Refeeding after the late increase in nitrogen excretion during prolonged fasting in the rat

Recovery of body mass, food intake and body composition was studied in the laboratory rat after the late increase in nitrogen excretion that characterizes prolonged fasting in mammals and birds. The rats lost 43% of their body mass during 13 days of food deprivation. They all regained their prefasting body mass within a shorter period of 11 days of refeeding. These results confirm that the late increase in nitrogen excretion in rats, as in spontaneously fasting birds, is reversible and is a part of the physiological adaptations to long-term food deprivation. Water intake of the rats continuously decreased during fasting, and the animals virtually stopped drinking as protein utilization increased. On refeeding, changes in water intake paralleled those in food intake. The refed rats progressively increased their daily food intake, that was always higher than the prefasting value (8.0-10.4 vs. 6.7% of body mass). The comparison of organ weights between fed and ad lib refed rats of similar body weight indicates that muscle mass was regained earlier than body fat during refeeding. The laboratory rat therefore appears to be a good experimental model to investigate the metabolic and behavioural changes that occur during spontaneous anorexia and refeeding in wild animals.