WASH comment on IOM report
21 May 2013
Blood pressure throughout its range is the major cause of death responsible for just under 10,000,000 deaths per year. Lowering blood pressure has been shown in numerous trials over the last 50 years to be immensely beneficial in preventing strokes, heart attacks and heart failure, as well as the development of kidney disease.
Salt intake is one of the major factors that put up blood pressure and many trials throughout the world have shown that lowering salt intake lowers blood pressure. Indeed recent papers in the BMJ confirm that reducing salt intake down to 3g a day causes falls in blood pressure both in those patients with high blood pressure and also those with normal blood pressure. These benefits would translate into very large reductions in strokes, heart attacks and heart failure.
The Centre for Disease Control in the United States asked the IOM for a review of the evidence that reducing sodium intake did reduce clinical outcomes. It was not a review of the evidence of salt on blood pressure.
A previous IOM report Evaluation of biomarkers and sodium outcome in 2010 made very strong statements that blood pressure stands apart as a valid proxy for hard outcomes i.e. anything that lowers blood pressure will be beneficial. The limited nature of the IOM report is correct in stating that when salt intake is below 5g there are no hard clinical outcomes as no properly controlled studies have been done.
There was a weak association between the measurement of the lower salt intake and increased mortality. However these studies were extensively reviewed previously in the American Heart Association. As stated there, and the IOM report agrees, these studies were methodologically flawed, particularly the method whereby sodium intake was assessed / inadequate and likely to be wrong. But most importantly of all, that reverse causality had not been considered - that is that patients who are ill lose their appetite and do not eat salt and will therefore have a lower salt intake however badly it is measured. They are more likely to die not because of the low salt intake but because they are ill. This is particularly true in the studies that this refers to as most of these patients either had severe renal failure, severe diabetes and high blood pressure. Many of them were on treatments that included drugs that blocked the renin angiotensin system and diuretics, and many of them had low blood pressures and in spite of this, had further blood pressure therapy added in. These patients do not represent the average person of the population, and are likely to be in a particular danger of having salt intake further reduced if they are already sodium depleted, unless the dose of the drugs that they are on is adjusted. This is something that is well known in clinical medicine and one would be very careful about advising a low salt diet in someone who has low blood pressure and on multiple blood pressure lowering drugs including diuretics.
The only randomised controlled trials that have been done are one group in Italy, which has focused on heart failure. These studies were done in patients with severe heart failure who were on very large doses of diuretics – drugs that get rid of salt and water – as well as drugs that inhibit the renin angiotensin system. It is well known in this situation it is extremely dangerous to reduce salt intake unless patients are carefully assessed, because many of them will be salt deficient before being put on the low salt diet and will need the dose of diuretics to be adjusted.
The trials that were done by the group in Italy did not change the dose of diuretics and they were randomised to either a normal or low salt diet. Many would consider these trials to be unethical as this is not what would happen in clinical practice in these very ill patients. If salt intake was going to be reduced, very careful assessment of patients would be made and their dose of drugs would be adjusted. Not surprisingly the study did show that where the doses of the diuretics were not reduced, salt restriction did lead to some adverse effects in these very ill patients.
Indeed it has been known for many years that in these types of patients, patients with Addison’s Disease who have adrenal failure, patients on dialysis need to be cautioned about restricting their salt intake too severely without medical advice.
These studies in any case are completely irrelevant to the public health implications of modest to moderate salt reduction in the whole population, not in these very ill patients.
Salt reduction remains a critically important component of public health efforts designed to promote cardiovascular health and prevent cardiovascular disease.
For full IOM report please click here [PDF 1,183KB]