Harvard Heart Letter

Take it with a grain of salt

Cutting back on our most common seasoning is a necessity for some people, but not for everyone.

"The history of the Americas is one of constant warfare over salt."

"" Mark KurlanskySalt: A World History

Battles for the control of salt once pitted country against country, Cape Cod militiamen against British regulars, even the Union against the Confederacy. Although the physical fights are history, the salt wars still rage in the pages of medical journals and government recommendations.

The controversy boils down to one question: Should we all eat less salt?

Experts have been arguing about this for decades. It's been a bitter and passionate fight, with little middle ground. One side says everyone needs to cut back on salt and that doing so would substantially reduce heart disease. The other side says universal salt reduction would have little effect on public health and would be a needless deprivation for most people.

Is salt a crystalline demon? A harmless treat for the taste buds? Or something in between? As we're learning about so many things in medicine, there isn't a simple right answer. How salt affects your blood pressure and health depends on your genes, your age, and your medical conditions.

Salt in circulation

To a chemist, salt is what you get when positive and negative ions enter each other's orbit. To most everyone else, salt is sodium chloride, the white crystals left over when seawater evaporates. It's the sodium in salt that causes most of the problems. One teaspoon of sodium chloride "" table salt "" contains 2,300 milligrams (mg) of sodium.

The human body can't live without some sodium. It's needed to transmit nerve impulses, contract and relax muscle fibers (including those in the heart and blood vessels), and maintain a proper fluid balance. It doesn't take much to do this. The Yanomamo people of the Amazon rainforest get by on just 200 mg of sodium a day (about the amount found in one-tenth teaspoon of salt). By comparison, the average American gets 3,400 mg (about 1½ teaspoons of salt), while in northern Japan the daily intake is a whopping 26,000 mg (more than 11 teaspoons of salt).

When sodium is in short supply, a host of chemical and hormonal messages signal the kidneys and sweat glands to hold onto water and conserve sodium. When you get more sodium than you need, the kidneys flush out the excess by making more, or saltier, urine. If they can't get rid of enough sodium, though, it accumulates in the fluid between cells. Water inevitably follows sodium, and as the volume of this fluid increases, so does the volume of blood. This means more work for the heart and more pressure on blood vessels. Over time, this can stiffen blood vessels, leading to high blood pressure, heart attack, or stroke. It can also lead to heart failure.

There is also some evidence that salt can directly affect the heart, aorta, and kidneys without necessarily increasing blood pressure.

Some people are exquisitely sensitive to salt "" their blood pressure rises and falls as a direct result of how much salt they get. Others don't seem to be affected at all. Unfortunately, there isn't an easy test to determine who is salt-sensitive.

A DASH of evidence

Hundreds of studies have looked at the connections between salt intake and blood pressure, heart disease, stroke, and mortality. In general, they show that cutting back on salt lowers blood pressure and reduces the chances of having a heart attack or stroke. The trouble with these studies is that virtually every one has flaws, which are pointed out immediately by those who disagree with the study's conclusions. They are too short, too small, not like the real world, or influenced by factors other than sodium.

Some of the most compelling evidence that eating less salt can lower blood pressure comes from the Dietary Approaches to Stop Hypertension (DASH) trials. The first DASH trial showed that a diet rich in fruits, vegetables, low-fat dairy products, whole grains, beans, nuts, fish, and poultry lowered blood pressure in people with normal and high blood pressure. A follow-up trial added salt into the mix. It compared a DASH diet with a control diet that was much like the average American diet. Both also had three levels of sodium: high sodium, at 3,500 mg of sodium a day; moderate sodium, 2,300 mg; and low sodium, 1,100 mg. More than 400 volunteers followed their assigned diets for a month.

The DASH diet again proved better for blood pressure than the control diet. And across the board, the less sodium, the lower the systolic (the top number of a blood pressure reading) and diastolic (the bottom number) pressures. Lower sodium had the greatest effect in people with high blood pressure on the control diet, blacks on the control diet, and women on the DASH diet.

The low-sodium DASH diet worked best of all. Among volunteers assigned to this combination, systolic blood pressure was 8.9 mm Hg lower than it was among those on the high-sodium control diet, while diastolic pressure was 4.5 mm Hg lower. In an individual, that's the equivalent of taking a medication to lower blood pressure. The study didn't last long enough to see if this translated into less heart disease. Similar reductions, though, have been shown to reduce the risk of heart disease by 20% and stroke by 35%.

We're all different

The overall results from the DASH trials and others harbor a fair amount of variation. In almost every experiment of salt reduction, while most volunteers' blood pressure dropped, some participants experienced no change, and others actually saw their blood pressure rise.

The increases could be chalked up to variations in how, or when, blood pressure was measured in the studies. They might reflect the sometimes substantial day-to-day variations in blood pressure that we all have. They could also be real. In some people, lowering sodium intake could force the body to make more renin which, in turn, can increase levels of angiotensin, a protein that can boost blood pressure.

How low should you go?

There isn't a one-size-fits-all recommendation for daily sodium intake. Some people definitely benefit from getting less. For others it won't make much of a difference on blood pressure.

If you are under age 50, your blood pressure is in the healthy range (under 120/80), and your health is good, you have little reason to worry about dietary salt right now. That said, weaning your taste buds from their dependence on salt might be a good idea for down the road.

A lower-sodium diet is good for people who are older, who are of African American descent, who have high blood pressure or diabetes, or whose blood pressure is gradually creeping upward. The Institute of Medicine, the Dietary Guidelines for Americans, and the American Heart Association recommend limiting your sodium intake to no more than 2,300""2,400 mg a day.

If you have heart failure, salt can cause or increase swelling. Too much salt can lead to hospitalization for powerful intravenous drugs to remove excess fluid. People with heart failure are usually advised to keep their sodium intake under 2,000 mg a day. People with kidney disease are usually instructed to do the same.

Sources of salt

Cutting back

Little of the salt we eat comes from the salt shaker, either in the kitchen or at the table (see "Sources of salt"). The bulk comes from food processing. Luncheon meats, pasta sauce, canned and dried soup, even commercially prepared baked goods pack plenty of sodium (see "Sodium in food"). Fast foods can be just as bad. At Burger King, a Whopper with cheese and a medium order of fries deliver 2,000 mg, or nearly a day's worth of sodium.

Sodium in food

Most of the sodium we eat comes from packaged, processed, or prepared foods. Some items contain staggering amounts of it.

Food

Sodium

Swanson's Hungry Man XXL Roasted Turkey, 1 package

5,410 mg

Dunkin' Donuts salt bagel, 1 bagel

4,590 mg

Fried rice, 1 serving

2,680 mg

Salt, 1 teaspoon

2,300 mg

Oscar Mayer Lunchables Deluxe Turkey and Ham with Swiss and Cheddar, 1 package

1,940 mg

Soy sauce, 1 tablespoon

1,260 mg

Baking soda, 1 teaspoon

1,260 mg

Prepared pasta sauce, 1 cup

1,200 mg

Chicken noodle soup, diluted 1:1 with water, 1 cup

1,070 mg

Mott's Mr. & Mrs. T Bloody Mary Mix, 6 ounces

1,050 mg

Sources: US Department of Agriculture National Nutrient Database and Center for Science in the Public Interest

The American Public Health Association, American Medical Association, and others have called for a national cutback on sodium. They want the FDA to revoke salt's status as a "generally recognized as safe" ingredient. Doing that would let the agency regulate salt content in food. The organizations also want the food industry to reduce sodium in processed and restaurant food by at least 50% over the next decade.

Such measures face an uphill battle. The food industry has substantial clout on Capitol Hill. And consumers haven't shown much interest in buying lower-salt foods.

For the foreseeable future, you are on your own if you want to "" or need to "" cut back on salt. Here are a few basic tips:

  • Read food labels and choose foods low in sodium.

  • Limit the use of canned, processed, and frozen foods.

  • When eating out, ask if items are prepared with salt; in fast food restaurants, ask for a nutrition information sheet.

  • Cook with herbs and spices instead of salt.

What about salt substitutes? Some are good, others can be tricky for some people.

Keep in mind that sodium is just one of many factors that influence your blood pressure and cardiovascular health. For the greatest gains in both, a broader focus on healthful eating, exercise, weight control, and stress reduction will have a bigger payoff.