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For Debate

The salt dilemma: some answers, many questions

M Gary Nicholls and A Mark Richards

MJA 1999; 170: 178-180
  

Back icon  The 100-year conflict: Introduction
Back icon  Universal recommendations for sodium intake should be avoided
Back icon  Restriction of salt intake is needed to ameliorate the cardiovascular disease epidemic
Introduction The many studies linking dietary sodium intake with blood pressure present conflicting views on the benefits and risks of salt restriction. Reviews, and more recently meta-analyses (which, as discussed by Swales,46 must be viewed with caution), have become commonplace in the struggle to reach firm conclusions about the importance of dietary sodium in contributing to raised blood pressure levels and the cardiovascular complications of hypertension. The continuing debate reflects inadequacies in the information.

We present a personal view of, firstly, whether moderate dietary sodium restriction in Western society reduces arterial pressure; secondly, whether such dietary sodium restriction reduces the complications of hypertension; thirdly, whether sodium restriction has adverse effects; and finally, whether a population approach to reducing dietary sodium is practical.

Does moderate restriction of dietary sodium reduce arterial pressure?
Most studies of dietary sodium restriction have been short term. Furthermore, vital methodological features (eg, using objective techniques for measuring blood pressure, and randomisation of dietary regimens) have often been ignored. Many of these trials would be considered unacceptable were antihypertensive drugs rather than dietary sodium being scrutinised.

From our overview of studies in Western countries, we conclude that restricting a population's dietary sodium intake to about 80-100 mmol/day has a minor and variable effect on arterial pressure. A majority of people show a small fall in arterial pressure, but a sizeable minority exhibit no change or even a slight rise. An antihypertensive effect is most readily seen in the elderly, in African Americans and in people with moderate to severe hypertension.37,47-49 Younger subjects and those who are normotensive or have mild hypertension generally exhibit little or no fall in arterial pressures.49,50 Individual blood pressure responses relate in part to activation of the renin-angiotensin and sympathetic systems as dietary sodium intake is reduced.41,51-53

Effects of dietary sodium restriction on complications of hypertension
We are not aware of interventional data demonstrating that moderate dietary sodium restriction either reduces or increases the complications of hypertension, namely stroke, myocardial infarction, cardiac failure or renal failure.

Does dietary sodium restriction have adverse effects?
Dietary sodium restriction stimulates the renin-angiotensin system, plasma aldosterone and circulating catecholamines,12 most obviously in young subjects. Whether long term activation of these neurohormonal systems is hazardous is a vexed question. Some researchers (with Laragh and colleagues usually leading) consider that heightened activity of the renin-angiotensin system constitutes a risk factor for coronary heart disease, stroke, retinopathy and left ventricular hypertrophy, especially in hypertensive patients.14,21,44,54-57 Others have found no evidence to support the view that the renin-angiotensin system constitutes a cardiovascular risk factor in hypertensive or normotensive subjects.58-60 Recently, the report from the first National Health and Nutrition Examination Survey (NHANES I) added fuel to the controversy by documenting an inverse association between all-cause mortality and sodium intake in a representative sample of 11 346 adults in the United States.6 Vigorous debate followed in the letters columns of journals, much of it focused on the adequacy (or otherwise) of using a single assessment of sodium intake based on diet recall.

There is no lack of animal and experimental information demonstrating the vascular and cardiac toxicity of angiotensin II,61 which, when added to the accumulated data from New York in particular,14,21,44 raises questions about the advisability of restricting dietary sodium intake over years and decades. These data, however, are difficult to reconcile with evidence that a high dietary sodium intake (which should suppress renin secretion) is claimed to have adverse effects on blood vessels and the heart beyond, and perhaps independent of, any action via an increase in arterial pressure.62-64 Information from a small number of laboratories suggests that raised levels of aldosterone, like angiotensin II, may have adverse effects, particularly on the heart.65,66 Whether a chronic threefold elevation in aldosterone levels with restriction of dietary sodium12 could offset to a greater or lesser extent any benefits from a concomitant fall in arterial pressure is unknown.

It can be argued that diuretic-based antihypertensive treatment, the effects of which are in many ways similar to those of dietary sodium restriction, has been shown to reduce the cardiovascular complications of hypertension notwithstanding stimulation of angiotensin II, aldosterone and catecholamine levels.67-69 However, these were intervention trials of about three to five years' duration and often in elderly people with hypertension. Extrapolation from short term, diuretic-based trials in elderly patients to longer term dietary sodium restriction in young and middle-aged subjects could be hazardous.

Dietary sodium restriction has been claimed to increase plasma levels of total and low density lipoprotein cholesterol, to reduce insulin sensitivity, to impair sleep and worsen fatigue, and to be undesirable in pregnancy.12,70-73 Whether such changes are significant or important in the long term is unknown. Some bravely dismiss such possibilities outright.74

The practicality of reducing dietary sodium in whole populations
Although there are exceptions,75 most researchers have found it difficult to achieve a substantial reduction in dietary sodium consumption in a majority of subjects in Western populations.76,77

Many foods, particularly processed foods, have a high sodium content, and presumably this is preferred by many. If food companies reduced the sodium content of foods considerably, a daily intake in the range of 80-100 mmol/day would be more readily attained. But should industry be encouraged, even forced, to reduce the sodium content of foods when evidence of benefit over risk remains so tenuous?

Conclusions
  • Dietary sodium restriction is advisable in all patients with hypertension who are receiving antihypertensive drugs, with the possible exception of monotherapy with a calcium channel antagonist, as the antihypertensive action of this drug is arguably little altered by the level of dietary sodium intake.

  • Dietary sodium restriction is advisable as a trial over a period of months in all patients with mild to moderate hypertension, especially the elderly,49,78 as part of a "hygienic" approach to reduce blood pressure, which should include weight loss in the obese, restriction of alcohol intake in heavy drinkers, and regular exercise in the sedentary.

  • The realities of life dictate that few patients can alter more than one or two lifestyle habits in the long term, and, given a choice of which to pursue based on evidence of benefit versus risk, we recommend weight loss, restriction of alcohol and regular moderate exercise ahead of dietary sodium restriction. Other lifestyle changes might repay further investigation.79

As noted already, the medical literature relating to dietary sodium, blood pressure and its complications is vast and conflicting. The controversy is well reflected in a recent overview in Science, in which the real difficulties attending interpretation of the Intersalt study,80 and especially reanalysis of the original data,81 are discussed.3 More information of an objective nature is required before we admonish whole populations to restrict dietary sodium intake, as, in our view (which has not changed in 14 years82), the balance between benefit and risk is unknown. Regarding lifestyle matters and general health (not just in relation to hypertension), energy is better directed to areas where the evidence is more secure, such as smoking cessation, prevention of obesity and regular moderate exercise.

Conflict of interest
None. We do not have any connection with or receive funds from the food and salt industries or any related commercial interests.

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Authors' details
Department of Medicine, Christchurch Hospital, Christchurch, New Zealand.
M Gary Nicholls, FRACP, Professor of Medicine.
A Mark Richards, FRACP, Professor of Medicine.

Reprints will not be available from the authors.
Correspondence: Professor M G Nicholls, Department of Medicine, Christchurch Hospital, PO Box 4345, Christchurch, New Zealand.
Email: bgriffinATchmeds.ac.nz

©MJA 1998
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