BMJ 7041

BMJ No 7041 Volume 312 Saturday 18 May 1996

Commentary: Evidence on salt is consistent

Malcolm Law

A rise in blood pressure is not an inevitable consequence of aging. In some societies blood pressure remains at about 110/70 mm Hg throughout life.(1)(2) Dietary salt, low dietary potassium, alcohol, and body weight contribute to the increase in blood pressure with age in Western countries. The Intersalt study quantifies the effect of each of these factors while allowing for the others.(1) The new Intersalt paper is a valuable addition to the evidence on salt and blood pressure.(3) In the original Intersalt paper the different analyses on sodium and blood pressure varied, some suggesting a strong association, others a weaker one.(1)

Now the position has been clarified; all the Intersalt analyses confirm salt as an important determinant of blood pressure. The average increase in blood pressure with age (from 25 to 55 years) was greater in centres with higher sodium intake, by 10 mm Hg systolic for a 100 mmol/day higher sodium.(1)(3) More directly, median blood pressure increased with median sodium intake across the 52 centres.(1)(3) Because these two analyses could have been disproportionately affected by the four centres with very low sodium intake, subset analyses omitting the four were done. This made virtually no difference to the result of the first analysis; the second was consistent with the original result, although not statistically significant.(1) The average association between blood pressure and sodium among the 200 individuals within each centre was consistent with the across centre analysis, an important observation well documented two years ago.(3-5)

The Intersalt estimates of the association between salt and blood pressure are similar to the estimates for the same age groups from our overview of the published data other than Intersalt,(2-6) as shown in figure 1. An important finding that refutes the possibility of confounding is that these observational estimates predict the changes in blood pressure found in trials of salt reduction remarkably well.(7) Other evidence shows the importance of a modest change in the average blood pressure. At age 55 the blood pressure difference for a 100 mmol/24h sodium difference is about 10 mm Hg systolic (or 5 mm Hg diastolic)(1)(3): this is associated with a 34% difference in mortality from stroke and a 21% difference in mortality from ischaemic heart disease. Excess dietary salt is a serious public health hazard.

Fig 1-Estimates of differences in systolic blood pressure at ages 40 and 55 years for urinary sodium difference of 100 mmol/day from Intersalt(1)(3) and from overview of other studies(2)(6)

Not surprisingly the US Salt Institute, the organisation representing salt producers, disagrees.(9)1 Its response enshrouds the salt Issue in confusion. The institute objected to the statistical method used to relate blood pressure to age in Intersalt: the Intersalt authors effectively rebut this quibble, showing that whatever method is chosen has little effect. The Salt Institute with the assistance of its lawyers then obtained secondary Intersalt data (which has mostly been published(10)). The paper on these data is so opaque that it obfuscates rather than elucidates; it might urge some readers into concluding that such a complex and unintelligible analysis must have some validity.

The first part of the paper proposed a bizarre hypothesis - in communities where people have a high blood pressure at age 60 they had a low blood pressure at age 20, whereas people with low blood pressure at age 60 started with a high blood pressure. Such crisscrossing of blood pressure is implausible. The Intersalt authors adjusted for blood pressure at age 20-29 and this made no difference.(3) Contrary to the statement by the Salt Institute, blood pressure at age 20-29 was higher on average in the centres with higher blood pressure at age 50-59, and among the centres specified in the article, higher at 20-29 in Portugal and China than in Brazil and New Guinea.(9)

The other analyses confirm the suspicion that this is not a competent analysis. A plot of R-squared on slope is presented, which is uninterpretable because the two are measures of the same thing (the extent to which one variable predicts another). Another plot is taken as evidence against an effect of salt because blood pressure is higher at age 50-59 than at age 40-49 while urinary sodium is slightly (3%) lower, ignoring the fact that a similar high sodium intake across the entire age distribution has an effect on blood pressure that increases with age.(2) This analysis is of service only to illustrate the lengths to which a commercial group will go to protect its market when presented with clear evidence detrimental to its interest.

Department of Environmental and Preventive Medicine
Wolfson Institute of Preventive Medicine
St Bartholomew's and the Royal London School of Medicine
London EC1M 6BQ

Malcolm Law, reader

References

1 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;297:3l9-28.
2 Law M R, Frost C D, Wald N J. By how much does dietary salt reduction lower blood pressure? I. Analysis of observational data among populations. BMJ 1991;302:811-5. 3 Elliott P, Stamler J, Nichols R, Dyer A R, Stamler R, Marmot M, et al.
Intersalt revisited: further analyses from the Intersalt study on 24 hour sodium and blood pressure. BMJ 1996;312:1249-53
4 Dyer A R, Elliott P, Shipley M. Urinary electrolyte excretion in 24 hours and blood pressure in the Intersalt study. II. Estimates of electrolyte blood pressure associations corrected for regression dilution bias. Am J Epidemiol 1994;139:940-51.
5 Dyer A R, Elliott P, Shipley M, Stamler R, Stamler J. Body mass index and associations of sodium and potassium with blood pressure in Intersalt. Hypertension 1994;23:729-36
6 Frost C D, Law M R, Wald N J. By how much does dietary salt reduction lower blood pressure? II. Analysis of observational data within populations. BMJ 1991;302:819-24. MacMahon S, Peto R, Cutler J, Collins R, Sorlie P, Neaton J, et al. Blood pressure, stroke, and coronary heart disease. Part I, prolonged differences in blood pressure: prospective observational studies corrected for the regression dilution bias. Lancet 1990;335:765-74.
9 Hanneman R. Intersalt: hypertension rise with age revisited. BMJ 1996;312:1283-4 10 Intersalt Cooperative Research Group. The Intersalt study: further results. J Hum Hypertens 1989;3:279-407