According to The Guardian to-day (15th April 2014)(1) the reduction in salt in recent years has been a key factor in the large fall in the number of people dying from heart attack or stroke. This is the conclusion of research which has just been published in BMJ Open (2).

The article pointed out that the mortality rate from heart disease in England had fallen from 232/100,000 to 139/100,000 between 2003 and 2011. The corresponding values for stroke were 134/100,000 and 78/100,000. Over the same period the average salt intake in adults fell from 9.5g to 8.1g per day and the average reduction in blood pressure (BP) for adults in England decreased by 3.0/1.4mm Hg.

Based on this reduction in BP the authors conclude strokes would be reduced by approximately 11% and heart attacks by 6%. They go on to state that therefore salt reduction is likely to have played an important role in the decreases in the incidence of strokes and in the mortality from heart attacks. They also claim that this conclusion is supported by evidence from prospective cohort studies and outcome trials, which have demonstrated that a reduction in salt intake is related to a decrease in the risk of cardiovascular disease. However it is important to appreciate the fact that these figures only represent about 25% of the total reductions actually achieved. While the researchers recognise that other factors such as the reduction in smoking and increased consumption of fruit and vegetables also make important contributions, the fact remains that this paper is being used as a justification for the campaign to reduce salt intake. One of the authors, Professor Graham MacGregor, said the results vindicated the action by the Food Standards Agency (FSA) to put pressure on the food industry to reduce the levels of salt in food. A spokesperson for the pressure group, World Action on Salt and Health, said:

It would now be a gross breach of ethical and corporate responsibility for companies not to reduce salt as the benefits of salt reduction are now so clear.”

Before we get carried away with the hype surrounding this article it is important to understand that these results are based on a relatively short period. Furthermore as with so much of the debate on diet and health this work simply presents an association. It certainly does not demonstrate “cause and effect”. In other words it does not prove that if you reduce salt that this will reduce your chances of dying of stroke or a heart attack.

In order to put his into perspective it is worth considering what happened during an earlier period. Data from the National Food Survey (3) show that between 1985 and 2000 in Great Britain the salt intake was virtually constant at about 6.3g per person per day. (This figure is lower than that cited above because a different basis was used to calculate it. Nevertheless the crucial factor is that it did not change.)

The best source of data on deaths from cardiovascular diseases is undoubtedly the British Heart Foundation (4) and so I have used this information to compare the periods 1985 to 2000 when there was no change in salt intake with the period 2000 to 2009 when there was a reduction in the intake of salt. (Note the BMJ Open paper continued until 2011 but the BHF does not go beyond 2009 at present).

It will be evident to anyone who views the BHF publication that for the last 50 years or so there has been a steady decline in the death rates due to the cardiovascular diseases (ie a straight line!). The actual values for the rates of change in death rates are shown in Table 1.

TABLE 1 Comparison in death rates from coronary heart disease and stroke in 1985-2000 and in 2000-2009, expressed as the annual decrease in death rates per 100,000 population

Cause of death Men
  1985-2000 2000-2009
Coronary Heart Disease 12 10
Stroke 3.3 2.7
  Women
Coronary Heart Disease 5 5
Stroke 2.7 2.2

 

These results confirm that there is no difference in the rate of decline between the 2 periods. As there was no change in the intake of salt during the first one it is difficult to understand why anyone can actually reach the conclusion that the reduction in the death rates could be due in any way to a change in the consumption of salt. To have any credence it would be essential to show that there had been a significant increase in the rate at which the deaths were falling. It is absolutely clear that this has not happened. It is difficult to understand why the authors of the recent report have not considered how the death rates have been changing over the much longer time, especially as the data are available.

The reality is that doubts have emerged about the desirability of reducing salt too far (5). These have been spelled out in a recent review published by the prestigious Institute of Medicine (IoM) in the USA. This  considered studies indicating that some sections of the population in the USA may not be consuming adequate quantities of salt, especially those over 51 years, African-Americans, those with hypertension, diabetes, chronic kidney failure or congestive heart failure. The review concluded there was a genuine danger that some people were not consuming enough salt to meet their requirements fully.

Unfortunately because of the activities of campaign groups and the response by the Government and health professionals it has become the accepted wisdom that “salt is bad”. While it is probably correct that very high levels of salt are damaging to health, there is certainly no agreement on how much is required. The danger is that some people may not be getting enough and will suffer as a result. Ironically it is probably those who are attempting to eat a healthy diet by acting in accordance with the official advice who will be especially at risk!

REFERENCES

  1. http://www.theguardian.com/society/2014/apr/14/research-finds-link-between-drop-in-salt-consumption-and-fall-in-heart-attack-deaths
  2. http://bmjopen.bmj.com/content/4/4/e004549.full
  3. Household Nutrient Data 1940 to 2000 Sourced at:

http://webarchive.nationalarchives.gov.uk/20130103014432/http://www.defra.gov.uk/statistics/foodfarm/food/familyfood/nationalfoodsurvey/

4.Trends in Coronary Heart Disease, 1961-2011. British Heart Foundation

5.http://vernerwheelock.com/?p=293