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BMJ No 7106 Volume 315 Letters Saturday 23 August 1997 Intersalt dataCross cultural studies such as Intersalt study cannot be used to infer causalityEditor,The practical issue of whether moderate dietary salt restriction can lower blood pressure was resolved in the classic Glyncorrwg community study in south Wales, which found it to be ineffective.(1) Clearly then the claims of the Intersalt investigators that their findings support 'the reduction of salt intake to control adverse blood pressure levels' must be wrong - and the reasons are not hard to find.(2) Firstly, cross cultural studies such as the Intersalt study cannot be used to infer causality as they are based on the false assumption that the populations of diverse societies - irrespective of their genetic and cultural composition - have the same susceptibility to environmental factors. Indeed, it is this very difference in susceptibility that explains why it is not possible to show a cross cultural correlation between smoking and lung cancer. For both the countries of northern Europe and of the Mediterranean there is a clear dose-response relation (confirming causality), but the adverse effects of smoking are much more pronounced in the countries of northern Europe.(3) The multiplicity of confounding variables in cross cultural studies makes it almost inevitable that these studies will fail to reflect genuine cause and effect relations (such as smoking and lung cancer), while the associations they do identify are likely to be spurious. Hence the only epidemiological method for determining whether salt intake is implicated in raised blood pressure is to use within population studies, in which the problem of differing susceptibilities does not arise. Here the results of the Intersalt study are both trivial and contradictory: a fall of 1 mm Hg in diastolic pressure for every extra 100 mmol sodium excretion in 24 hours for men and women between the ages of 20 and 39, and a change of similar magnitude but in the reverse direction for those between the ages of 40 and 59. This leaves only the clinical studies of the effects of salt reduction on blood pressure to be considered. Malcolm Law's assertion, in his commentary, that these studies show that 'dietary salt intake is a serious health hazard' is based on a reference to his own meta-analysis,(4) whose methods, he fails to point out, have been subjected to critical scrutiny and conclusions rejected.(5) The important question that emerges from these papers is why the combined intellects of so many distinguished epidemiologists should maintain that the evidence incriminating salt in hypertension is so convincing when clearly it adds up to very little. Readers of the BMJ may rightly wonder how many of the other epidemiological discoveries of recent years linking diet with disease are similarly insecure. James Le Fanu General practitioner Mawbey Brough Health Centre, References 1 Watt G C M, Edwards C, Hart J T, Hart M, Walton P, Foy C J W. Dietary sodium restriction for mild hypertension in general practice. BMJ 1983;286:432-5. 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 Keys A. Seven countries: a multivariate analysis of death and coronary heart disease. Cambridge, MA: Harvard University Press, 1980. 4 Hanneman R L. Intersalt: hypertension rise with age revisited. [With commentaries by M Law and J Stamler et al.] BMJ 1996;312:1283-7. 5 Swales J D. Dietary salt and blood pressure: the role of meta-analyses. J Hypertens 1991;9(suppl 6):S42-6.
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