Effect of salt intake on renal excretion of water in humans

Two studies were performed to determine the quantitative relationship betwe en salt intake and urinary volume (U-v) in humans. In study 1, 104 untreate d hypertensives were studied on the fifth day of a high- and a low-salt die t. The 24-hour U-v was 2.2 L (urinary sodium [U-Na] 277 mmol) on the high-s alt 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 decreas e in 24-hour U-Na (P<0.001) and predicts that a 100-mmol/d reduction in sal t intake would decrease 24-hour U-v by 367 mL. In study 2, 634 untreated hy pertensives were studied on their usual diet. There was a significant relat ionship between 24-hour U-v and U-Na (P<0.001). This predicts that a 100-mm ol/d reduction in salt intake would decrease 24-hour U-v by 454 mL. The Int ernational Study of Salt and Blood Pressure (INTERSALT) of 1731 hypertensiv es and 8343 normotensives on their usual diet showed that 24-hour U-v was s ignificantly related to U-Na (P<0.001) and predicted that a 100-mmol/d redu ction in salt intake would decrease 24-hour U-v by 379 and 399 mL in hypert ensives and normotensives, respectively. These findings document the import ant effect that salt intake has on U-v. The recommended reduction in salt i ntake in the general population is from 10 to 5 g/d. This would reduce flui d intake in the population by approximate to 350 mL/d per person. This woul d have a large impact on the sales of soft drinks, mineral water, and beer.