Letters

Intersalt data

BMJ 1997; 315 doi: https://doi.org/10.1136/bmj.315.7106.484 (Published 23 August 1997) Cite this as: BMJ 1997;315:484

We received many letters commenting on the cluster of papers on salt that we published in May last year. We had several problems with the letters but have now resolved these and are publishing the letters now. These 10 letters and three others are available on the BMJ's website (http://www.bmj.com/).

Cross cultural studies such as Intersalt study cannot be used to infer causality

  1. James Le Fanu, General practitionera
  1. a Mawbey Brough Health Centre, London SW8 2UD
  2. b Department of Epidemiology and Social Medicine, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, NY 10461-1602, USA
  3. c Cae'n y Bwlch Isaf, Talsamau, Gwynedd LL47 6YB
  4. *Alexander Macnair acts as a specialist adviser to several food manufacturers that add salt to their products.
  5. d MRC Biostatistics Unit, Institute of Public Health, Cambridge CB2 2SR
  6. e University of Bristol, Department of Social Medicine, Bristol BS8 2PR
  7. f University Department of Primary Care and Population Sciences, Royal Free Hospital and School of Medicine, London NW3 2PF
  8. g University Department of General Practice, Woodside Health Centre, Glasgow G20 7LR
  9. h International Section, Department of Primary Health Care, Royal Free Hospital Medical School, London NW3 2QU
  10. i School of Computing and Mathematical Sciences, University of Greenwich, London SE18 9PF
  11. j Department of Epidemiology and Public Health, Imperial College School of Medicine, St Mary's Hospital, London W2 1PG
  12. k Department of Preventive Medicine, Northwestern University Medical School, Chicago, IL 60611-4402, USA
  13. l Department of Epidemiology, St Raphael University Hospital, Leuven, Belgium
  14. m Department of Epidemiology and Public Health, University College London Medical School, London WC1E
  15. n Shapiro Center for Evidence-Based Medicine, Hennepin County Medical Center, Minneapolis Medical Research Foundation, 914 South 8th Street, D-2, Minneapolis, MN 55404, USA
  16. o Center for Science in the Public Interest, 1875 Connecticut Avenue NW, Washington DC, 20009-6728, USA

    Editor—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 …

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