BACKGROUND:

A water diuresis occurs when a large volume of water is ingested rapidly. Nevertheless, water conservation is required to provide a source of water for evaporative heat dissipation throughout the day. Therefore, the objective was to define conditions that permit the retention of ingested water.

METHODS:

Volunteers collected urine q2h plus an overnight specimen; water loading was conducted after overnight food and water restriction; paired arterialized and venous blood samples were analyzed.

RESULTS:

When 20 mL water/kg was consumed in <15 minutes, the peak urine flow rate was 11 +/- 0.6 mL/min. The volume of water retained after water intake stopped, and when the urine was hyperosmolar, correlated directly with the daily excretion of sodium plus potassium (r(2)= 0.63). The plasma sodium concentration (P(Na)) was 4.0 +/- 0.5 mmol/L lower in arterialized than paired venous blood 30 to 40 minutes after water ingestion began (P < 0.01). In preliminary studies, the smallest water load consumed in 15 minutes that would reproducibly cause a water diuresis was defined in each subject. This same acute water load was retained, however, if it contained 150-mmol/L fructose, but not glucose, or if it was consumed slowly (sipping). The arterialized P(Na) was not significantly lower than in paired venous samples when water was sipped.

CONCLUSION:

A large fall in arterialized and not venous P(Na) best reflected the signal to induce a water diuresis. Although a very large water load can induce a water diuresis, smaller water loads can be retained for future heat dissipation.