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BMJ No 7106 Volume 315 Letters Saturday 23 August 1997 Intersalt data Conclusions drawn in paper 'revisiting' Intersalt data are of questionable validity Editor, Despite editorial assertions to the contrary,(1) the conclusions drawn in the paper by Paul Elliott and colleagues 'revisiting' the data from the Intersalt study are of questionable validity.(2) The original report showed that 24 hour urinary sodium excretion was not significantly related to the prevalence of hypertension.(3) This second reanalysis depends on two statistical manoeuvres to draw the opposite conclusion. The first manoeuvre is to correct for regression dilution bias. The authors have assumed that the differences between true mean blood pressures and the measured blood pressures were greater than those allowed for in the first analysis. Since they have no means of knowing what the true blood pressures were at the time of the Intersalt measurements, there is no valid basis for their assumption. Secondly, the effect of body mass index is removed from the multiple regression analysis. This was not an allowance for an assumed error but the withholding of an accurately measured variable which in the original report showed a strong and significant independent relation with blood pressure. The reason given for this startling decision is a supposed correlation between body weight and sodium excretion. There surely is no logical basis for believing that fat people eat more salt than thin people, and the exclusion of body mass index from the multiple regression analysis is without justification. Without all this statistical prestidigitation, the third visit to the Intersalt study still does not stand up. As an ad hoc adviser to several food manufacturing interests, including some that add salt to their products, I take exception to the suggestion that my views are necessarily more 'commercial' than those of the academics. Surely the science speaks for itself. Alexander Macnair Private consultant in preventive medicine* Cae'n y Bwlch Isaf, Talsamau, Gwynedd LL47 6YB *Alexander Macnair acts as a specialist adviser to several food manufacturers that add salt to their products. References 1 Thelle D S. Salt and blood pressure revisited. BMJ 1996;312:1240-1. 2 Elliott P, Stamler J, Nichols R, Dyer A R, Stamler R, Kesteloot H, et al for the Intersalt Cooperative Research Group. Intersalt revisited: further analyses of 24 hour sodium excretion and blood pressure within and across populations. BMJ 1996;312:1249-53. 3 Intersalt Cooperative Research Group. Intersalt: an international study of electrolyte excretion and blood pressure. Results for 24 hour urinary sodium and potassium excretion. BMJ 1988;297:319-328. Home | Current issue | Past issues | Classified ads | Career Focus | Feedback Collections | About this site | About the BMJ | BMA | Medline