Water scarcity and conservation and their role in obesity in nature and in humans.

Increasing temperatures and water scarcity pose threats to animals living in the wild and humans. Here, we review biological mechanisms animals use to prevent dehydration. Fat and glycogen generate water during metabolism that can be used by many animals as a source of water. In hibernating animals, fat production is stimulated in the autumn by a vasopressin-dependent, carbohydrate-based metabolism that leads to thirst, increased water intake, and storage of glycogen and fat. As fall advances, the animals switch to fat-based metabolism with falling vasopressin levels, and actual entrance into torpor can be triggered when water becomes unavailable and/or unpredictable. Once in torpor, metabolic water is generated by fat metabolism along with a suppression of vasopressin and fall in serum osmolality that blocks thirst. We suggest that water production from fat does not keep up with demands, and that respiratory acidosis also develops as a consequence of hypoventilation, and this leads to the necessity of interbout arousals (IBA), in which the animal rewarms with a switch to carbohydrate metabolism that causes a rapid increase in water availability from the breakdown of glycogen that facilitates the ventilation needed to correct the acidemia. The animal then drops its metabolic rate again, allowing fat metabolism to continue. The observation that water deficit may be a stimulus for fat storage in hibernation carries significance for human obesity, especially in response to salt and sugar, as it suggests that hydration may be protective. These studies also provide an understanding of how glucagon-like peptide-1 agonists may cause weight loss.