Implications of the INTERSALT study.

Abstract

It has been said that two people can sleep in thesame bed but have different dreams. The samephenomenon is sometimes seen when the same data are observed by two people who come to opposite conclusions as to their meaning. A case in point is the contrast in evaluating the implications of the INTERSALT study,1 as voiced by two figures well known in international research on hypertension. Dr. Simpson of New Zealand writes as follows: "... the INTERSALT data on the slope of blood pressure with age in relation to sodium intake are unique and more encouraging to the idea that salt is important."2 But Dr. Swales of England has written: "Salt has only small importance in hypertension. "3 Two words have been italicized in this latter quotation, "small " and "hypertension, " since they provide a clue that helps explain what I judge to be an erroneous interpreta-tion of INTERSALT and of the whole issue of salt and blood pressure. The error arises from the impos-ing of a strictly clinical approach when the question under study is about implications for the population. Patients and Populations A clinician, observing a patient in his or her office, will note that the blood pressure may change 2, 3, 5, or even 10 mm Hg from one measurement to the next. The clinician will, understandably, call such differences small in magnitude and unimportant for health. But is it appropriate to look at findings on populations, such as in the INTERSALT study,1 in the same way? The INTERSALT data indicate that if a population, on average, had a daily sodium intake lower by 100 mmol (about 1 teaspoon salt), average population systolic blood pressure would be lower by 2-3 mm Hg (Table 1). If other relevant aspects of lifestyle were improved in the population (higher potassium intake so that the sodium/potassium ratio was 1 instead of 3 as observed in INTERSALT, average weight lower by about 6 kg [13 lb], and excess alcohol avoided), then population average systolic pressure could be lower by at least 5 mm Hg. Is this difference really small when applied not to the vari-ations of blood pressure of a patient but to a poten-tially lower average blood pressure in a population?