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Halt your salt: The latest research on salt's link to heart, cancer and brain diseases

As its role in premature death becomes clearer, the case against sodium is hardening. Yet, not getting enough of the right kind of salt brings risks of its own.

Salt is still killing us. Despite decades of dietary advice to cut down, New Zealanders’ average salt intake of about 9g a day remains about 50% higher than the recommended upper limit of just under 6g.

Although another white powder, sugar, has snaffled the health headlines in recent years, our high sodium intake has well-established links with high blood pressure, a major risk factor for cardiovascular diseases such as heart attack, stroke, congestion and circulatory conditions. In New Zealand, cardiovascular disease is our biggest killer, accounting for a third of deaths annually. Every 90 minutes, another New Zealander dies from heart disease.

“Cutting down the amount of salt we consume is one of the most important factors when it comes to reducing blood pressure,” says Dave Monro, food and nutrition manager for the Heart Foundation.

Salt is linked to a number of other health problems including stomach cancer, osteoporosis and kidney disease.

A 2018 study published in the International Journal of Epidemiology found “a direct linear relationship” between sodium and risk of death once people consume more than 1200mg of sodium (about half a teaspoon of salt) a day – well below the typical daily intake for New Zealand adults of about 3400mg.

There’s new evidence linking salt to cognitive decline and Alzheimer’s disease. Researchers at New York’s Weill Cornell Medical College using mice found a high-salt diet led to a gut reaction that compromised the brain’s blood vessels and caused a build-up of tau, the protein linked to Alzheimer’s. They suspect excessive salt may negatively affect brain health regardless of its effect on blood pressure.

Read more: Is sea salt more healthy than table salt?

Dave Monro. Photo/Gino Demeer/Supplied

Hidden exposure

The problem with salt is its insidiousness. Unlike other culprits in cardiovascular disease such as alcohol, tobacco and saturated fats, it’s not so easy to take individual responsibility by cutting down at home. That’s because salt is so ubiquitous in our food supply: three-quarters of the salt we consume is from processed food – from bread, breakfast cereals and processed meats to soups and snack foods. Manufacturers add salt not only for its preservative and food-processing roles but also because they know we crave it.

“For most of human existence, we were only getting the pinch of salt a day naturally found in whole foods,” says US nutrition expert Dr Michael Greger in his latest book, How Not to Diet. “Now, thanks mostly to processed foods, we’re exposed to 10 times more than our bodies were meant to handle.”

A staggering 15,000 tonnes of salt is used each year to feed our national salt habit. Even though more than 320 tonnes of salt was removed from New Zealand’s food supply in 2019 – thanks to the Heart Foundation’s food reformulation project with industry – it’s a small drop in our salty habits. Countering the patient effort to persuade food manufacturers to use less salt is the trend to buy more convenience foods and to eat out more.

There’s also increasing scientific interest in the potential direct link between salt and weight. For decades, studies have linked salt intake to excess body fat, but this was thought to be “because high salt and calories tend to travel together in the same foods”, writes Greger. Salt also promotes thirst, he says, “and what do most Americans drink on a given day? Sugar-sweetened beverages such as soda.”

But there is growing evidence that there may be a more direct link between sodium intake and obesity. Studies controlling for total calorie intake, including sweetened drinks, still found a link between salt intake and body fat or obesity, he says, though he acknowledges the studies’ limitations. “After controlling for a number of potential confounding factors, those who ate the most salt appeared to shift their body compositions to more fat and less lean tissue.”

One experiment found that when people switched to a low-salt diet, levels in their blood of a gut hormone called ghrelin, the so-called hunger hormone, dropped. When participants shifted to a high-salt diet, their ghrelin levels shot up.

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Diets high in sodium, low in whole grains and low in fruit together accounted for more than half of all diet-related deaths globally in 2017, according to the latest Global Burden of Disease Study, published last year. High sodium intake was the leading dietary risk for death and disease in China, Japan and Thailand. In China, researchers have found evidence of a genetic predisposition to salt sensitivity.

But just as a little sugar has health benefits, salt is not all bad news. It contains an essential mineral that helps with fluid balance, nerve transmission and muscle function. There’s concern that health- and environmentally conscious consumers turning away from meat, dairy and processed foods and eschewing iodine-fortified salt unknowingly risk a return of diseases linked to iodine deficiency, such as energy-sapping hypothyroidism and goitre, not seen widely in New Zealand since the 1930s. People at particular risk of iodine deficiency include vegans, those avoiding mass-market breads and possibly even those switching to plant-based milk products.

Although some recent research questioned the link between sodium and health problems, these studies were later found to have methodological issues. “The Heart Foundation and many other leading academics still view the totality of the evidence as supporting salt reduction to lower the risk of heart disease at both a population and individual level,” says Monro.

From the multinational “Intersalt” study in the 1980s onwards, the evidence is clear that, for most people, a reduction in dietary sodium intake lowers blood pressure. Reducing our blood pressure is one of the key things we can do to reduce cardiovascular disease risk, along with not smoking, being active and changing the way we eat. Other cardiovascular disease risk factors, such as age, gender, ethnicity and family history of heart disease, are beyond our control.

Rachael McLean. Photo/Supplied

But do we all need to reduce our salt intake? In short, yes, says Dr Rachael McLean, a public health physician at the University of Otago and a member of the Ministry of Health’s sodium expert working group. “One of the things we’ve known since the 1980s is that even people with relatively normal blood pressure will still lower their risk of having a heart attack or a stroke by lowering their blood pressure just a little bit more. So, if we can lower everybody’s risk of having cardiovascular disease, then that makes sense from a population health perspective.”

And the path to do this is clear, says McLean. “We’ve done a lot of research in trying to help people to reduce their salt intake, and it’s really difficult for them to do it on their own.”

Just dining out for lunch once a week and eating bread or breakfast cereal every day could be enough to tip you over the recommended limit for sodium. “There’s quite a lot [of sodium] in breakfast cereals. So, unless you prepare every single thing you eat from scratch, it’s very difficult to control how much sodium you consume.”

That means a two-pronged attack on salt is required if we’re to truly tackle our excessive intake, with both individuals and the food industry playing key roles. “The best things we can do to lower the population’s salt intake are to change the diet and to decrease the amount of salt in processed food – then at least people get a choice,” McLean says.

“If we can reduce salt in the food supply, then probably everybody will reduce their salt intake, but particularly the people who need to the most.”

University of Otago, Wellington public health researchers looking at the potential for salt-reduction interventions have argued the benefits would be relatively greater for Māori than non-Māori, “so sodium reduction is a way of reducing health inequalities generated by higher cardiovascular disease rates among Māori”, they wrote in a 2015 study. Christina McKerchar, a lecturer in Māori health at the university’s Christchurch campus, suspects Māori may be more at risk through eating specific foods, such as shellfish, which are high in salt.

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Targeting manufacturers

The Heart Foundation’s Monro agrees that reducing our consumption of processed foods and the amount of salt they contain is the key to reducing overall salt intake and sodium-related diseases. In 2007, the foundation began a pilot food reformulation project with bread bakers that reduced the average salt content of loaves in the first year alone by about 15%, says Monro, removing about 150 tonnes of salt from the targeted products.

Since then, the food reformulation project has been expanded to 17 different food categories. “The leading sources of salt in our diet continue to be grain products, particularly breads, breakfast cereals and processed meat products such as bacon, sausages and ham.”

The goal is for manufacturers of the leading 80% of products (by sales volume) in each food category to meet the reduced-salt targets within the agreed timeframe. “The focus of the work continues to be on high-volume, lower-cost products for maximum public health impact,” Monro says.

Removing salt from processed foods is not as simple as it may sound, however, because salt is added not only to enhance taste, but also for preservative and processing functions. Salt slows microbial growth by reducing water activity at particular concentrations. Its use as a preservative is particularly important in meat and meat products, pickled vegetables, fermented products, sauces and chilled foods. It delays mould formation in baked products such as bread and cakes, increasing their shelf life.

Salt also plays an important food-processing role: for example, in the development of dough and in the structure of bread.

There are different constraints and technical barriers between, say, bread and processed meat.

Manufacturers are also wary of changing the taste profile of their popular products: salt reduction may not be something they want to advertise. “Some consumers don’t necessarily want their favourite product changed in any particular way,” says Monro.

Humans naturally prefer salty tastes. However, that preference is related to salt intake over the previous 8-12 weeks, meaning we can adjust salt preferences to either a high- or low-salt diet accordingly, University of Otago researchers maintained in a 2011 New Zealand Medical Journal article. The Heart Foundation conducted consumer research that found salt differences of up to 10% in bread were typically not detectable. The fact that we generally eat bread with a spread or filling helps to reduce the chances of flavour changes being detected.

By gradually reducing sodium levels over time, the public is less likely to pick up any flavour changes, says Monro. The foundation encourages major companies within a food category to do salt reduction at the same time. “Consumers don’t tend to notice.”

Some products are at the “least healthy” end of the spectrum, he says. “So, if we were to go out with companies talking about a particular product now being healthier because it has 15% or 20% less sodium, we wouldn’t want the consumer thinking they can eat more of that particular product. It’s a difficult communication message.”

The foundation is continuing to revise its salt targets downwards to gradually shift New Zealanders to a lower sodium intake. “The bread target, for example, has been revised twice since originally being set in 2007.”

Before the project started, some New Zealand breads had sodium levels of about 550mg per 100g. They’re now 30% lower at about 380mg, says Monro.

“We’ve also seen similar reductions in cereals such as cornflakes and rice bubbles.

“We now have more than 30 different salt-reduction targets, which are customised to the different food categories. And, in 2016, the work programme was expanded to include sugar-reduction targets for categories such as breakfast cereals, sauces and yogurt.”

The categories for sodium reduction will continue to expand and evolve as the market changes. “If you think of [food] categories such as rice crackers, even 15 years ago we might not have had them in the market,” says Monro. “Categories of food evolve, and we get new lines of products coming in.”

The foundation is monitoring other changes in the way we eat. With more consumers shifting to a low-meat diet that includes vegetarian meals, it’s aware it may soon need to track and develop targets for vegan and vegetarian meat alternatives.

“Traditionally, you had tofu, but now you have other products made out of soy protein and various things such as chicken-free chicken, made out of pea flour,” that are potential sources of added dietary salt.

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Less salt, less iodine

But as the foundation continues work to reduce salt levels in numerous food categories, and consumers themselves shift from processed foods towards lower-sodium wholefoods, the potential for a key dietary deficiency is likely to rear its head. Again.

In the 1920s and 30s, a national health crisis occurred when widespread iodine deficiency resulted in many people developing goitre (enlargement of the thyroid gland) or hypothyroidism.

Iodine is critical for the healthy growth of the developing brain in particular. An iodine deficiency during pregnancy can cause stillbirth and intellectual disabilities. Unfortunately, New Zealand soils are naturally low in this essential mineral.

Iodine fortification of salt from the 1920s, with a further boost in 1938, significantly reduced rates of iodine deficiency in New Zealand. However, by the early 21st century, the problem had re-emerged as consumers heeded calls to reduce their escalating salt intake for heart health.

Consequently, from 2009, New Zealand bread bakers were required to use iodised salt in most breads.

A subsequent study by Sheila Skeaff, a professor of human nutrition, and colleagues at the University of Otago, conducted in 2012 and published in 2016 in the European Journal of Clinical Nutrition, found that bread was the main source of iodine in the diet of New Zealand adults (47%) and that their iodine status was likely to be adequate following bread fortification. However, since then, not only has the salt content of bread been reduced (effectively reducing its iodine content, too), but also many people are  choosing to eat less bread.

That’s a real concern given that bread is the only New Zealand food fortified with iodised salt, says Skeaff. “If they’re not eating bread, and that’s the main source, and they’re not eating iodised salt, where are they going to get the iodine from, unless they’re eating a lot of shellfish and fish – and most people don’t really do that.”

Skeaff’s concern seems well founded, with a recent pilot study by Massey University finding that middle-aged women with low bread intake are at risk of inadequate iodine levels. The cross-sectional study, involving 46 New Zealand women aged 40 to 63 who consumed less than one slice a day of iodine-fortified commercial bread, found their average iodine intake was below the recommended dietary intake of 150 micrograms a day. Low levels of iodine excretion in urine confirmed the women had iodine deficiency. Although only a small study, it supports Skeaff’s concern about how diet trends and changes in the way many of us eat – made in the name of good health – can unintentionally lead to nutritional deficiencies.

Sheila Skeaff. Photo/Sharron Bennet/Supplied

Skeaff is also wary of the growing popularity of plant-based milks, such as soy, almond, rice and oat milks. “They’re not necessarily going to be good sources of iodine, because they’re not coming from a cow. The mammary gland in the cow, like a human, actually bioconcentrates iodine from the diet, so it’s a relatively good source of iodine. “If you’re not using cows’ milk, then that might drop iodine intake as well, although it’s hard to know.”

Certainly, if past research is any indication, there is a real risk that our growing fondness for plant-based diets could harm our iodine status. A Slovakian study, published in 2002 in the Annals of Nutrition & Metabolism, investigated iodine status in vegans, vegetarians and adults on a mixed diet. It found that a quarter of vegetarians and 80% of vegans were iodine deficient, compared with 9% in the mixed-diet group. It concluded that a diet exclusively or prevailingly of plant origin, with no fish or other sea products, and reduced iodine intake in the form of salt, may have been responsible for the high prevalence of iodine deficiency among meat-free eaters.

“When you think about replacing one food with another food that sometimes seems like a healthier alternative, you have to think about all the other things,” says Skeaff. The side effect of a potential iodine deficiency for consumers choosing more plant-based foods and less dairy and meat for health and environmental reasons is receiving little recognition.

“If bread is the only thing that we’ve really fortified other than iodised salt, and people aren’t eating the other kind of foods that are natural moderate sources of iodine, their iodine status is going to be what it was [before bread fortification],” says Skeaff. Which means we need another plan to improve our iodine status, as, clearly, increasing iodised salt intake is not an option.

Skeaff would like to see other foods being used as a vehicle for delivering iodised salt to the New Zealand population. “You’ve got to look at what people are eating. I want us to put it in pasta. Even though that’s still [traditionally] gluten based, you do use a lot of salt when you’re making dry pasta.” Skeaff also thinks that using iodised salt in tinned vegetables that are staple foods might be an option.

The Massey University research team says larger studies are required to assess if low bread intake is placing New Zealanders at risk of inadequate iodine levels. But it agrees alternative strategies may be needed to deliver sufficient iodine to the population, including fortification of other foods. In the meantime, any salt added by cooks or at the table should be iodised table or iodised rock salt.

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Back to nature

Of course, salt isn’t the only dietary factor that influences our heart health. “There are a range of dietary approaches that can be heart healthy. It’s not just one formula,” says Monro. But there is one consistent theme, he adds, and that’s eating more vegetables, fruit and intact whole grains while cutting down on processed foods.

“While we work with processed food companies to reduce sugar and salt levels, we also have these important dietary messages about cutting down on processed foods and actually choosing foods as close to how they’re found in nature as possible.”

The most important thing is to look at your total diet, he says, rather than fixating on one or two nutrients. Although choosing a lower-sodium bread and breakfast cereal is a great starting point, it’s also important to eat more fruit, vegetables and whole grains  and minimise processed foods.

What that diet consists of can be based on personal preferences and circumstances – wholefoods can be relatively expensive. But whether it’s a Mediterranean style of eating, a Nordic diet, or following the Dash (Dietary Approaches to Stop Hypertension) diet, all have been shown to reduce blood pressure – unsurprisingly, since all of these dietary patterns are rich in fruit, vegetables, whole grains, legumes, seeds, nuts, fish and dairy and low in meat, sweets and alcohol. Add to that a lifestyle that includes regular activity and it’s a winning formula for cardiovascular health.

McLean says about 10% of our intake is from sodium naturally in foods, and there’s some in water as well, which is enough to keep us going. All the rest of the sodium we’re consuming through processed foods is unneeded. “It’s almost impossible to have a low sodium intake in our current food environment.”

As an alternative to adding salt for flavour in home cooking, herbs and spices can be successfully used, with taste buds quickly adapting. In a US trial in 2014, participants reduced their average sodium intake by 1000mg a day.

It’s not impossible to change an entire nation’s high-salt habits. In the UK, a major salt-reduction programme that began in 2003, pushed through under threat of regulation by the Department of Health, resulted in substantially less sodium in the food supply. Over seven years, there was a 15% decrease in the average salt intake of the population at a relatively small cost, with potentially major healthcare savings and reductions in disease and premature death.

Says McLean: “We would love the Government to do a bit more and set some targets for the food industry. The Heart Foundation is doing a really good job within its scope.  But the places, the companies, around the world that have seriously reduced the salt intake of their population have had some degree of regulation and a much more active government involvement in sodium reduction.”

With a grain of salt

Only 2% of NZ shoppers were able to correctly identify the amount of salt in a can of beans.

A quick check of the nutrition information panel on food packages reveals there is no detail on salt content, only sodium. Salt is, in fact, sodium chloride and sodium makes up just under 40% of its content.

However, a 2006 New Zealand study found more than 58% of shoppers believed sodium and salt were interchangeable, and that the sodium figure given on the nutrition information panel represented the salt content of the food.

Consumers need to multiply the amount of sodium listed in the nutrition information panel by 2.5 to work out how much salt a product contains.

This isn’t common knowledge, with the same research team finding that only 2% of New Zealand shoppers were able to correctly identify the amount of salt in a can of beans based on the nutrition information panel.

The Ministry of Health recommends an upper daily limit for sodium of 2.4g, equivalent to about 6g of salt. To optimise diets and lower chronic disease risk, however, it suggests lowering sodium intake further to 1.6g a day, or 4g of salt.

And the World Health Organisation recommends a daily sodium intake of less than 2g for adults, equivalent to 5g (just under a teaspoon) of salt from all sources – water, whole foods and processed foods.

Genes suspect in salt sensitivity

Some people who consume too much salt are more at risk of developing hypertension than others.

In recent years, there has been much talk about how genetics alters disease risk, including our risk of high blood pressure from a high-salt diet.

“It seems highly likely that there is a genetic predisposition to being more salt sensitive,” says Dr Rachael McLean of the University of Otago. “And some people seem to have a greater blood pressure reaction to high-salt diets than others. But we don’t have a good way of telling who those people are on an individual level.”

In 2010, Chinese researchers conducted a large family-based dietary trial in rural North China. “What they did was give people high- and low-salt diets and identify individuals who seemed to have a greater reaction in terms of raised blood pressure to a high-salt diet,” says McLean. The Genetic Epidemiology Network of Salt Sensitivity (GenSalt) study found that 39% of Chinese adults were sodium sensitive: their blood pressure was raised more on a high-salt diet than that of other individuals. It also revealed that sodium sensitivity was more common in women, older adults, individuals with metabolic syndrome and those who typically had higher blood pressure.

“In some ways, developing hypertension is a good indication that you may be more salt sensitive than other people,” says McLean. For instance, in randomised controlled trials there is clear evidence that lowering salt intake has a bigger effect on the blood pressure of people with pre-existing hypertension than those with normal blood pressure. By reviewing genetic variations in the more sodium-sensitive individuals, the Chinese research team compiled a list of candidate genes that may play a factor in determining sodium sensitivity.

A number of different genes may have a role, says McLean. There are also many potential mechanisms by which sodium sensitivity might occur, whether the effect is on the kidneys, the lining of the blood vessels or a combination of both.

“There certainly doesn’t seem to be one individual gene with one individual mechanism that dominates things.

“We’re a long way from understanding how genes are involved in salt sensitivity and we’re even further from being able to identify individuals that are more salt sensitive than others, either on a genetic or a clinical basis. Until we know a bit more about salt sensitivity, it makes sense for everybody to reduce their salt intake.”

This article was first published in the January 18, 2020 issue of the New Zealand Listener.

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