Patterns of physiological flexibility in response to fasting are well established, but much less is known about the contribution of water deprivation to the observed effects. We investigated body composition and energy and water budget in three groups of zebra finches: birds with access to food and water, food-deprived birds having access to drinking water and food-and-water-deprived birds. Animals were not stimulated by elevated energy expenditure and they were in thermoneutral conditions; thus, based on previous studies, water balance of fasting birds was expected to be maintained by increased catabolism of proteins. In contrast to this expectation, we found that access to water did not prevent reduction of proteinaceous tissue, but it saved fat reserves of the fasting birds. Thus, water balance of birds fasting without access to water seemed to be maintained by elevated fat catabolism, which generated 6 times more metabolic water compared with that in birds that had access to water. Therefore, we revise currently established views and propose fat to serve as the primary source for metabolic water production. Previously assumed increased protein breakdown for maintenance of water budget would occur if fat stores were depleted or if fat catabolism reached its upper limits due to high energy demands.
The authors declare no competing or financial interests.
All authors took part in collecting the data and interpreting the results. J.R. carried out the statistical analyses and lead writing of the manuscript; E.T.S. carried out BMR measurements and tissue analyses; M.C. helped draft the manuscript; U.B. conceived, designed and coordinated the study and helped draft the manuscript. All authors gave final approval for publication.
The study was financed by the Jagiellonian University (Uniwersytet Jagielloński w Krakowie) statutory funds: DS 757 (K/ZDS/004151).
Supplementary information available online at http://jeb.biologists.org/lookup/doi/10.1242/jeb.138966.supplemental
- Received February 15, 2016.
- Accepted June 14, 2016.
- © 2016. Published by The Company of Biologists Ltd