Digestive flexibility during fasting in fish: a review.

Digestive flexibility is important because it allows an animal to maximize energy and nutrient return from the diet consumed, and also to reduce the maintenance costs associated with one of the body's most expensive systems in terms of energy and protein requirements. Two different patterns of digestive flexibility have been described for vertebrates, one for species in which metabolic costs of homeostasis are relatively high and the gut is rarely empty (e.g., mammals and birds), and one for species in which metabolic costs of homeostasis are relatively low and the gut usually spends long periods of time empty (e.g., amphibians and reptiles). In this review we analyze the information on digestive tract down-regulation during fasting in fish, in order to evaluate the extent to which digestive flexibility in fish conforms to that in other species. We found that: (1) gut size decay during long-term fasting in fish appears to be almost linear with time, even for very long fasting periods. Thus, gut size temporal dynamics in fish during long-term fasting resemble those observed in some mammals species; (2) by contrast, histological changes during fasting in fish are more similar to those described for amphibians and reptiles; and (3) data on enterocyte turnover rates indicate that cell turnover times in fish are relatively short, and although longer than those observed in mammals, they are not very different from those reported for birds. In conclusion, current data suggest that both mechanisms, cell turnover rates and change in epithelial configuration, probably are involved in digestive tract regulation in fish.