Is too little salt just as unhealthy as too much?
by - Jessica Brown BBC Future
Last year a video of Turkish chef Nusret Gökçe lovingly seasoning a massive steak with a pinch of salt amassed millions of views online and earned him the nickname ‘salt bae’. But it wasn’t just his attention to detail that captured attention.
We’re obsessed with salt – despite warnings we’re consuming too much of it and harming our health in the process. But a counter-argument is gaining ground, casting doubt on decades of research and shedding light on the questions that still remain unanswered about our favorite seasoning.
Sodium, the key element found in salt, is essential for our bodies to maintain its overall fluid balance, transport oxygen and nutrients, and allow our nerves to pulse with electricity. But most populations have historically eaten more salt than advised, and health officials around the world have had their work cut out for them convincing us to cut down.
Guidelines recommend adults have no more than 6g of salt per day. In the UK we consume closer to 8g; in the US, 8.5g.
But only a quarter of our daily intake comes from salt we add into food ourselves – the rest is hidden in the food we buy, including bread, sauces, soups and some cereals.
Adding to the confusion is that on food labels, manufacturers often refer to sodium content rather than salt, which can make us think we’re consuming less salt than we are. Salt is made up of both sodium and chloride ions. In 2.5g of salt, there is about 1g of sodium. “The general public isn’t aware of this, and just think sodium and salt are the same thing. No one tells you this,” says nutritionist May Simpkin.
Research has found that too much salt causes high blood pressure, which can lead to strokes and heart disease, and experts broadly agree that the evidence against salt is compelling. Our bodies retain water when we eat salt, increasing blood pressure until our kidneys flush it out. Too much salt over a long period of time can put strain on our arteries and lead to prolonged high blood pressure, known as hypertension, which causes 62% of all strokes and 49% of coronary heart disease events, according to the World Health Organization.
One meta-analysis of 13 studies published over 35 years found a 17% greater risk of total cardiovascular disease and a 23% greater risk of stroke from consuming an extra 5g of salt per day.
As you might expect, cutting salt intake can have a reverse effect. In one eight-year data analysis of blood pressure, other cardiovascular disease risk factors and mean salt intake, researchers found that a decline in salt intake by 1.4g per day was likely to have contributed to a fall in blood pressure – which in turn contributed to the 42% decline in fatal strokes and 40% drop in heart disease-related deaths.
But in a common theme in observational studies such as this one, the researchers also concluded that it was difficult to entirely separate the effects of less salt from other diet and lifestyle behaviors. Those who are more conscious of their salt intake are more likely to eat healthier overall, exercise more and smoke and drink less.
Long-term, randomised trials comparing people who eat a lot versus a little salt could establish cause and effect. But very few such studies exist because of the funding requirements and ethical implications. “Randomised trials showing salt’s effect on the body are almost impossible to carry out,” says Francesco Cappuccio, professor of cardiovascular medicine and epidemiology at the University of Warwick’s medical school and author of the eight-year review.
“But there are also no randomised trials for obesity, or smoking, which we know kills you.”
Meanwhile, observational evidence is in abundance. After the Japanese government launched a campaign to persuade people to reduce their salt intake in the late 1960s, intake decreased from 13.5g to 12g per day. Over the same period, there were falls in blood pressure and an 80% reduction in stroke mortality. In Finland, daily salt intake dropped from 12g in the late 1970s to as little as 9g by 2002, and there was a 75-80% decrease in deaths from stroke and heart disease in the same period.
An additional complicating factor is that the effects of salt consumption on blood pressure and heart health differ from one individual to another. Studies have found that our sensitivity to salt varies from person to person – depending on factors as varied as ethnicity, age, body mass index, health and family history of hypertension. Some studies have found that those with higher salt sensitivity are more at risk of salt-associated high blood pressure.
In fact, some scientists are now arguing that a low-salt diet is just as much of a risk factor for developing high blood pressure as high salt consumption. In other words, there is a J- or U-shaped curve with a threshold at the bottom where risks starts to go back up.
One meta-analysis, for example, found a link between low salt intake and cardiovascular-related events and death. The researchers argued that consuming either less than 5.6g or more than 12.5g a day is associated with negative health outcomes.
A different study involving more than 170,000 people had similar findings: a link between ‘low’ salt intake, defined as less than 7.5g, and increased risk of cardiovascular events and death in people both with and without hypertension, compared to a ‘moderate’ intake of up to 12.5g per day (between 1.5 to 2.5 teaspoons of salt). That moderate intake is up to double the UK recommended daily intake.
The study’s lead author, Andrew Mente, a nutritional epidemiologist at McMaster University in Ontario, concluded that reducing salt intake from high to moderate reduces the risk of high blood pressure, but there are no health benefits beyond that. And increasing salt intake from low to moderate might help too.
“The finding of a sweet spot in the middle is consistent with what you would expect for any essential nutrient… where at high levels you have toxicity and at low levels you have a deficiency,” he says. “An optimal level is always found somewhere in the middle.”
But not everyone agrees - Cappuccio is unequivocal that a reduction in salt consumption reduces blood pressure in everyone – not just people who eat far too much. He says the wave of studies in recent years concluding contrary findings are small, include participants who are already unwell, and rely on flawed data – including Mente’s study, which used a fasting spot urine test on participants instead of the ‘gold standard’ of spreading several tests over a 24-hour period.
Sara Stanner, science director at the charity the British Nutrition Foundation, agrees that the evidence that reducing salt intake in those with hypertension lowers blood pressure and risk of heart disease is strong. And there aren’t many people are consuming levels as low as 3g, the level at which some of this research calls dangerously low.
This would be difficult to achieve, Stanner says, due to the levels of salt in foods we buy.“So much of the salt we consume is in everyday foods,” she says. “This is why reformulation across the food supply is the most successful approach to cut down on national salt levels, as has been the case in the UK.”
Experts also have conflicting views on whether high salt intake can be offset by an otherwise healthy diet and exercise. Some, including Stanner, say that a diet rich in potassium, found in fruit, vegetables, nuts and dairy, can help to offset salt’s adverse effects on blood pressure.
Ceu Mateus, senior lecturer in Health Economics at Lancaster University, advises that we should prioritise being aware of hidden salt in our diets rather than trying to avoid it altogether.
“The problems we have with too much salt could be similar to those linked with too little, but we still need to do more research to understand what goes on here. In the meantime, a healthy person is going to be able to regulate small amounts,” Mateus says.
“We should be aware that too much salt is really bad, but don’t eliminate it completely from your diet.” Despite recent studies arguing the potential dangers of a low salt diet, and individual differences in salt sensitivity, the most established takeaway from existing research is that too much salt definitely increases blood pressure.
Take the rest with a pinch of… well, you know.