The INTERSALT study : an international co-operative study of the relation of electrolyte excretion to blood pressure. Design, methods, some results and implications.

Evidence relating salt and blood pressure is critically reviewed including the problems of bias and confounding in the epidemiological studies. Overviews of controlled trials of sodium reduction and within-population epidemiological studies give similar estimates of effect. In the trials, average blood pressure was lowered by 2.9/1.6 mm Hg systolic/diastolic for daily sodium excretion less by 70-75 mmol, whereas the epidemiological studies gave a regression estimate (corrected for reliability) of 3.7/2.0 mm Hg systolic/diastolic per 100 mmol sodium. INTERSALT is an international co-operative study of the relation of salt and other factors to blood pressure in 10,079 men and women from 52 centres and 32 countries; field methods, and procedures of the Coordinating Centre are specified. The analysis of multicentre studies is illustrated using INTERSALT data relating systolic pressure to body weight. There was significant heterogeneity of systolic pressure-weight regression coefficients which is explored. Implications for the design and analysis of future studies are discussed. In INTERSALT, the corrected pooled within-centre regression estimate, adjusted for age and sex, was 3.5/1.5 mm Hg systolic/diastolic per 100 mmol, reducing to 2.2/0.1 with adjustment for confounders. Age and sex specific results suggested a stronger relation at older ages, and for women compared with men. Across the centres, average sodium excretion was positively related to slope of blood pressure with age, such that 100 mmol/day lower sodium was associated with a 9 mm Hg lower rise in systolic pressure from age 25 to age 55. In the three United Kingdom centres, there was relatively high urinary sodium excretion (average 152 mmol/24-hour), sodium/potassium ratio, body mass index and prevalence of heavy alcohol drinking in men. These findings are used to illustrate the potential for non-pharmacological control of blood pressure in the community, and recommendations are made on more desirable levels of sodium intake.