If you’re confused about whether salt is really the number one enemy of public health, you’re probably not alone. And it’s the fault of people like me. I’ll say it plainly: a lot of the journalism about sodium intake is crap.
These stories often start by reporting on new observational studies — in isolation, of course — and end with some inane recommendation about whatever the latest piece of the scientific puzzle demonstrates, instead of talking about what we should glean from all the research that has been done.
There’s now enough high-quality research that you don’t need to be confused. The truth, according to science right now, is pretty straightforward: a very high sodium diet (two teaspoons or more of salt each day) is probably harmful; a very low sodium diet (less than one teaspoon per day) may also be harmful; about one teaspoon (or 2300 mg) seems to be just right for most people.
Many Americans get more than that, mostly from salt added during food processing. The interesting thing about the salt debate is this: for some people, consuming extra salt probably doesn’t matter all that much. (More on this below.) For others, though, it does, and sodium intake — unlike obesity or stress — is one of the few risk factors we can control when it comes to heart health. This is why salt has become the focus of so much public-health campaigning.
So what does this mean for you? Here's all you need to know about salt intake and health. And unless some monumental bit of research comes out that shakes up all the previous science on salt, these conclusions won't change much any time soon.
Why does salt matter for health?
Blood pressure refers to the force your heart uses to pump blood through your circulatory system. When you have high blood pressure (known as hypertension), the blood is pumping too hard, sometimes through arteries that are too narrow, which stresses your heart and causes it to work harder than it should.
How does this relate to salt? Blood is basically made up of platelets, and red and white blood cells, suspended in a salt-water solution. (Salt is also present in urine and tears and many other fluids in the body, such that the average adult contains the equivalent of about three or four saltshakers worth of sodium.)
When you eat a lot of salt, it’s necessarily absorbed into the body, but your body retains water along with it so that the ratio of sodium to water holds stable. (That's why you crave water when you eat very salty foods.) Because your body is retaining water along with the salt, your cells — including blood cells — expand. The volume of your blood, therefore, swells, boosting the pressure on your arteries.
Some people’s bodies do a good job of filtering excess sodium but others’ don’t. "Salt sensitivity" increases as we get older and our arteries harden. It's also more common among African Americans, obese people, and those who have chronic kidney disease. (These are all "high risk" groups for hypertension.)
The reason this matters for health is simple: high blood pressure is associated with heart attacks and heart failure, which are among the leading causes of death in America. The thinking is that if we cut back on salt, we’d reduce the risk of high blood pressure, and all the death and destruction that it can eventually wreak.
Why is salt so controversial?
The question of how much of an impact reducing one's salt intake can have on tangible health outcomes like death and cardiovascular disease has been the subject of fervent debate among researchers for two reasons.
One: there are a number of factors that can affect blood pressure: genetics, exercise, body weight, alcohol consumption, stress, age, overall diet. So it's not only about salt.
Two: many of the studies linking salt consumption to hard end points like disease and death have only tracked changes in blood pressure over a short period of time. But blood pressure is just an indicator — or "surrogate endpoint" — in health research. It’s not a real health outcome, like heart attacks or death. Doing an experiment — assigning thousands of people to a particular level of salt intake over many years to figure out how their diet related to their risk of developing cardiovascular disease and death — is much more difficult and expensive.
So the controversy lies in the question of whether the temporary boosts in blood pressure we've seen from the studies we have actually end up taking a toll on the heart and overall health in the long term.
To truly answer this question, some argue we'd need a big, randomized trial that looks at various levels of sodium restriction and the impact on cardiovascular disease and mortality in 20,000 patients for five years. As yet, no one has funded such a study.
One of the best trials the big health agencies rely on — the DASH study — only ran for 30 days, comparing the blood pressure levels in two groups of people on a regular diet or low-sodium diet. (So, again, they didn't look at disease or death in the long term.)
One 2014 meta-analysis, which examined all the best research on sodium intake, concluded, "There was weak evidence of benefit for cardiovascular events, but these findings were inconclusive and were driven by a single trial among retirement home residents, which reduced salt intake in the kitchens of the homes." Another, published in 2011, found, "we do not know if low salt diets improve or worsen health outcomes," and "more research on reduced salt intake is required."
How much salt should you eat?
Several years ago, the Institute of Medicine — an independent, non-profit organization that provides medical advice — convened an expert panel to review all the evidence on dietary sodium and health outcomes.
In 2013, the group published a report in which they agreed that it made sense to aim at reducing sodium intake around the world, but they also said they couldn't find any evidence that a very low sodium diet was helpful to anyone, not even those high-risk groups (despite what the Centers for Disease Control and Prevention now recommends for high-risk populations).
"The only conclusion we could draw is that there haven’t been enough studies to make any recommendation for lowering your sodium intake below 2,300 mg/day [or about a teaspoon]," said Maria Oria, an Institute of Medicine scientist who worked on the report. So the IOM said that, based on the best-available evidence, about 2,300 mg/day seemed fine for most people.
This is something every expert I spoke to for this story agreed on. "There’s no evidence lowering sodium below 2,300 mg will do you any good," said sodium expert and Albert Einstein College of Medicine professor, Michael Alderman.
He also pointed out that there’s never been a nutrient with a linear relationship to health that ends in zero. In other words, the health benefits of nutrients tend to exist in a "J-shape": if sodium intake was plotted on the y-axis, and cardiovascular events were plotted on the x-axis, you'd see people who have the most health fall somewhere in the middle.
Emory University's Andreas Kalogeropoulos — the author of a new sodium study in JAMA Internal Medicine — looked at the effects of salt intake on mortality, cardiovascular disease, and heart failure in older people (ages 71 to 80). (This was an observational study based on data gathered for another purpose, not a big experiment.) He also found the extremes weren't beneficial.
"There seems to be a ‘sweet spot’ of salt intake restriction, after which it is difficult to observe extra benefit with more restriction," he said. "On the other hand, in all studies, including ours, high salt intake (say, two teaspoons of salt or more daily) was harmful."
What’s the easiest way to control salt intake?
"If it’s fresh," said UT Southwestern blood-pressure researcher Norman Kaplan, "you don’t have to worry about the sodium. The fact that nothing in nature is high in salt should tell people something."
You'll notice, in the handy Vox chart at the top of the story, that restaurant foods have an exaggerated amount of sodium, and fruits and vegetables, very little.
About 80 percent of the sodium you eat comes from salt that's added during food processing. So the easiest way to cut back on salt is to avoid prepackaged foods and restaurant meals wherever possible. When you eat food that you prepare yourself, you shouldn't have to worry about salt (unless you're sprinkling your meals with a snowy coating of the white stuff).
This may not be easy in an environment where many of us rely on quick, ready-made foods to get by, which is why some public-health officials continue to call on governments and industry to find ways of cutting salt during food processing. People like Alderman think that this could create unintended consequences, which we saw when the low-fat craze led food manufacturers to pack sugar into their foods to compensate for lost flavor. But again, you can avoid any potential harms and the negative health effects by just preparing more food at home.
For more on how to eat better at home, see the Vox Anti-Detox Diet.