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Reduction in salt intake DOES lower blood pressure and disease risk
Eating less salt DOES lower blood pressure and the risk of heart disease and stroke, according to a new review which challenges paradoxical studies
A new review by Action on Salt, published in Nature Reviews Cardiology this May, has challenged the previously-published studies of poor scientific quality which have generated controversy and cast doubt on the importance of reducing salt intake on public health. The review, Role of salt intake in prevention of cardiovascular disease: controversies and challenges (1) reiterates that there is strong evidence supporting salt reduction as a key public health strategy to lower blood pressure and reduce the risk of cardiovascular disease (CVD).
Salt consumption has long been shown to have a negative impact on health, with studies of various types consistently showing a high dietary salt intake is the major cause of raised blood pressure. Raised blood pressure can lead to CVD, including strokes and heart failure, which is the leading cause of death and disability worldwide.
A high salt diet is also linked to a number of other conditions, such as osteoporosis, stomach cancer and kidney disease, with new evidence on the harmful effects of salt emerging all the time.
The WHO has set a global target of reducing population salt intake from the current level of approximately 10g/day to <5g/day. This recommendation however, has been challenged by several studies suggesting a J-shaped relationship between salt intake and CVD risk – an increased risk of CVD with lower intakes as well as higher intakes of salt.
This review highlights the severe methodological problems of these studies, such as reverse causality and measurement error due to poor assessment of salt intake via spot urine.
Reverse causality often arises in studies where the people who already have medical conditions, such as chronic kidney disease or diabetes mellitus, are included. These studies propose that a lower salt intake is associated with a higher CVD risk, when in fact the people in the study are likely to eat less food and less salt due to their poor health, or may already have been advised to reduce salt intake.
Therefore, the finding that lower salt intake is linked to higher risk of CVD is likely to be due to the underlying disease. It is not the low salt intake that is causing their death, but rather their illness that is causing their low salt intake.
Measurement error occurs in cohort studies that use a single spot urine sample to estimate an individual’s usual salt intake. However spot urine measurements are influenced by several factors, including the amount of fluid consumed and the amount of salt in the last meal they ate, and therefore do not reflect salt intake accurately.
For studies investigating the association between salt intake and health outcomes, multiple nonconsecutive 24-hour urine collections are essential. To date, only three studies have assessed individuals’ salt intake using such methodology, and all showed a direct, linear relationship between salt intake and CVD risk.
Gradual, stepwise salt reduction as recommended by the WHO remains an achievable, affordable, effective, and important strategy to prevent CVD worldwide. In most developed countries, salt
reduction can be achieved by a gradual and sustained reduction in the amount of salt added to food by the food industry (i.e. reformulation). The UK has pioneered a successful salt-reduction programme by setting incremental targets for >85 categories of food; many other developed countries are following the UK’s lead.
Evidence for the effect of salt on health is similarly if not more robust than that for other lifestyle changes, such as eating more fruit and vegetables, increasing physical activity, and reducing smoking. What’s more, unlike these other factors, industry-led salt reduction via reformulation does not require individual behavior change and is more likely to be successful across the whole population, including the more socially deprived.
From a public health perspective, salt reduction is one of the most cost-effective, feasible, and affordable measures to prevent CVD and other noncommunicable diseases (NCD).
Dr Feng He, lead author of the Review and researcher at the Wolfson Institute, Barts & The London, Queen Mary University of London, says:
“Studies of poor scientific quality have unfortunately tried to cause controversy in what is recognised as a robust public health strategy. However, the totality of evidence shows reducing our salt intake will be immensely beneficial in preventing strokes, heart attacks and heart failure, the commonest causes of death and disability in the world. The question is not ‘should’ we reduce salt intake, but ‘how’.”
Professor Graham MacGregor, Professor of Cardiovascular Medicine at Queen Mary University of London, Chairman of Action on Salt and Blood Pressure UK, and co-author of the Review, says: “Salt reduction has been identified as a cost effective measure to reduce NCDs for both developed and developing countries. The UK was once leading the world with an effective strategy to reduce salt, which led to a reduction in population salt intakes and consequently a fall in average blood pressure and fewer deaths from heart attacks, heart failure and stroke. Progress of late has been fragmented and slow, and therefore needs reinvigorating by Public Health England".
Mhairi Brown, Nutritionist for World Action on Salt & Health, adds:
“Eating too much salt puts up our blood pressure, the major cause of cardiovascular disease which is the leading cause of death and disability worldwide. This issue is recognised worldwide, with many countries adopting a salt reduction strategy similar to the UK. If we were able to achieve the WHO global target of <5g/day, then we would prevent more than 1.6 million CVD related deaths per year, as well as major cost savings to individuals, their families and health services.”
The UK was once leading the world on salt reduction, which was shown to be the most cost-effective public health programme. Indeed, up to 2011, the UK salt reduction programme had already saved 18,000 strokes and heart attacks per year, 9,000 of which were fatal, with £1.5 billion a year in NHS healthcare saving costs, according to NICE.
In 2016, PHE assumed responsibility for UK salt reduction, however there has been little action so far, with no progress report on whether the last set of salt targets (due to be met by the end of 2017) have been reached, nor any plans to set new targets. Every 1g reduction in salt intake prevents 7,000 deaths, 4,000 of which are premature, from strokes and heart disease.
The full paper is available to download below.
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1. He FJ, MacGregor GA. Role of salt intake in prevention of cardiovascular disease: controversies and challenges. Nature Reviews: Cardiology. 2018. https://doi.org/10.1038/s41569-018-0004-1