Sodium chloride, known to all by the generic term ‘salt’, is the principal source of sodium in today’s diet. Through multiple lines of research including epidemiology, migration studies and population-based interventions, it is clear that sodium in our diet – our salt intake – is directly linked to changes in blood pressure. The more salt we eat, the higher our blood pressure.
“Though the human body does need a very small amount of sodium to function properly, most people nowadays consume too much salt,” remarks Dr Monique Tan, NIHR Advanced Fellow at Queen Mary University of London. “Of all dietary risk factors, our current high salt intake is estimated to be responsible for most deaths and disability worldwide – meaning that our excess salt consumption is more harmful to health than our low fruits or vegetable intake or our soft drink consumption, for example.”
The link is strong enough for national governments and global health organisations to issue guidelines on salt intake and initiate programmes to reduce it. WHO Member States, for example, have agreed to reduce the global population’s salt intake by 30% by 2025. China’s central government is aiming for a 20% reduction by 2030 as a key component in its ‘Healthy China 2030’ initiative.
In the UK, work on salt reduction began back in 2004 when the Scientific Advisory Committee on Nutrition recommended average salt intake should be reduced to 6g per day to reduce the risk of high blood pressure and cardiovascular disease (CVD).
CVD causes around one quarter of all deaths in the UK and is the largest cause of premature mortality in deprived areas. In China, where salt intake is among the highest in the world at around 11g per day, CVD accounts for 40% of all deaths.
Time for change
Although the link between sodium and blood pressure has been investigated before, research has not affected wholesale change in global salt consumption. Indeed, a vocal minority has always decried the link between salt and CVD, though it is hoped the latest research will quieten those voices.
“It has been a confusing space. There are two main types of study that can be done to investigate the link between sodium intake, blood pressure and CVD,” says Dr Bruce Neal, executive director at The George Institute for Global Health Australia and professor of medicine at the University of New South Wales Sydney. “You can do an observational study, where you ask people how much salt they eat and track them over time to see their health outcomes. The results from these studies can be confounded by other factors and give misleading results.
“For example, if you have had a prior heart attack or stroke you may well have reduced the amount of salt you eat,” he adds. “In an observational study, these people are counted as low salt consumers, despite years of eating high salt before their heart attack. And because they have now already had one heart attack this means they are at very high risk of having another one. This sort of problem can skew the results because it makes it look like people who eat less salt have higher, not lower, risks.”
A physician by training, Neal has been a researcher for 20 years. His long-standing interest has been in blood pressure and how excess sodium consumption affects it, and he has been eager to replicate the process of drug therapy trials in the food and nutrition space.
Consequently, he was involved in China’s recent Salt Substitute and Stroke Study (SSaSS), which sought to emulate the kind of randomised trials that pharmaceutical companies perform when seeking regulatory approval for a new drug treatment.
“The second stronger type of study is a randomised trial,” he remarks. “By randomly assigning people to either salt or a salt substitute you know that everything was equal between the two groups when you started. And that means you can infer whatever results you see at the end to be the effect of the salt substitute. In addition, we wanted to study effects on stroke and heart attacks, not just blood pressure. This meant we had to go big because even in high-risk people these sorts of outcomes are fortunately fairly infrequent. No one had done a large enough study to define effects on stroke and heart attack before.”
“If you have had a prior heart attack or stroke you may well have reduced the amount of salt you ear. In an observational study, these people are controlled as low salt consumers, despite years of eating high salt before.”
Dr Bruce Neal
30%
WHO Member States have agreed to reduce the global population’s salt intake by 2025.
WHO
Spanning five years, SSaSS is one of the largest dietary interventions ever conducted. It enrolled more than 20,000 participants from 600 rural villages in China. All of the participants had a history of stroke or were aged over 60 with high blood pressure.
“We did the study in rural China because they eat a lot of salt there, and the salt is largely added when preparing food in the home,” says Neal. “This means you can change the type of salt people eat quite easily. In the UK, 80–90% of salt comes in processed and packaged food, so it is hard to change to a salt substitute.”
“Also, China has a rural health insurance scheme in which everyone in the study was enrolled, so we could search hospital records every six months for stroke, heart attack and death,” he adds.
In the study, one group continued to consume normal salt (sodium chloride), while the second group was provided enough salt substitute (25% potassium chloride) to cover all household cooking and food preservation requirements – about 20g per person per day – free-of-charge. After five years, the results were striking. In the second group, the risk of stroke fell 14%, the risk of major cardiovascular events by 13% and the risk of death by 12%.
“To ensure a level playing field, clear timeframes were given for the salt targets to be met, and industry compliance was closely and independently monitored.”
Dr Monique Tan
20,000
China’s recent Salt Substitute and Stroke Study, spans five years and involves this many participants from 600 rural villages in China.
Salt Substitute and Stroke Study (SSaSS)
“We’ve been trying to change the amount of salt people eat for 20 years in Australia and elsewhere, but it’s been very hard to get people to change their behaviour and we have almost universally failed,” Neal remarks.
“Potassium-enriched salt looks the same, tastes the same and can be used just like regular salt, so it’s really easy for people to make the switch and stick with it. At the end of the study, 93% of people were still using the salt substitute.”
From China to the world
As part of a larger programme – ‘Action on Salt China’ – a global health research unit funded by the UK National Institute for Health and Care Research and a partnership between Queen Mary University of London, The George Institute China, and several Chinese health authorities has set out to develop salt reduction strategies. Tan plays a key role in that research and, in a study published in BMJ Nutrition, Prevention & Health, found that reducing daily salt intake in China by even 1g could prevent almost 9m cardiovascular events by 2030.
To derive their results, the team extracted the effect of salt reduction on systolic blood pressure (SBP) from a meta-regression of randomised trials and a population study, and that of SBP on CVD risk from pooled cohort studies. Those findings can now be a platform for pushing forward change on a global scale.
“In China, most of the salt that people consume is the salt they add themselves when they cook or prepare food, either in the form of regular table salt or of high-salt condiments such as soy sauce,” says Tan.
“In terms of lowering salt in manufactured foods, what would be needed is for the government to set salt targets for the industry to comply with,” she continues. “This has been done with great success in the UK, for example, which has pioneered this approach by setting progressively lower salt targets for over 85 food categories. To ensure a level playing field, clear timeframes were given for the salt targets to be met, and industry compliance was closely and independently monitored.”
Reinventing the supply chain
Tan believes that a gradual reduction in salt intake across the whole population, as recommended by the WHO, is achievable, affordable, and cost-effective. In part, this is because low-sodium, high-potassium salt substitutes are cost-effective. They cost approximately $1.62 per kilo versus $1.08 per kilo for regular salt.
Furthermore, while sodium raises blood pressure, potassium lowers it. Neal believes that the data from SSaSS could lead to a complete reworking of the global salt supply chain. That sounds like a huge challenge, but it has been done before.
“Firstly, ingredient suppliers need to have potassium-enriched salt on shelf at minimal cost as a wholesale product,” he says. “From the food manufacturer’s perspective, salt is typically just 1% or 2% of the final product and a very low-cost ingredient. So even if the salt substitute is a bit more expensive it should have almost no impact on food production costs.”
“The global supply chain was already switched once in the past 50 years from regular salt to iodised salt – before this switch people in many areas were not getting enough iodine, which leads to enlargement of the thyroid and neurological impairment in children,” he adds. “Salt is a great vehicle for getting iodine to people but it’s also problematic. You are putting a good thing into a bad thing, which gives salt a veneer of respectability it doesn’t warrant. If we now make another switch from iodised salt to iodised and potassium-enriched salt, we can resolve that anomaly.”
From rural China to low-income and mature economies across the world, a huge change could be coming, and food manufacturers must play a pivotal role in making that change happen.
