Harvard Men's Health Watch

Salt and your health, Part I: The sodium connection

By now, most health-conscious American men understand the difference between bad fats (saturated fats and trans fats) and good fats (omega-3s and other poly- and monounsaturates). Many also recognize the difference between bad carbohydrates (simple sugars and refined grain products) and good carbs (dietary fiber and whole-grain products). In addition, savvy consumers are finally switching from red meat to fish, poultry, and legumes to get the protein they need.

It's heartening progress, but it overlooks another nutrient that's responsible for more than 100,000 American deaths a year, about three times more than prostate cancer. Perhaps because the problem nutrient is hidden away in processed foods and receives massive support from companies that manufacture and market these foods, the average American consumes 55% more of it today than in 1980. The hidden nutrient is not a fat or carbohydrate, and it doesn't pack any calories. The forgotten nutrient is salt.

A grain of history

In today's world, salt is abundant and cheap, but it wasn't always that way. Salt was hard to come by for our earliest ancestors, who got along quite nicely on about a tenth of today's average use in the United States. In time, people learned how to find salt and extract it from the earth. But it was hard work and salt was scarce, so it became a valuable commodity that was used for currency. In fact, the word salary is derived from the Latin word for salt. Perhaps because it was rare and expensive, salt carried a certain prestige; even today, a successful man is "worth his salt" and a good man is "the salt of the earth."

After the Industrial Revolution, salt became inexpensive and plentiful. It found a valuable role as a food preservative, and the average consumption soared to as much as 7,000 milligrams (mg) a day in the 19th century. Salt has long since outlived its use as a preservative, but our hankering for sodium lingers on, with daily consumption in America averaging 3,436 mg. Because of this acquired preference, salt is a big business: every year, the world consumes about 187 million tons, which is both mined from the earth and claimed from the sea.

Salt and sodium: A glossary

Each molecule of ordinary salt is composed of an atom of sodium (Na) joined to an atom of chloride (Cl); the chemical designation is NaCl. Because chloride is heavier than sodium, it contributes more to the weight of the molecule. But when it comes to health, it's the sodium that counts, whether it comes from table salt or from other sources, such as baking soda (sodium bicarbonate) or MSG (monosodium glutamate).

Because sodium is what matters, food labels list the content of sodium, not salt; it's expressed as milligrams (mg) of sodium. Most current dietary guidelines also specify milligrams of sodium, and it's the designation used by this and many other publications. But some nutritional information is still expressed in milligrams (mg) or grams (g) of salt. And to make things even more confusing, many research papers use another unit, millimoles (mmol).

Milligrams will do nicely for most of us; it's complex enough, especially if you're not used to the metric system. But if you encounter the other terminology, you can make your own conversions using these round numbers:

1,000 mg sodium = 1 g sodium

1 g sodium = 2.5 g salt

1 mmol sodium = 23 mg sodium

It is confusing, but it shouldn't shake your determination to keep track of the sodium in your diet.

It's the sodium, stupid

For chemists, a salt is any molecule that forms when positively and negatively charged atoms bond with each other. But when the atoms are sodium and chloride, the compound takes the name salt all to itself. For physicians, sodium is the key element in salt. It's a crucial as well as controversial substance; perhaps that's why its name evolved from the Arabic suda, "a splitting headache."

A taste of physiology

Make no mistake about it: salt is essential for human health. The average adult's body contains 250 grams (g) of sodium — less than 9 ounces, or about the amount in three or four saltshakers. Distributed throughout the body, salt is especially plentiful in body fluids ranging from blood, sweat, and tears to semen and urine.

Sodium is absorbed from the gastrointestinal tract, always bringing water along with it. It is the major mineral in plasma, the fluid component of blood, and in the fluids that bathe the body's cells. Without enough sodium, all these fluids would lose their water, causing dehydration, low blood pressure, and death.

Fortunately, it only takes a tiny amount of sodium to prevent this doomsday scenario; in fact, some isolated population groups manage perfectly well on just 200 mg a day. About one-quarter of the tongue's taste buds are devoted to recognizing salt; like other animals, humans can — and do — seek out salt when they need it. And when dietary salt is in short supply, the body can conserve nearly all its sodium, dramatically reducing the amount excreted in urine and shed in sweat. Remember that water always follows sodium, and you'll understand why your skin is dry and your urine scant and concentrated when you are dehydrated and conserving sodium.

To be sure its supply of salt and water is just right, the body has developed an elaborate series of controls. The blood vessels and brain signal the kidneys to retain or excrete sodium as needed; they also fine-tune the sensation of thirst so you'll provide water in amounts that match the body's sodium supply.

The body, in its wisdom, can make do with remarkably small amounts of sodium. But human behavior can thwart nature's checks and balances by taking in much more sodium than we need. The major consequence is a rise in blood pressure, which leads to a heightened risk of heart attack and stroke.

Other benefits

Reducing dietary salt will lower blood pressure, protecting against heart attack and stroke. That's reason enough to shake the salt habit, but there's more. Even modest salt restriction improves vascular reactivity and reduces urinary albumin loss, which protects the kidneys and the heart. Salt restriction also lowers the risk of kidney stones by reducing the amount of calcium in the urine. And the DASH diet appears to protect against diabetes, at least in Caucasians.

Sodium and blood pressure

Scientists know that sodium has an important influence on blood pressure, but they are not sure exactly how it works. It's no surprise, since the systems that control blood pressure include dozens of complex vascular, neurological, and hormonal elements. Although the body can rid itself of excessive dietary sodium, it seems likely that eating salt expands your blood volume, at least to a subtle degree. In turn, the extra volume may signal your kidneys to trigger a cascade of hormonal and vascular effects that raise blood pressure. And some experts suspect that these hormones may have adverse effects on vascular health even if blood pressure remains stable. In fact, a 2009 Australian study reported that a low-sodium diet improves arterial function independent of any effect on blood pressure.

Sodium and hypertension

The first person to suspect that eating salt might contribute to high blood pressure may have been Emperor Huangdi of China; about 5,000 years ago he wrote: "If too much salt is used in food, the pulse hardens." The scientific discourse, however, dates only from 1972, when Dr. Lewis Dahl presented evidence that a diet high in sodium contributes to high blood pressure. His hypothesis was soon questioned by other researchers, and the sodium controversy has raged ever since.

Why did the link between sodium and blood pressure generate so much heat? Part of the reason stems from the body's intrinsic complexity: sodium is but one of an enormous number of factors that affect blood pressure — and for all its importance, blood pressure is only one of the many things that determine vascular health. And the complexities of human behavior are just as daunting as those of human biology; dietary potassium, calcium, and many other nutrients influence blood pressure, as do exercise, body weight, alcohol use, and stress.

Additional challenges result from the methods scientists use to study the link between diet and hypertension. Blood pressure can fluctuate widely from minute to minute; if sustained, even small changes in blood pressure can have a large impact on lifetime risk. Plus, sodium consumption can vary substantially from day to day. Studies that rely on dietary history can differ from those that measure the amount of sodium in a person's daily urine, which should be a more accurate reflection of how much sodium has been consumed on a given day. Some people are more sensitive to sodium than others. And experiments that subject volunteers to a high or low consumption of sodium are necessarily brief, at least compared to the months and years it takes for blood pressure to affect health.

Little by little, though, a consensus has emerged. Most researchers, scientific advisory boards, and government agencies agree that reducing dietary salt will lower blood pressure, reduce the risk of heart attack and stroke, and save lives — up to 150,000 lives a year in the United States alone, according to the American Medical Association Council on Science and Public Health. And in this era of rapidly rising health care costs, it's important to note that cutting down on salt would save us up to $24 billion a year.

Salt and resistant hypertension

Many excellent antihypertensive drugs are available. Still, some 10% to 30% of patients fail to achieve good blood pressure control even while they're taking three such medications. But that doesn't mean these patients with resistant hypertension are beyond help. An important 2009 study of resistant hypertension reported that a low-sodium diet reduced systolic blood pressure by a whopping 22.7 millimeters of mercury (mm Hg) and diastolic blood pressure by 9.1 mm Hg.

Sodium restriction will never replace blood pressure medications — but it sure will help.

Impressive evidence

Although not all studies agree, a large body of evidence points to sodium as an important contributor to high blood pressure. After reviewing the results of animal experiments, population studies, and clinical trials, the World Health Organization described the evidence that high dietary sodium causes hypertension as "conclusive." Instead of wading through all the studies, pro and con, let's focus on just three landmark investigations.

Epidemiologic evidence

The International Study of Salt and Blood Pressure (INTERSALT) compared sodium intake, as measured by urinary levels, with blood pressure in 10,079 people between the ages of 20 and 59 in 52 population samples around the world. To check other factors that affect blood pressure, each subject was also evaluated for obesity, alcohol use, and dietary potassium. The result demonstrated a clear link between dietary sodium and blood pressure: in communities where the average sodium consumption was low, only 1.7% of the subjects had high blood pressure, but in places where sodium consumption was high, 13.4% were hypertensive.

It didn't take long for scientists to spot a weak link in the chain between sodium and blood pressure: although the relationship was clear when one society was compared with another, there was little if any correlation between dietary sodium and blood pressure within any one community. That means Americans who eat a lot of salt don't necessarily have higher blood pressure than those who eat less. It's a legitimate criticism, but INTERSALT responded by showing that age is an important element in the equation. Even within a single country, such as the United States, blood pressure rises more steeply with age in people who take in large amounts of sodium than in people who eat less salt.

This means you

It's easy to dismiss salt as the other guy's problem. That may be okay if you're a lean twenty-something with a blood pressure of 110/60, but even these men are likely to face issues with salt as they grow older.

Current guidelines say no adult should consume more than 2,300 mg of sodium a day, and that people with hypertension, all middle-aged and older adults, and all African Americans should consume no more than 1,500 mg a day.

Where do you fit in? At present, about two-thirds of all American adults have hypertension or prehypertension, and the average 50-year-old has a 90% chance of developing hypertension as he ages. So if you're like the rest of us, you'll benefit from cutting your dietary salt.

DASHing doubts

Demonstrating a link between dietary sodium and blood pressure is one thing, showing that cutting down on salt will lower blood pressure is another. Early trials of reduced sodium diets produced mixed results, largely because the patient populations, test diets, and experimental designs varied so greatly. That led many people to take advice about dietary sodium with a grain of salt. That skepticism was understandable, at least until 1997, when a major trial put things in perspective.

The first conclusive evidence that diet can lower blood pressure came from the Dietary Approaches to Stop Hypertension (DASH) study. Researchers evaluated three diets: a typical American, or "control," diet that was low in fruits, vegetables, and dairy products, with a fat content typical of the American average diet; a test diet rich in fruits and vegetables; and a "combination" diet (now known as the DASH diet) rich in fruits, vegetables, and low-fat dairy products.

Both of the test diets lowered blood pressure, but the DASH diet was the clear winner, reducing systolic and diastolic blood pressure by 5.5 and 3 millimeters of mercury (mm Hg), respectively. People with high blood pressure benefited even more, reducing their systolic blood pressure by 11.4 mm Hg and their diastolic pressure by 5.5 mm Hg.

The DASH diet is high in potassium (4,700 mg a day) and dietary fiber (31 g a day), moderate in calcium (1,240 mg a day), and moderately low in fat (27% of the total calories). All three diets contained a similar number of calories, so weight loss did not account for the benefit of either therapeutic diet. But did sodium explain the blood pressure improvements?

Surprisingly, perhaps, the answer is no; the study was designed to provide the same amount of sodium — 3,000 mg a day — in each diet. So while the original DASH study provided important evidence that a good diet can lower blood pressure, it offered no evidence about sodium one way or the other.

Critics of the salt hypothesis argued that DASH provided evidence against the importance of sodium. It's not true: the study showed that a good diet can lower blood pressure even if it contains more sodium (3,000 mg a day) than is currently recommended (1,500 to 2,300 mg a day), but it doesn't address whether additional sodium restriction can provide additional benefits. That's the question scientists looked at in the second DASH study.

Like the original research, the second DASH trial compared a control (typical American) diet with the combination DASH diet. But researchers also compared the effects of high-, intermediate-, and low-sodium intakes within each dietary group.

As before, the DASH diet produced a lower blood pressure than the typical American diet. But in both groups, sodium restriction produced additional benefits (see box below). Virtually everyone benefited from sodium restriction, including people with hypertension and those with normal blood pressures, African Americans and whites, men and women. The best results were observed in people with high blood pressure who followed the DASH diet and also reduced their consumption of sodium to the lowest levels.

DASHing hypertension

Blood pressure drop when various amounts of sodium are incorporated in a typical American diet or DASH diet. The blood pressure in a typical American diet containing 3,400 mg sodium/day is used for comparison.

Diet (sodium/day)

Systolic

Diastolic

Typical (2,300 mg)

2.1 mm Hg

1.1 mm Hg

Typical (1,150 mg)

6.7 mm Hg

3.5 mm Hg

DASH (3,450 mg)

5.9 mm Hg

2.9 mm Hg

DASH (2,300 mg)

7.2 mm Hg

3.5 mm Hg

DASH (1,150 mg)

9.9 mm Hg

4.5 mm hg

Source: Archives of Internal Medicine 2007, Vol. 167, p. 1,463.

Studies confirm that adherence to the DASH diet is associated with substantial protection against coronary artery disease (24% reduction in risk), strokes (18% reduction), and heart failure (37% reduction). These results should be enough to convince you to shake the salt habit, but if they don't, consider another important trial.

Cutting sodium, cutting risk

The two DASH trials evaluated diet and blood pressure; although subsequent studies suggest this diet produces protection against disease, DASH itself did not evaluate clinical events. But the two larger, longer Trials of Hypertension Prevention (TOHP) studies demonstrate that salt restriction both lowers blood pressure and prevents disease.

The first trial (TOHP I) evaluated 744 volunteers between 1987 and 1990, and the second (TOHP II) enlisted 2,382 people between 1990 and 1995. All the participants were between the ages of 30 and 54, and all had prehypertension (diastolic blood pressure of 80–89 mm Hg). In both studies, volunteers were randomly assigned to continue their normal diet or to reduce their sodium consumption. In TOPH I, participants who cut their average dietary sodium by 1,012 mg a day reduced their average systolic blood pressure by 1.7 mm Hg and their diastolic pressure by 0.8 mm Hg. In TOHP II, a more modest sodium reduction of 920 mg a day lowered the average systolic blood pressure by 1.2 mm Hg without producing a significant change in diastolic blood pressure readings.

The blood pressure reductions in TOHP I and II were so modest that they didn't do much to advance the cause of sodium restriction. Fast forward a decade, though, and you'll see the power of even modest sodium restrictions and small blood pressure reductions. When the TOHP researchers re-evaluated their subjects 10 to 15 years after the trials ended, they discovered that the volunteers who restricted their dietary sodium during the 18 to 48 months the studies lasted enjoyed a 25% to 30% reduction in their long-term risk for cardiovascular events.