Salt intake that is often deemed high may actually have benefits, scientists say.
“We humans eat more salt than is necessary. But we all do it. So the question is: why?” asks Paul Breslin, a professor of nutritional sciences who researches sodium appetite at New Jersey’s Rutgers University.
In the past, people thought that salt boosted health — so much so that the Latin word for “health” — “salus” — was derived from “sal” (salt). In medieval times, salt was prescribed to treat a multitude of conditions, including toothaches, stomachaches and “heaviness of mind.”
While governments have long pushed people to reduce their intakes of sodium chloride (table salt) to prevent high blood pressure, stroke and coronary heart disease, there are good reasons why cutting down on salt is not an easy thing to do.
Scientists suggest that sodium intake may have physiological benefits that make salt particularly tempting — and ditching the salt shaker difficult. It comes down to evolution.
“In biology, if something is attractive and we invest in gaining it, it must be beneficial, adaptive in evolutionary terms,” says Micah Leshem, a professor of psychology at Haifa University in Israel, who spent decades researching salt’s unique appeal.
People tend to consume about the same amount of sodium no matter where they live, and this amount hasn’t changed much in decades. Those facts hint at the biological basis of our sodium appetite.
A 2014 analysis of data that spanned 50 years and dozens of countries (including the United States, France, China and several African nations, including Zimbabwe and South Africa) found that the quantity of sodium that most people consume (and then excrete) falls into a historically narrow range of 2.6 to 4.8 grams per day. (And then there are extremes: In 16th-century Sweden, for example, people ate 100 grams a day, mostly from fish that had been salted to preserve it.)
“Over the last five decades, salt content of commercial food in our food [in the United States] has gone up. But if you look at people’s 24-hour urinary sodium excretion, you see that the amounts of salt people consume have been constant,” he says. Irrespective of age, sex or race, between 1957 and 2003 Americans have been eating on average 3.5 grams of salt a day. “This suggests that we are somehow regulating the amount of salt we are eating,” Breslin says.
And, in fact, salt is good for us. Sodium is necessary for preventing dehydration, for proper transmission of nerve impulses and for normal functioning of cells. If we ate no sodium at all, we would die. When they become sodium-deficient, many animals go out of their way to find the mineral. That’s why, for example, sweaty clothes of alpinists tend to attract mountain goats.
Sodium depletion can develop after severe sweating, diarrhea or vomiting or, if you are a lab rat, after it is induced by a scientist. Pharmacology professor Alan Kim Johnson and colleagues from the University of Iowa gave rats diuretics and found that sodium-depleted rodents acquired a strong attraction for salted chips. In other experiments, sodium-deficient animals hungrily drank ultra-salty solutions that they would otherwise find disgusting.
Once sodium deficiency is experienced, salt cravings can last a lifetime. That happens with humans, too — but only if the deficiency strikes in very early childhood, or even before birth.
If your mother suffered frequent vomiting in pregnancy or if you lost significant amounts of sodium as a baby (due to vomiting or diarrhea, for example), chances are good that you eat more salt than other people do, even by as much as 50 percent, as one of Leshem’s studies has shown. This is probably because sodium depletion alters our central nervous system so that we develop long-lasting preference for the mineral, Johnson says.
In one of Leshem’s studies, babies who had low concentrations of sodium in their blood in the first weeks of their lives grew up to be teenagers with a penchant for salt, even salt that is seemingly hidden in processed foods. “Even if you can’t taste the salt, apparently your body does. It’s working on an unconscious level to condition a preference for sodium,” Leshem explains.
This makes sense from an evolutionary perspective, says Daniel Fessler, a UCLA anthropologist: If a mother or her infant experiences sodium hunger, it could mean that salt is hard to come by in their environment — and so it is better to be on a constant lookout for it. That is also how humans may have evolved their general liking of salt, Johnson says: “Mankind spent a lot of evolutionary history on the hot African savanna where salt was very scarce and readily lost from the body. Since severe sodium depletion can cause circulatory collapse, there was a selection for mechanisms to save sodium and to drive us to consume more of it.”
Eating salt may also help calm us, or reduce our stress. In animal studies, the effects are pretty clear. An experiment published in 1995 showed, for example, that when rats are put in stressful situations, they choose to drink salty water rather than unsalted water. In another study, when wild rabbits were stressed, their sodium intake shot up.
The possibly stress-reducing, or mood-enhancing, effects of salt in humans are not as well documented, but there is some evidence. In a 2014 study involving about 10,000 Americans, Leshem and his colleagues found a relationship between salt intake and depression: Women whose diets were high in sodium were less depressed than other women. “Maybe people are self-medicating with salt,” he reasons. “But that’s a small effect; salt is not going to cure anyone of depression.”
Breslin believes there may be another evolution-based reason why we love salt: “Salt accelerates sexual maturation in animal models, resulting in more offspring,” he says. Male rats on high-sodium diets, for example, have increased sperm counts. And in a 1991 experiment, men whose sodium intake was lowered to 2.4 grams a day complained of erectile dysfunction more often than those who consumed three grams a day. “The most problematic was a combination of a diuretic and a low-sodium diet,” says epidemiologist Sylvia Wassertheil-Smoller, the study’s lead author.
What’s more, women from the Yanomami tribe in Brazil, famed for their low salt intake (23 milligrams per day — less than 1 percent of what the average American consumes), have fewer children than could be expected, and they often miscarry. Yet according to Tilman Drüeke, a nephrologist who researches fertility and sodium intake at the French Institute of Health and Medical Research, this observation should be taken “with a grain of salt” because, he says, ”the relatively low fertility and high rate of pregnancy loss in Yanomami women clearly cannot be attributed to their very low salt intake alone. This is only one hypothesis among several others, including the higher prevalence of infectious diseases.”
It’s also possible that sodium aids growth. As scientists from New Jersey Medical School found out, if you put rats on low-salt diets, their bones and muscles fail to grow as fast as they normally would. In one of his experiments, Leshem found that children in general reach for more salt than adults do — independent of calorie intake — which may be explained by the needs of their growing bodies.
Finally, there are a few diseases that can turn a few of us into salt gluttons. About 15 percent of people with adrenal insufficiency (Addison’s disease) — which can cause weakness, anemia and low blood pressure — experience acute salt cravings. Hiding saltshakers from them may not be a good idea.
In 1940 the case of a little boy was described in the Journal of the American Medical Association. From the time he was a year old, the boy would go out of his way to eat massive amounts of salt. When he started speaking, one of his first words was “salt.” During a hospital stay (unrelated to his dietary habits), he was put on a low-sodium diet. To prevent him from sneaking around the hospital and stealing salt, he was strapped to his bed. He soon died. The reason? Due to severe and undiagnosed cortico-adrenal insufficiency, his kidneys were unable to retain sodium. Only eating huge amounts of salt had kept the boy alive.
Yet most of us do not need huge amounts of salt to survive. Just the opposite: About half of humans are what is called salt-sensitive: If they consume lots of sodium, their blood pressure will go up. But if we do have internal regulatory mechanisms that tell us to load up on salt when our bodies need it (for growth, for mood improvement or to simply prevent dehydration), does it even make sense to encourage people to try to reduce their dietary sodium? It does, Breslin says, but only to a point.
“If people are regulating their sodium intakes, they are not going to be able to reduce it a lot — say, by 50 percent or more. It would be like putting someone in a room and cutting the amount of oxygen by half: Your body will try to maintain the level of oxygen in your blood and will make you breathe faster.” And so, as Johnson suggests, when it comes to salt intake, “moderation is probably ideal.”
Cut your sodium intake if your health condition requires it and your doctor recommends it, but don’t look at salt as an evil that should be banned from your plate completely: There may be valid reasons why your body craves it.
Zaraska is a writer based in France.