Body mass index and associations of sodium and potassium with blood pressure in INTERSALT

This report further examines the relation of body mass index (BMI) to associations of 24-hour urinary sodium, potassium, and sodium-potassium ratio with blood pressure in INTERSALT, a 52-center international study of electrolytes and blood pressure. Analyses without adjustment for BMI indicated average systolic pressure greater by 6.00 mm Hg per 100 mmol higher sodium and diastolic by 2.52 mm Hg. With adjustment for BMI, these values were reduced to 3.14 and 0.14 mm Hg, respectively. For the sodium-potassium ratio, blood pressure associations were stronger when not adjusted for BMI, and for potassium, adjustment generally had little effect. To explore possible interactions of these variables with BMI in relation to blood pressure, the 52 centers were divided into two groups of 26 based on whether the center median for BMI was less than or greater than or equal to 24.5 kg/m², and individuals within each of the 52 centers were classified into lower- or higher-BMI groups based on individual BMI less than or greater than or equal to 24.1 kg/m². Sodium and the sodium-potassium ratio were positively and significantly and potassium inversely and significantly related to systolic pressure in all four of these subgroups, and the sodium- potassium ratio and potassium were related to diastolic pressure in two and three subgroups, respectively. Electrolyte-blood pressure associations did not differ significantly between the two subgroups of centers or between the two subgroups based on individuals. Although these results indicate that adjustment for BMI has an important effect on INTERSALT associations of sodium and the sodium-potassium ratio with blood pressure and may represent an overadjustment, they also indicate that each of the three variables is related to blood pressure throughout the BMI range. Thus, they do not support the concept of important interactions of sodium and potassium with BMI in relation to blood pressure.