oxidative stress during fasting.