The INTERSALT Study: Results for 24 hour sodium and potassium, by age and sex

The relations by age and sex of urinary sodium, potassium and sodium/potassium ratio to blood pressure were examined in the 5,045 men and 5,034 women of the INTERSALT Study. With adjustment for age, and combining (pooling) regression coefficients over the 52 centres of the study, sodium excretion was positively and significantly related to the blood pressure of individuals in both men and women. These positive and significant relationships were also found in seven of eight age-sex specific analyses. In most analyses, ζ-scores suggested stronger associations in women than in men: the size of regression coefficients in women was as much as twice that of the corresponding coefficients in men. In age-specific analyses for men and women combined, sodium excretion was positively related to blood pressure, significantly so for systolic pressure at all ages and for diastolic pressure at ages 50-59; regression coefficients tended to be larger at older compared to younger ages. Results for sodium/potassium ratio in individuals were similar to those for sodium, being stronger in women than in men, and (at least for systolic pressure) at older compared to younger ages. With adjustment for confounding variables, potassium excretion was negatively and significantly related to the blood pressure of individuals; again these relationships tended to be more marked at older ages. Across the centres, median sodium excretion was positively and significantly related to the slope of systolic and diastolic blood pressure with age in men, in both the 52 and 48 centre analyses, and with and without adjustment for confounding variables. For women, relationships with slope of blood pressure with age were positive in all analyses, and were significant in the analyses across 52 centres. Other cross centre relationships were less consistent. These results extend the findings of INTERSALT to age and sex specific sub-groups. They suggest that previous estimates of the effects on blood pressure of changes in electrolyte excretion are conservative, at least at older ages, and hence underestimate the potential gain to the community of appropriate dietary and lifestyle changes.