Salt: A shakeup of the science

How much salt is too much? Expert opinion is broad. Photo / 123RF

It is a diet "truth" that we're used to hearing: we eat too much salt and need to cut back.

We know, thanks to ongoing public health campaigns that a high-salt diet can lead to raised blood pressure that can lead to heart attacks and strokes, particularly in the over-50s.

The World Health Organisation recommends 5g per day of salt, or a single teaspoon, about half of what New Zealanders consume a day.

What makes it harder to visualise is that most salt in the diet comes from processed foods, rather than a liberal sprinkling at the table.

However, a growing number of academics say that, actually, the targets are too low.

"Current recommendations on salt stem from the assumption that there's a straight-line relationship between salt and health," says Prof Andrew Mente, the principal investigator of the population health programme at the UK's Population Health Research Institute.

"Yes, there are straight-line relationships for things like tobacco, where we know that the optimal level is zero, because tobacco is a pollutant. Salt, on the other hand, is an essential nutrient. If you didn't eat any sodium, you'd die."

As Prof Mente explains, some sodium is essential for functions including fluid balance, nerve conduction and muscle contraction. So, new theories suggest, there's actually a "sweet spot" of the amount of sodium that we should consume which provides benefits, without attracting mortality risks.

The good news for those of us who like our food to taste of something, is that it's higher than health experts recommend. Prof Mente's research, undertaken with colleagues at McMaster University in Canada, involved looking at the sodium intake of more than 90,000 people in more than 300 communities in 18 countries.

He thinks that the current recommendation - 2g of sodium, equivalent to 5g of salt - is too low, not only as an achievable target, but also as a health outcome.

"Indeed, what the data shows both in our study and about a dozen other studies, is that there's a sweet spot: being in the middle is comfortable. In fact, reducing sodium may cause harm."

He says that the same result is found "whether you're looking at the general population, or subgroups of people with conditions including hypertension, diabetes, or existing vascular disease".

Prof Mente found that the harmful effects of sodium only occurred in countries like China, where the liberal use of soy sauce leads to the equivalent of 12g of salt a day (soy sauce can be 20 per cent salt by weight).

But in Western countries, including the UK, US and Canada, the WHO targets aren't so necessary.

"Most people in the West eat only moderate amounts," he says.The main reason being that the anti-salt campaigns have worked.

In 1994, a national target of 6g salt per day was recommended by the UK's committee on medical aspects of food and nutrition policy and followed up in 2004 by a Food Standards campaign to raise awareness among the public and the food industry.

It was a huge success: within a year, the public had cut their salt intake by 10 per cent and supermarkets had reduced the salt content of their foods by almost a third.

However, last week, an advert for Farmdrop, a fresh-produce delivery service, became the first to fall foul of a new ban on promoting junk food on the London Underground. Its posters, which contained a photograph that included bacon, butter and eggs, were considered not to be "high fat, sugar, and salt (HFSS) compliant".

"Salt intakes have fallen steadily over the past 50 years in the UK for a variety of reasons," says Prof Tom Sanders, emeritus professor of nutrition and dfietetics at King's College in London.

"We consume more fresh foods and fewer salt-preserved foods. More recently, salt intakes have fallen because the food industry had been persuaded to cut salt in processed foods, especially bread, a major source."

Public Health England recently published its first report into the food industry's progress towards meeting the government's salt reduction targets and found, on the whole, good news.

Just over half of all average salt reduction targets were met, with four out of five foods meeting or falling below targets. Our reduced salt intake is undoubtedly a good thing, and Prof Mente is clear this is not a hall pass to up our salt intake.

"We're not saying go and eat all the salt you want - high amounts are harmful. But when you eat an all-round healthy diet, you don't need to worry about single nutrients like sodium."

A healthy diet, he suggests, is one that focuses on whole foods, rather than ready meals, crisps and biscuits, unprocessed meats and filling up on fruit, vegetables and dairy.This, of course, means swerving processed foods, which have been subject to a latest study of almost 45,000 middle-aged people, which found a clear link between death and ultra-processed foods including burgers, pizzas, biscuits and cakes.

"Sugar, refined carbs, trans fats are the problems in the food supply that we need to cut back on," Prof Mente says. Alarmingly for those who have adopted a vegan diet for health reasons, lots of meat substitutes can actually be higher in sodium than the real thing.

According to campaign group Action on Salt, based at Queen Mary University of London, some vegan products were found to be "saltier than seawater". Meanwhile, health bloggers have long been keen to promote pink Himalayan salt, the supposed benefits of which - based on a micronutrient composition that contains up to 84 different minerals and trace elements - range from "balancing the body's pH levels", to improving sleep and regulating blood sugar.

But Prof Mente demurs: "The short answer is we don't know yet. More research is needed on pink Himalayan salt, but mostly at this point it's based on conjecture, rather than evidence."

Overall, says Prof Mente, the message is to stick to real foods, rather than processed ones. "If anything, try to up your levels of potassium," he says. "A higher amount of potassium is found to be associated with a lower risk of heart attack and strokes and total mortality. So, if you have to worry about anything, it would be to eat less sugar and up your potassium, by eating fresh fruit and vegetables. If you eat a healthy diet, your sodium levels will take care of themselves."

Kiwis and salt

• The World Health Organization estimates that high blood pressure is responsible for 17% of all deaths in high-income countries (such as New Zealand).
• One in seven New Zealanders report taking medication for high blood pressure.
• At a population level, reducing sodium intake reduces both blood pressure and risk of heart disease.
• It has been estimated that reducing average sodium intake by 20% would save 930 lives in New Zealand each year.
• New Zealanders currently eat around 9 grams of salt a day; our body needs less than 1 gram to survive. To reduce risk of chronic disease it is recommended to eat no more than 4g salt a day, a little less than the WHO's 5g a day.
• Foods that are almost always high in salt include: bacon, cheese, ham, gravy granules, salami, stock cubes, smoked meat and fish, olives and prawns.