Lown Cardiovascular Research Foundation

Harvard Heart Letter | September 2007

Shake the salt habit for a longer life

Long-term study links cutting back on salt with less heart disease and longer survival.

The argument over how much salt in the diet is too much has been going on for decades. One camp says that we all need to eat less salt. The other scoffs at this tasteless idea, claiming that cutting back on salt would have little effect on public health. One thing missing from the sometimes heated debate is evidence about the long-term effects of lowering salt intake. Oodles of studies have shown that eating too much salt boosts blood pressure and getting less lowers it. But until now, there has been precious little information on what really matters: salt’s effect on heart disease and survival.

That’s changing, thanks to persistent work from a national team headed by researchers at Harvard-affiliated Brigham and Women’s Hospital. Their work suggests that modestly reducing salt intake can pay off in terms of fewer heart attacks, strokes, and premature deaths from cardiovascular disease. Whether this works for everyone, or just for people with blood pressures above the healthy range, remains to be determined.

Getting a handle on the long-term effects of cutting back on salt hasn’t been easy. The main barrier? No long-term trials. Few volunteers are willing to sign up for a trial lasting 10 years or longer and, even if they did, they would probably have trouble sticking with an assigned salt intake for that long. Plus, the National Institutes of Health, which comes up with the lion’s share of funding for this kind of research, rarely pays for such long-term studies. This kind of situation calls for creative approaches.

Taking the long view

Epidemiologist Nancy R. Cook and her colleagues turned to the Trials of Hypertension Prevention (TOHP), two trials they had conducted in the late 1980s and early 1990s. The first TOHP looked at four strategies for lowering blood pressure: weight loss, stress management, nutritional supplements, and salt reduction. The second one tested only weight loss and salt reduction. All told, more than 3,000 men and women with what is now called prehypertension — blood pressures that are above normal but not quite high enough to qualify as hypertension — were randomly assigned to one of two groups. Those in the sodium reduction groups learned how to select and prepare lower-sodium foods and monitor their salt intake. Those in the control groups were given general guidelines on healthy eating.

In TOHP 1, which lasted for 18 months, participants in the salt reduction group lowered their daily intake of sodium by just over 1,000 milligrams (mg), or about one-half teaspoon of salt. This was accompanied by a small but significant reduction in blood pressure. In TOHP 2, which lasted 36 months, sodium intake fell by 750 mg, or about one-third of a teaspoon of salt, while blood pressure barely changed.

The two trials didn’t make a huge splash, mostly because the intense dietary and behavioral counseling that was provided to the participants yielded fairly small reductions in blood pressure. The TOHP results were overshadowed a few years later by more compelling findings from the Dietary Approaches to Stop Hypertension (DASH) trial. It yielded bigger drops in blood pressure from a diet rich in fruits, vegetables, low-fat dairy foods, whole grains, beans, nuts, fish, and poultry. A lower-sodium DASH diet proved to be even better.

Fast-forward 10 years. Dr. Cook and her colleagues gathered health information on all the sodium or sodium-control TOHP participants they could track down — just over three-quarters of the volunteers. Among those who had been in the control groups, 9% had suffered a heart attack or stroke or died of cardiovascular disease in the years after the trial, compared with 7.5% from the sodium reduction groups. That doesn’t seem like much of a difference. But when you consider that more than 1.5 million Americans have heart attacks or strokes a year, this small difference could mean preventing thousands of these catastrophes annually.

Sodium overload

Table salt — sodium chloride to a chemist — isn’t entirely a demon. Your body needs some sodium to transmit nerve impulses, contract and relax muscle fibers (including those in the heart and blood vessels), and maintain a proper fluid balance. But it doesn’t take much to meet the body’s needs. The Yanomami people of the Amazon rainforest make do with just 200 mg a day. The 3,400 mg the average American gets each day (the amount in one and a half teaspoons of salt) is far more sodium than is needed to do the job.

In some people, the kidneys effortlessly flush excess sodium into the urine. In others, the kidneys can’t keep up. Sodium accumulates in the fluid between cells. Water follows sodium, leading to an increase in the amount of water in the body and the volume of blood in circulation. Blood pressure climbs, and the heart must work harder. Too much sodium in the body may harm the heart in other ways, too. There is some evidence that it blunts the ability of blood vessels to relax and contract with ease. Excess salt may also overstimulate the growth of heart tissue.

Stealth salt

The TOHP trials didn’t require drastic dietary changes. They weren’t about sodium restriction — cutting out as much salt as possible. Instead, they focused on sodium reduction. The volunteers ate their normal diets but learned how to look out for hidden salt and avoid it.

That’s an important strategy. Only about 10% of the salt we eat is added during cooking or at the table. More than 75% of it comes from processed or prepared foods. You can’t always taste hidden salt; a one-cup serving of Cheerios has more salt than a one-ounce serving of salted potato chips.

The American Public Health Association, American Medical Association, and other health organizations have called for a national reduction in sodium in prepared foods. That’s not likely to happen any time soon, so you are on your own if you want to cut back on salt. Here are three tips:

Check out the fine print. Choose foods low in sodium by reading food labels. Sodium is usually listed after total fat and cholesterol.

Make it at home. Limit the use of canned, processed, and frozen foods. Cooking from scratch lets you control the salt content.

Don’t hesitate to ask. When eating out, ask if items are prepared with salt; in fast-food restaurants, ask for a nutrition information sheet.