Adults who use less salt in their diet can experience a slight reduction in their blood pressure in the medium term. However, whether in the long term this can also reduce the risk of late complications in people with sustained high blood pressure, otherwise known as essential hypertension, and whether in the long term their anti-hypertensive medication can be reduced remains unresolved. This is the conclusion of a report by the German Institute for Quality and Efficiency in Health Care (IQWiG), Cologne and for which an English-language summary is now available.
This rapid report is part of a package commissioned by the Federal Joint Committee (G-BA), in which the benefit of various non-drug treatment strategies for high blood pressure are to be assessed. Stress management and more physical activity are also included, as well as giving up smoking and cutting down alcohol consumption. IQWiG has already completed a report on the effect of weight reduction on blood pressure.
Assessment was based on secondary literature
IQWiG's benefit assessments are generally based on systematic searches and analysis of clinical trials, in other words, primary literature. However, this rapid report was prepared on the basis of secondary literature. In principle, this can be done - and is included in IQWiG's General Methods - if current, high quality systematic reviews are already available on a given topic. This was the case with reducing salt intake in hypertension, as IQWiG's preliminary search revealed.
IQWiG searched for systematic reviews (these basically provide an analysis of studies in summary) that compared the following patients with hypertension: an intervention group, which was to follow a low-salt diet over a long period, versus a control group, which either did not have this target or whose salt reduction was not so great as in the intervention group. The minimum duration of the studies had to be 4 weeks. In order not to overlook any current and potentially relevant studies, IQWiG also conducted an update search of recently published primary studies.
IQWiG was able to include in its assessment 7 reviews, in which the results of between 520 and 3391 participants from a total of 62 randomized controlled trials were analysed together.
No conclusions on cardiovascular disease or mortality possible
IQWiG found that no conclusions on late complications could be drawn from the available data. The reason for this is that none of the studies had the primary goal of investigating the effects of a low-salt diet on cardiovascular disease or all-cause mortality. Moreover, most of the studies were only of a few months' duration and had low numbers of participants, which meant that possible differences in late complications might not have been detected with certainty.
Uncertainty whether the reduction in blood pressure is sustainable
However, the investigations consistently show that a reduction in salt intake can assist in lowering blood pressure: over a period of up to one year, there was a mean drop of 3.6 to 8 mmHg in systolic values and a mean drop of approximately 2 to 3 mmHg in diastolic values. This applied primarily to patients who did not take any additional anti-hypertensive drugs.
The sustainability of this effect, however, remains unclear. The authors of at least one review report that the observed advantage disappears when the analysis is restricted to studies of a longer duration (at least 6 months).
None of the reviews solely considered patients who were simultaneously taking anti-hypertensive drugs or separately analysed data for participants on concomitant medication. The additional blood pressure-lowering effect of a low-salt diet in these patients is therefore uncertain.
Basically, it is still not known whether people with essential hypertension can reduce their drug dosage through less salt intake.
Report preparation procedure
Rapid reports are intended to offer timely information on a current topic. They are not designed for G-BA guideline decisions. In order to guarantee a shorter delivery time, the report preparation procedure differs primarily in two ways from that of the other reports: working documents, report plans or preliminary reports are not published, nor is there a submission of comments procedure. Furthermore, the assessment is generally based on information already published, i.e. IQWiG is not concerned with obtaining unpublished study data from drug manufacturers, for example.
The report was produced in collaboration with external experts. A preliminary version was reviewed by a further independent research group and the final version was despatched to the G-BA on 22 June 2009.