Effect of changing dietary sodium on the airway response to histamine.

The airway response to histamine has been shown to be related to the 24 hour urinary excretion of sodium. To assess whether this relation is likely to represent a direct causal association a randomised double blind crossover trial of slow sodium (80 mmol/day) was compared with placebo in 36 subjects having a low sodium diet. The dose of histamine causing a 20% fall in FEV1 (PD20) was 1.51 doubling doses lower when the men were taking sodium than when they were taking placebo (p less than 0.05). On the basis of PD10 values, the difference in men was 1.66 doubling doses of histamine (p less than 0.05). There was no corresponding effect in women. Regressing PD10 against urinary excretion of electrolytes with data from the two occasions during the trial and the measurements made before the trial showed a significant association with sodium excretion after allowance had been made for any effect associated with potassium or creatinine excretion, the latter being a marker of the completeness of the urine collection. Again there was no corresponding effect among women. These findings are compatible with the differences in regional mortality data for England and Wales, which show a relation between asthma mortality and regional per person purchases of table salt for men but not for women.