Salt and blood pressure

Advisory report | 18-10-2000

It is generally assumed that dietary sodium raises blood pressure. Over the last few decades, a considerable amount of research has been undertaken into the correctness of this assumption. Cross-sectional studies have indicated that people with a lower sodium intake have a lower systolic blood pressure. This type of research has provided no evidence of a relationship between sodium use and diastolic blood pressure. The current mean sodium intake in the Netherlands is 3.7 grams per day. According to the results of intervention studies, a reduction in sodium intake by one gram daily — equivalent to about 2½ grams of salt — results in a mean systolic blood pressure reduction of at most 1 mmHg in normotensive people. For hypertensive subjects, the estimate is 2.5 mmHg. An effect on diastolic blood pressure is not convincingly demonstrated; the highest dose-effect estimates are a blood pressure reduction of 0.7 mmHg and 1.8 mmHg per gram reduction in sodium intake for normotensive and hypertensive subjects, respectively. As a substantial reduction in sodium intake results in only a slight reduction in mean blood pressure, the blood pressure of only a small group of people with mild hypertension will decrease to a level below the threshold above which the diagnosis of hypertension is established. Therefore, a reduction of sodium intake at the level of the general population as a means of reducing hypertension is only of limited value.

Although the request for advice is solely concerned with the prevention of (mild) hypertension, this report also discusses the extent to which the mortality from coronary heart diseases can be reduced by decreasing sodium intake. Results from research into the relationship between blood pressure and mortality indicate that a lowering of blood pressure provides health benefits at all blood pressure levels. It has been estimated that a reduction in systolic blood pressure of 1 mmHg can reduce deaths from coronary heart diseases by 1.5 to 3%. In addition to an effect on blood pressure, sodium restriction probably also has other effects, some of which may be beneficial and others detrimental. A balanced consideration of these effects requires studies in which sodium intake — rather than blood pressure — is directly related to illness and death. Such research is still scarce. The available data do not show a lower mortality from coronary heart diseases at a lower sodium intake. Further research of this type is desirable.

The mean sodium intake in the Netherlands is moderate in comparison with other Western European countries. According to a report issued by the Dutch Nutritional Council in 1986, this intake can be reduced by about 20%. This estimate was based on dietary measures restricting sodium intake, combined with the use of salt-substitute mineral mixtures. Recent research results confirm that salt-substitute mineral mixtures, if used in industrially prepared products, can in fact play an important role in reducing sodium intake. However, the domestic use of salt-substitute mineral mixtures, i.e. to replace cooking salt and table salt, has not shown to reduce sodium intake.

In intervention studies, a reduction in sodium intake has almost always come about through an intensive individual support programme. In this type of research it has been possible to reduce the mean sodium intake by 20% or more. The success of measures aiming at a restriction of sodium intake, however, generally decreases over the course of time. It is doubtful whether a substantial and sustained reduction in sodium intake can be achieved with interventional campaigns directed towards the general population as opposed to the individual. In any case this would require extensive co-operation between trade and industry, government and information agencies.

With respect to the prevention of (mild) hypertension, the use of salt-substitute mineral mixtures is probably more valuable than simply just reducing the salt content in the diet, because it increases the intake of several minerals, notably potassium and magnesium. From the available research results it is apparent that even if these mineral mixtures do not reduce the sodium intake, they nonetheless effectively lower the blood pressure of hypertensive subjects. No conclusion can be drawn about the effect of salt-substitute mineral mixtures in normotensive subjects, due to a lack of study data. This group accounts for 85% of the Dutch population. Research in subjects whose blood pressure is not raised is of major importance in assessing the desirability of general measures to promote the use of these mineral mixtures. Salt-substitute mineral mixtures must not be used by people with poor renal function, or by patients taking potassium-sparing diuretics, ACE inhibitors, or non-steroidal anti-inflammatory drugs because of the risk of hyperkalaemia.

It has recently become apparent that other dietary measures may result in a considerable reduction in blood pressure, in particular a dietary pattern in which a very high consumption of fruit and vegetables is combined with the consumption of low-fat dairy products and a low intake of saturated fatty acids and total fat. Moreover, in connection with the prevention of hypertension, it is important to prevent overweight and excessive alcohol consumption and to encourage physical activity.

The present report is specifically confined to the prevention of hypertension in the general population. Results of intervention studies indicate the existence of major differences between individuals in the effect of sodium restriction on blood pressure, referred to as ’salt sensitivity’. In particular, ’salt-sensitive’ patients with hypertension can benefit from a low-sodium diet and the use of salt-substitute mineral mixtures. The attending physician is responsible for assessing the patient’s degree of ’salt sensitivity’ and whether or not a low-sodium diet should be prescribed.

The aforementioned conclusions provide no grounds for arguing for a change in the current regulations governing so-called iodine prophylaxis. However, when replacing iodinated salt by a mineral mixture, the iodinated version of the mineral mixture should be preferred to the non-iodinated version.