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Alterations of cell volume induced by changes of extracellular osmolality have been reported to regulate intracellular metabolic pathways. Hypo-osmotic cell swelling counteracts proteolysis and glycogen breakdown in the liver, whereas hyperosmotic cell shrinkage promotes protein breakdown, glycolysis and glycogenolysis. To investigate the effect of acute changes of extracellular osmolality on whole-body protein, glucose and lipid metabolism in vivo, we studied 10 male subjects during three conditions: (i) hyperosmolality was induced by fluid restriction and intravenous infusion of hypertonic NaCl (2-5%, wt/vol) during 17 h; (ii) hypo-osmolality was produced by intravenous administration of desmopressin, liberal water drinking and infusion of hypotonic saline (0.4%); and (iii) the iso-osmolality study comprised oral water intake ad libitum. Plasma osmolality increased from 285±1 to 296±1 mosm/kg (P<0.001 during hyperosmolality, and decreased from 286±1 to 265±1 mosm/kg during hypo-osmolality (P<0.001). Total body leucine flux ([1-13C]leucine infusion technique), reflecting whole-body protein breakdown, as well as whole-body leucine oxidation rate (irreversible loss of amino acids) decreased significantly during hypo-osmolality. The glucose metabolic clearance rate during hyperinsulinaemic-euglycemic clamping increased significantly less during hypo-osmolality than iso-osmolality, indicating diminished peripheral insulin sensitivity. Glycerol turnover (2-[13C]glycerol infusion technique), reflecting whole-body lipolysis, increased significantly during hypo-osmolar conditions. The results demonstrate that the metabolic adaptation to acute hypo-osmolality resembles that of acute fasting, that is, it results in protein sparing associated with increased lipolysis, ketogenesis and lipid oxidation and impaired insulin sensitivity of glucose metabolism.