Salt, blood pressure and health

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<ul><li><p> 2001 British Nutrition Foundation Nutrition Bulletin, 26, 133139</p><p>*Editors note. John Swales (MA MD FRCP FmedSci), Professor ofClinical Medicine, was President of the British Hypertension Societyfrom 19824 and the author of five books and over 200 papers. Hewas seconded to the Department of Health as Director of R &amp; Dfor the NHS between 1996 and 1998. This article was originallysubmitted as a review paper. Sadly, Professor Swales passed away(aged 64 years) on 17 October 2000, after a short illness. The peer-refereeing process for this paper could not therefore be completed.Correspondence about this article is welcomed and should beaddressed to the Editor.</p><p>NEWS AND VIEWS: RESEARCH</p><p>Salt, blood pressure and health</p><p>John Swales*Clinical Sciences Building, Royal Infirmary, University of Leicester, UK</p><p>Introduction</p><p>The healthy individual has a right to feel confused bythe contradictory advice being given about dietary salt.Some experts advocate a reduction in salt intake foreveryone, while others say this advice is unjustified inthose who do not suffer from high blood pressure. It isnot surprising that this has been described as one of themost polarised fields in medical science. In spite of thedivergent views, evidence accruing over the last fewyears has been remarkably consistent. We should there-fore beware of older work claiming an enormous savingof life if we all reduced our salt intake. In 1998, expertsfor the leading American scientific journal, Science, eval-uated the most widely quoted study that made thisclaim. They concluded that the study was so flawed asto be effectively meaningless.</p><p>People with high blood pressure in our society eat nomore salt than anyone else. In clinical trials, even majorreductions in salt intake, larger than could be achievednormally, produce little or no reduction in blood pres-sure. Blood pressure rises with age in developed soci-eties, and this has been linked to salt intake in theIntersalt Study, but differences in blood pressurebetween the developed and the developing world couldequally well be due to the other multiple differences instress and life. Nuns living in closed orders, for instance,seem equally protected from this increase in blood pres-sure, although their diet contains as much salt as others.</p><p>It is not of course possible for most of us to avoidstress in such a radical way as this. Is there anything ahealthy individual can do to avoid the dangers of highblood pressure? Here, there is scientific agreement.Reducing weight, taking a diet high in fruit and vegeta-bles, indulging in regular physical exercise and avoidingexcessive alcohol intake are all sensible, health-promoting measures that help to lower blood pressureand also carry other health benefits. The tragedy of thesalt controversy is that it has tended to distract atten-tion from these sensible measures by unjustified andcontroversial claims.</p><p>The significance of blood pressure</p><p>The modern debate about salt and blood pressure isnow reaching the end of its fifth decade, although hintsof it can be traced back to ancient Chinese literature.The last few years must seem particularly bewilderingfor those without any special knowledge. Claims thatthere is a consensus amongst experts are countered bya distinguished scientific journalist writing if ever therewere a controversy over the interpretation of scientificdata, this is it (Taubes 1998).</p><p>I believe that the salt controversy is important toeveryone for two reasons. Firstly, reducing the burdenof high blood pressure would undoubtedly producemajor public health benefits. Secondly, it demonstratesthe pitfalls in trying to combine scientific accuracy andcaution with persuasive, credible public health advice.</p><p>High blood pressure is a major cause of ill-health anddeath in developing countries. Because it injures bloodvessels, it is the most important risk factor in strokes,an important contributor to heart attacks and kidneydisease and probably to deteriorating mental functionin the elderly. There is some exciting recent work sug-gesting that high blood pressure even plays a role inpatients with Alzheimers disease. Over the last 30 years,it has become clear that lowering blood pressure helpsto prevent strokes, heart attacks and kidney disease. No-</p><p>133</p></li><li><p>one now denies the importance of blood pressurecontrol in maintaining health, and we now have excel-lent drugs that can achieve this in most patients.</p><p>This is the nub of the problem that confronts thosewho provide health care. The risks associated withblood pressure apply to everyone. Risk of suffering astroke, for instance, can be reduced by a modest lower-ing of blood pressure even in those people who havewhat used to be called normal blood pressure. Thisrepresents a very small benefit to the individual whoseblood pressure is not particularly high, but added upacross the whole population would have a major impactupon the total number of strokes. Of course, mass medication is not a realistic prospect, and so, medicalresearch has had to find other, hopefully harmless, waysof reducing blood pressure through diet and lifestyle.We have to be sure of our ground before advocatingsuch changes. Advising us to alter the way we organiseour lives may be attractive to those who run the UKNational Health Service (NHS), as it costs the NHS littleor nothing and may save it some of the costs of treat-ing illness. However, there are personal and social costsfor even the simplest modification of diet. Those whoadvocate change have to behave with the same degreeof responsibility they would be expected to employ inprescribing a drug for a patient. Unfortunately, this hasnot always been the case amongst enthusiasts for thevarious healthy diets and writers of healthy recipebooks.</p><p>Scientific evidence</p><p>The UK NHS and other healthcare systems across theworld are undergoing a quiet revolution in the way inwhich scientific evidence is used in decision-making.This is the result of some tragic shortcomings in the tra-ditional approach, which became clear about 10 yearsago. It became quite evident that so-called expertadvice was likely to be deeply flawed on occasions,reflecting as it did conventional wisdom and prejudice.This was demonstrated in a seminal study from Harvardby Antman and his coworkers (Antman et al. 1992).Treatment of heart attacks has improved vastly in recentyears with the development of thrombolytic clot-busting drugs, which are now used routinely. In theearly days, there were fears about drugs that inhibitedone of the bodys normal mechanisms for preventingbleeding. Theoretically, these drugs could make mattersworse for victims of a heart attack, indeed, they areharmful in a small minority of patients. By the mid-1970s however, clinical trials had been performed thatshowed net benefit for patients and subsequent trials</p><p>134 John Swales</p><p> 2001 British Nutrition Foundation Nutrition Bulletin, 26, 133139</p><p>simply reinforced that conclusion. Antman et al.however, went on to look at the expert advice beinggiven to doctors through textbook chapters and reviewsin specialist journals. Thrombolysis was rarely men-tioned until the late 1980s and where it was referred toat all, it was as an experimental treatment. Otheradvice consistently recommended treatment that couldnot be supported at all from clinical trial evidence andthat may actually have been harmful. When the only evi-dence cited in favour of a treatment is that an expertgroup has recommended it, one should recall that such expert groups tend to consist of selected individ-uals of rather similar outlook with their own view ofaccepted wisdom. The track record has not been good,but what can be put in their place? The experts tendedto select trials that supported their view and ignored ordenigrated those that did not, giving a misleadingimpression of the overall conclusions of research.Modern databases now allow immediate access to allthe relevant information on methods of preventing andtreating disease. This provides the basis for the sys-tematic review, which figures increasingly in decision-making in the NHS. Of course, a broad range ofexpertise is still required in screening this evidence andinterpreting its relevance, but the day of the old-fashioned expert setting out with their own firmly heldviews and finding evidence to support them is nowpassing. The NHS now funds two groups, the CochraneCentre and the Centre for Reviews and Dissemination,which carry out systematic reviews to very high, care-fully defined standards.</p><p>The second seminal episode in medical history under-lined the caution required in interpreting epidemiologi-cal associations. Because two things tend to happentogether, we cannot assume that they are cause andeffect. I think the best illustration of this fallacy was provided by George Bernard Shaw in the DoctorsDilemma, when he wrote: </p><p>Thus it is easy to prove that the wearing of tall hatsand the carrying of umbrellas prolongs life and confersimmunity from disease, for the statistics show that theclasses which use these articles are bigger, healthier andlive longer than the class which never dreams of pos-sessing such things (Shaw 1911). </p><p>This may seem obvious. Nevertheless, a similarassumption proved fatal for many people in the 1980s.Many patients, after a heart attack, develop extra heart-beats (sometimes felt as palpitations, although oftenonly picked up by carrying out an electrocardiogram).When a particular form of these palpitations (ventricu-lar extrasystoles) is present, patients are much morelikely to have a second, fatal heart attack, probably as</p></li><li><p>Salt, blood pressure and health 135</p><p> 2001 British Nutrition Foundation Nutrition Bulletin, 26, 133139</p><p>a result of a more serious disturbance of heart rhythm.There is, in other words, an association between ven-tricular extrasystoles and death from heart disease. Formany years, doctors assumed, quite reasonably, that thiswas a causal relationship and gave patients drugs tosuppress the extra beats. It was only at the end of the1980s that a trial was set up to assess the benefits ofthis treatment (CAST Investigators 1989). The CAST(Cardiac Arrhythmia Suppression Trial) has stronglyinfluenced the evidence used as a basis for providingdoctors with guidance. The trial was never completed.In April 1989, it was discontinued. Deaths in the treatedgroup were over twice as great as in the control groupand it was no longer ethically acceptable to proceedwith it.</p><p>It has been calculated that more Americans died fromthis treatment than died in the Vietnam War. The lessonfor doctors was in some ways a hard one. Not only wasan epidemiological association a flawed basis for treat-ment, but clinical experience in quite carefully moni-tored groups of patients had failed to detect evidence ofharm. This is not surprising. The overall annual mor-tality rate is quite low even in these patients and an indi-vidual doctors experience of patients dying will be quitelimited. An increase of one or two patients in a year islikely to be attributed to chance. The message was quitesimple, and again points to the danger of unsupportedexpert views. Where it is feasible to carry out clinicaltrials, these should be done before recommendations aremade and, where several such trials have been carriedout, a proper systematic review from relevant databasesof evidence is needed. No regulatory agency wouldaccept unsupported clinical experience or carefullyselected clinical trials as sufficient evidence that a drugdid no harm.</p><p>Salt and blood pressure: the associations</p><p>Preventive medicine and public health have been muchslower to take on board these messages. It is to be hopedthat a much more scientific Food Standards Agency,funded to carry out its own research and reviews, willchange this. It will not be easy. It is much more difficultto carry out a randomised controlled trial in a healthypopulation than in a group of patients. Nevertheless, inthe case of blood pressure, we have something that iseminently measurable and can be studied easily in clini-cal trials. If claims are made that a particular dietarychange will influence blood pressure, these can be exam-ined with just as much rigour as is used for evaluatinga drug.</p><p>It is also true, however, that clinical trials are not so</p><p>good at picking up the net, very small, beneficial andharmful effects of an intervention when these occur overa long period of time. On other occasions, a trial maybe impossible. The role of smoking in lung cancer andheart disease is a good example of an important risk,the evidence for which is entirely based upon epidemi-ological associations. Why should we accept this andnot all associations? Austin Bradford Hill, who, withRichard Doll, discovered the tobacco lung cancer rela-tionship, as well as designing the first randomised clinical trial, laid down some important principles (Hill1965). Besides being biologically plausible, associationsneeded to be strong and consistent. They needed toshow a gradient, so that weaker influences were associ-ated with less effect, and they needed to be specific. Thiswas true both of the smoking/cancer association and thecholesterol/heart disease association. The association oftobacco with lung cancer, for instance, was required tobe unique to tobacco and not due to other factors thatare associated with tobacco, such as social class anddiet. The existence of such confounders is one of themajor pitfalls in ascribing causality to an association.</p><p>How far does the salt/blood pressure association meetthese criteria? The short answer is that it cannot,because there is no such simple association. Individualswith high blood pressure take no more dietary salt thananyone else. It has been impossible to show any associa-tion between blood pressure and measured salt intakewhen this has been rigorously sought in quite largestudies of populations such as the study carried out in Scotland some years ago (Smith et al. 1988). This isin striking contrast to, for example, the relationshipbetween cholesterol and heart disease where the evi-dence for causality is now strong.</p><p>It is surprising, under such circumstances, that somuch emphasis in the public debate has been placed ona different type of association study, Intersalt (IntersaltCooperative Group 1988). This looked at salt intakeacross a wide range of cultures extending from theUnited States to the Yanamamo Indians of SouthAmerica. The latter, like some of the other populationsexamined, take extremely small quantities of salt, wellbelow the level that could ever be achieved in developedsocieties. Even so, when other factors that can influenceblood pressure were taken into account, it provedimpossible to show any relationship between salt intake(measured by the sodium content of urine) and diastolicblood pressure. In addition, only a minuscule change insystolic blood pressure was shown over extremely largedifferences in salt intake. In the reports of Intersalt, thisfinding took second place to a different sort of analy...</p></li></ul>