For years, doctors and dietitians have delivered a firm message about salt: Watch your intake or your blood pressure could rise to unhealthy levels. Yet scientists have known for decades that there is great variation in how humans and other mammals respond to the sodium component of salt (chemically known as sodium chloride). People who are “salt sensitive” experience a rise in blood pressure if they increase their consumption of salty foods—and a dip in blood pressure if they cut back on them. But other people are “salt resistant,” meaning that the sodium content of their diet has little or no effect on their blood pressure.
Interestingly, only about half of all people with high blood pressure, or hypertension, are salt sensitive. Yet roughly one-quarter of people with normal blood pressure are salt sensitive, too. That matters because having blood pressure that rises and falls in response to the amount of salt in your diet—even if it doesn’t exceed the threshold for a diagnosis of hypertension— appears to increase the risk of heart attacks, strokes, and other forms of cardiovascular disease. People who are salt sensitive also die younger on average than their salt-resistant counterparts.
Researchers are taking a growing interest in salt sensitivity. Here’s what they know—and don’t know—about it.
An uncertain mechanism
Sodium helps regulate the amount of water in the body, among many other important roles. When there’s too much sodium in the bloodstream, it draws water into the blood vessels, raising pressure inside. Over time, that excess pressure can harm the walls of the vessels, increasing the risk of heart attacks, strokes, and other adverse events.
It’s debated why salty diets increase blood pressure in some people but not others. A defect in the kidneys, which filter the blood, was long thought to cause salt sensitivity, by allowing too much sodium to remain in circulation. However, many other explanations have been proposed with varying degrees of science to support them. Some experts today believe that salt sensitivity begins with the blood vessels, which fail to dilate adequately in response to rising sodium levels in the blood, causing pressure to build up.
There is no established threshold to define salt sensitivity, as there is for hypertension (which is diagnosed if systolic blood pressure is 130 or higher or if diastolic blood pressure is 80 or higher). But in one often-cited study, researchers identified participants as salt sensitive if their blood pressure dropped by 10 points or more within one day of being fed a low-sodium diet and receiving a diuretic (a drug that helps the body excrete sodium). In other research, people were classified as salt sensitive if their blood pressure rose 10 percent or more after being switched from a week-long low-sodium diet to a high-sodium diet.
Some studies suggest that salt sensitivity is inherited, but it also increases with age (generally after 45) and is more common in women, African Americans, and people with metabolic syndrome, a cluster of risk factors including high blood pressure, high fasting blood sugar, abnormal blood fats, and a large waistline (indicating excess abdominal fat).
A cardiovascular concern
There is still a lot to learn about salt sensitivity, but the American Heart Association (AHA) found it worrisome enough to issue a scientific statement on the condition in 2016, calling it “as powerful” a risk factor for cardiovascular disease as high blood pressure. In its statement, the AHA cited two older studies as evidence for concern:
- In a study in the Lancet in 1997, Japanese researchers compared 62 people with hypertension who were salt sensitive to 94 people with hypertension who were salt resistant. Over a seven-year period, heart attacks and other cardiovascular events occurred twice as often in the salt-sensitive group.
- A study in Hypertension in 2001 followed 596 adults for up to 27 years, testing their blood pressure, salt sensitivity, and other factors at baseline. In the end, 73 percent of those who were identified as salt sensitive had died, compared to 45 percent of those who weren’t salt sensitive, regardless of whether they had hypertension. Among the participants who were over 25 and had normal blood pressure when the study began, those who were salt sensitive were more likely to have died than those who were salt resistant. And being salt sensitive was as big a risk factor for dying as hypertension.
No easy test
If salt sensitivity is such a risk, then why doesn’t your doctor check for it at your annual physical exam, along with your blood pressure and cholesterol, for instance? Unfortunately, there’s no quick and cost-effective way to measure salt sensitivity and no hard rules about how to do it. The standard method used in research requires a three-day hospital stay, which is impractical and too expensive for the general population. Instead, scientists are trying to identify a protein or other substance in the blood that could serve as an indicator, or biomarker, of salt sensitivity.
Best bet all around: reduce salt
Even if your doctor can’t test you for salt sensitivity, there’s generally no downside—and plenty of potential benefit—to cutting back on foods that are high in sodium, whether or not you have hypertension. Other potential harms from too much salt include increased bone loss, impaired kidney function, and elevated risk of some digestive-tract cancers, according to prior research. (If you have very low blood pressure, or hypotension, however, you should discuss your sodium intake with your doctor before making changes.)
There’s a good chance your diet is high in sodium, since Americans consume about 3,440 milligrams of sodium a day, on average, which is more than the daily limit of 2,300 milligrams recommended by the Dietary Guidelines for Americans and more than double the limit of 1,500 milligrams for people who have hypertension or other risk factors for cardiovascular disease.
About 75 percent of the sodium we consume comes from processed, prepared, and restaurant foods; the rest is naturally present in food (about 10 percent) or is added during cooking or at tableside (about 15 percent). For example, canned soup often has 800 or more milligrams of sodium per one-cup serving, while a fast-food cheeseburger typically has more than 700 milligrams. Thus, the best way to reduce your sodium intake is to cook your own meals as much as possible, using whole, unprocessed foods.
You can also follow the DASH (Dietary Approaches to Stop Hypertension) diet or other lower-sodium diets that emphasize vegetables, fruits, whole grains, nuts, legumes, low-fat dairy products, poultry, and fish. The standard DASH diet provides 2,300 milligrams of sodium a day while the lower-sodium version limits sodium to 1,500 milligrams a day.
What’s more, following many of the basic rules of healthy living can help you manage your blood pressure and overall cardiovascular risk: Don’t smoke (vaping is not a safe alternative), exercise regularly, keep your weight under control, and limit or avoid alcohol.
This article first appeared in the UC Berkeley Wellness Letter.
Also see How to Shake the Salt Habit.