Effect of Salt Intake on Renal Excretion of Water in Humans

Abstract

Two studies were performed to determine the quantitative relationship between salt intake and urinary volume (U v ) in humans. In study 1, 104 untreated hypertensives were studied on the fifth day of a high- and a low-salt diet. The 24-hour U v was 2.2 L (urinary sodium [U Na ] 277 mmol) on the high-salt diet and decreased to 1.3 L ( P <0.001) (U Na 20.8 mmol) on the low-salt diet. The reduction in 24-hour U v was significantly related to the decrease in 24-hour U Na ( P <0.001) and predicts that a 100-mmol/d reduction in salt intake would decrease 24-hour U v by 367 mL. In study 2, 634 untreated hypertensives were studied on their usual diet. There was a significant relationship between 24-hour U v and U Na ( P <0.001). This predicts that a 100-mmol/d reduction in salt intake would decrease 24-hour U v by 454 mL. The International Study of Salt and Blood Pressure (INTERSALT) of 1731 hypertensives and 8343 normotensives on their usual diet showed that 24-hour U v was significantly related to U Na ( P <0.001) and predicted that a 100-mmol/d reduction in salt intake would decrease 24-hour U v by 379 and 399 mL in hypertensives and normotensives, respectively. These findings document the important effect that salt intake has on U v . The recommended reduction in salt intake in the general population is from 10 to 5 g/d. This would reduce fluid intake in the population by ≈350 mL/d per person. This would have a large impact on the sales of soft drinks, mineral water, and beer.