Plasma sodium: ignored and underestimated.

Salt intake is a major regulator of blood pressure. There is evidence that those who develop high blood pressure have an underlying defect in the ability of the kidney to excrete salt. It has been suggested that this results in a greater tendency to retain sodium and an increased compensatory response that is responsible for the rise in blood pressure. There is also evidence suggesting that small increases in plasma sodium may directly affect blood pressure, independent of the associated expansion in extracellular volume. We reanalyzed 3 types of studies of changing salt intake. (1) An acute and large reduction in salt intake from 350 mmol/d to 10 to 20 mmol/d for 5 days in hypertensives and normotensives was associated with a fall in plasma sodium of approximately 3 mmol/L (P<0.001). (2) Progressive increases in salt intake from 10 to 250 mmol/d by a daily amount of 50 mmol in normotensives caused increases in plasma sodium (P<0.001). (3) Longer-term modest reduction in salt intake in hypertensives was studied in double-blind randomized crossover studies; 1 month of usual salt intake ( approximately 170 mmol/d) compared with reduced salt intake ( approximately 100 mmol/d). There was a decrease in plasma sodium of 0.4+/-0.2 mmol/L (P<0.05), which was weakly but significantly correlated with the fall in systolic blood pressure (r=0.18; P<0.05). These studies demonstrate that an increase or a decrease in salt intake causes changes in plasma sodium. Small changes in plasma sodium alter extracellular volume, which may influence blood pressure. Changes in plasma sodium may also affect blood pressure directly.